Fossils of the Burgess Shale
Encyclopedia
The fossils of the Burgess Shale, like the Burgess Shale
itself, formed around in the Mid Cambrian
period. They were discovered in Canada
in 1886, and Charles Doolittle Walcott
collected over 60,000 specimens in a series of field trips up from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to discover new species, and statistical analysis suggests discoveries will continue for the foreseeable future. Stephen Jay Gould
's book Wonderful Life
describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution
is largely superseded.
The fossil beds are in a series of shale
layers, averaging 30 millimetres (1.2 in) and totalling about 160 metres (524.9 ft) in thickness. These layers were deposited against the face of a high undersea limestone
cliff. All these features were later raised up 2500 metres (8,202.1 ft) above current sea level during the creation of the Rocky Mountains
.
These fossils have been preserved in a distinctive style known as Burgess shale type preservation
, in which fairly tough tissues such as cuticle are preserved as thin films, very soft tissues are preserved as solid shapes and so quickly that decay had too little time to destroy them, while moderately soft tissues such as muscles are lost. Scientists are still unsure about the processes that created these fossils. While there is little doubt that the animals were buried under catastrophic flows of sediment
, it is uncertain whether they were transported by the flows from other locations, or lived in the area where they were buried, or were a mixture of local and transported specimens. This issue is closely related to whether conditions around the burial sites were anoxic or had a moderate supply of oxygen
. Anoxic conditions are generally thought the most favourable for fossilization, but imply that the animals could not have lived where they were buried.
In the 1970s and early 1980s the Burgess fossils were largely regarded as evidence that the familiar phyla
of animals appeared very rapidly in the Early Cambrian, in what is often called the Cambrian explosion
. This view was already known to Charles Darwin
, who regarded it as one of the greatest difficulties for the theory of evolution he presented in The Origin of Species
in 1859. However, from the early 1980s the cladistics
method of analysing "evolutionary family trees" has persuaded most researchers that many of the Burgess Shale's "weird wonders", such as Opabinia
and Hallucinogenia, were evolutionary "aunts and cousins" of present-day types of animal rather than a rapid proliferation of separate phyla, some of which were short-lived. Nevertheless there is still debate, sometimes vigorous, about the relationships between some groups of animals.
in Canada's Rocky Mountains
by a construction worker, whose reports of them reached Richard McConnell of the Geological Survey of Canada. McConnell found trilobite
beds there in 1886, and some unusual fossils that he reported to his superior. These were misdiagnosed as headless shrimp
s with unjointed appendage
s, and were named Anomalocaris
because of their unusual appendages – but turned out to be pieces of a puzzle that took 90 years to solve.
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Similar fossils were reported in 1902 from nearby Mount Field
, another part of the Stephen formation
. These may have been why Charles Doolittle Walcott
visited Mount Field in 1909. While taking photographs there Walcott found a slab of fossils that he described as "Phyllopod crustacean
s". From late August to early September 1909 his team, including his family, collected fossils there, and in 1910 Walcott opened a quarry which he and his colleagues re-visited in 1911, 1912, 1913, 1917 and 1924, bringing back over 60,000 specimens in total. Walcott was Secretary of the Smithsonian Institution
from 1907 to his death in 1927, and this kept him so busy that he was still trying to make time for analyzing his finds two years before his death. Although he drew attention to the exceptional detail of the specimens, which were the first known fossils of soft-bodied animals from the Cambrian
period, he also had other research interests: the Early Paleozoic
stratigraphy
of the Canadian Rockies, which took up the great majority of his time there; and Precambrian
fossils of algae
and bacteria
, to which he assigned as much importance as to the fossils of animals. He managed to publish four "preliminary" papers on the fossil animals in 1911 and 1912, and further articles in 1918, 1919 and 1920. Four years after Walcott's death his associate Charles Resser produced a package of additional descriptions from Walcott's notes. Walcott's classification
s of most of the fossils are now rejected, but were supported at the time, and he accepted a change for one of the few where his conclusion was disputed. Many of the later comments were made with the benefits of hindsight, and of techniques and concepts unknown in Walcott's time.
Although in 1931 Percy Raymond opened and briefly excavated another quarry about 20 metres (65.6 ft) above Walcott's "Phyllopod bed
", there was very little interest in the Burgess Shale fossils from the 1930s to the early 1960s, and most of those collected by Walcott were stored on high shelving in back rooms at the Smithsonian Institution. Between 1962 and the mid-1970s Alberto Simonetta re-examined some of Walcott's collection and suggested some new interpretations. Beginning in the early 1970s Harry Whittington
, his associates David Bruton and Christopher Hughes, and his graduate students Derek Briggs
and Simon Conway Morris
began a thorough re-examination of Walcott's collection. Although they assigned groups of fossils to each member of the team, they all decided for themselves which fossils to investigate and in what order. Their publications and Stephen Jay Gould
s' popularization of their work in his book Wonderful Life
aroused enduring scientific interest and some public interest in the Cambrian explosion
, the apparently rapid appearance of moderately complex bilaterian animals in the Early Cambrian
.
The continuing search for Burgess Shale fossils since the mid-1970s has led to the description in the 1980s of an arthropod Sanctacaris
and in 2007 of Orthrozanclus
, which looked like a slug
with a small shell at the front, chain mail
over the back and long, curved spines round the edges. Recent digs have discovered species which have yet to be formally described and named. They have also unearthed more and sometimes better fossils of animals that were discovered earlier, for example Odontogriphus
was for many years known from just one poorly preserved specimen, but the discovery of a further 189 formed the basis for a detailed description and analysis in 2006. Re-examination of Walcott's collection also continues, and has led to the reconstruction of the large marine animal Hurdia
in 2009.
deposits spread over a vertical distance of hundreds of metres, extending laterally for at least 50 kilometres (31.1 mi). The deposits were originally laid down on the floor of a shallow sea; during the Late Cretaceous
Laramide orogeny
, mountain-building processes squeezed the sediments upwards to their current position at around 2500 metres (8,202.1 ft) elevation in the Rocky Mountains.
The rocks containing the fossils are on the border between two partially overlapping bands of rock that run along the western face of the Canadian Rockies. On the eastern side of this border is the Cathedral Formation, a platform of limestone
formed by algae
. The western surface of the Cathedral Formation is steep and consists of the resistant rock dolomite
, which was originally part of the limestone platform, but between the Mid Silurian
and Late Devonian
was transformed by hydrothermal flows of brine at up to 200 °C (392 °F), which replaced
much of the limestone's calcium
with magnesium
. A layer of shale
lies partly on top of and partly to the west of the Cathedral Formation. This shale layer used to be called the "thin" Stephen Formation where it lies above the Cathedral Formation and the "thick" Stephen Formation where it lies to the west; but the "thick" Stephen Formation is now generally known as the Burgess Formation.
The shale is made of alternating fine-grained layers of siliceous mudstone (compressed, hardened mud originally made of ground-up silicate
rock) and calcisiltite
originally animal shells). The calcisiltite layers contain relatively unremarkable shells and occasional non-biomineralized fossils (such as priapulid tubes). The soft-bodied organisms for which the Burgess Shale is famous are fossilized in the mudstone layers, which are between 2 and 170 mm (0.078740157480315 and 6.7 in) thick, averaging 30 millimetres (1.2 in), and have well-defined bases. Opinions vary about how the mudstone layers were produced: perhaps by mudslides from the top of the "Cathedral" limestone platform, when its edge collapsed occasionally; or possibly by storms which created back-currents that abruptly washed large volumes of mud off the platform. Each mudstone layer is the result of one such catastrophe. The Greater Phyllopod Bed, a 7 metres (23 ft) thick sequence consisting of Walcott
's famous "Phyllopod Bed" plus the 5 metres (16.4 ft) below that, contains at least 36 layers, deposited over 10 to 100 thousand years, during which the environment was essentially stable.
to sustain animals. The traditional view is that soft bodies and organs could only be preserved in anoxic conditions, otherwise oxygen-breathing bacteria
would have made decomposition
too rapid for fossilization. This would imply that the sea-floor organisms could not have lived there. However, in 2006 Caron and Jackson concluded that the sea-floor animals were buried where they lived. One of their main reasons was that many fossils represented partially decayed soft-bodied animals such as polychaete
s, which had already died shortly before the burial event, and would have been fragmented if they had been transported any significant distance by a storm of swirling sediment. Other evidence for burial where the animals had lived includes the presence of tubes and burrows, and of assemblies of animals preserved while they fed – such as a group of carnivorous priapulids clustered round a freshly moult
ed arthropod
whose new cuticle
would not yet have hardened. Fossilized swimming organisms were also buried immediately below where they lived.
Fossil tracks
are rare and no burrows under the sea-floor have so far been found in the Burgess Shale. These absences have been used to support the idea that the water near the sea-floor was anoxic. However it is possible that the water just above the sea-floor was oxygenated while the water in the sediment below it was anoxic, and also possible that there simply were no deep-burrowing animals in the Burgess Shale. Some fossils, such as Marrella, are almost always the right way up, which suggests they were not transported far if at all. Others, such as Wiwaxia
, are often at odd angles, and some fossils of animals with shelly or toughened components very rarely contain remains of soft tissues. This suggests that the distances over which corpses were transported may have varied between genera
, although most were buried where they had lived.
Fossils known as Girvanella
and Morania
may represent members of microbial mat
communities. Morania appears on about a third of the slabs Caron and Jackson studied, and in some cases presents the wrinkled "elephant skin" texture typical of fossilized microbial mats. If such mats were present, they may have provided food for grazing animals and possibly helped to preserve soft bodies and organs, by creating oxygen-free zones under the mats and thus inhibiting the bacteria that cause decomposition.
The Burgess Shale animals were probably killed by changes in their environment either immediately preceding or during the mud-slides that buried them. Proposed killing mechanisms include: changes in salinity
; poisoning by chemicals such as hydrogen sulfide
or methane
; changes in the availability of oxygen; and changing consistency of the sea floor. The death event was not necessarily related to the burial, and there may have been multiple death events between burial events; but only organisms killed immediately before a burial event would stand any chance of being fossilised, instead of rotting or being eaten.
Burgess shale type preservation
is defined as the fossilization of non-biomineralized organisms as flattened carbon
aceous films in marine shales. When the animals started to decompose, their tissues collapsed under the weight of the sediment that buried them. The typical flattened fossils are outlines of tougher parts such as cuticle
s and jaws, which resisted decomposition for long enough to be fossilized. Soft elements, such as muscles and gut contents, were sometimes squeezed out of the decomposing organism to produce dark stains on the fossils. Organisms that lack tougher structures, such as flatworm
s, nemerteans and shell-less molluscs, were not preserved by this process. Very soft but chemically active tissues may be preserved by different processes. For instance during decomposition, bacteria
modify the chemically unusual mid-gut glands of some organisms into the durable mineral phosphate
. This change happens extremely quickly, before the corpse is compressed, and leaves a three-dimensional mold
of the tissues. Gills can also be preserved in something close to their original three-dimensional shape by this process. Both preservation mechanisms can appear in the same fossil. In Burgess-like shales, organisms and parts that are only quite soft, such as muscles, are generally lost, while those that are extremely soft and those that are fairly tough are preserved. The preservation of different body parts in different ways may sometimes help palaeontologists, by suggesting whether a body part was fairly tough like an arthropod limb (preserved as flat film) or very soft and chemically active, like a part of the gut (preserved as a solid piece of mineral). These differences may also help to identify fossils, by excluding from consideration organisms whose body parts do not match the combination of types of preservation found in a particular fossil bed.
It has often been suggested that this type of preservation was possible only when sediments were not disturbed
by burrowing animals or the anchors
of plants. However a similar type of preservation has been found in fossils from the Late Riphean period, about , but in no known fossils between the end of that epoch and the start of the Cambrian. This suggests that such bioturbation has little to do with the appearance and disappearance of Burgess Shale type preservation
. Such preservation may depend on the presence of clay
-like minerals that inhibit decomposition, and ocean chemistry may only have favoured the production of such minerals for limited periods of time. If so, it is impossible to be sure when the animals known as "Burgess Shale fauna" first appeared or when they became extinct. A few fossils of animals similar to those found in the Burgess Shale have been found in rocks from the Silurian
, Ordovician
and Early Devonian
periods, in other words up to 100 million years after the Burgess Shale.
in 1986 and by Caron and Jackson in 2008. Caron and Jackson commented that Conway Morris had to rely on a set of specimens which may not have been representative, since their excavators discarded specimens they found uninteresting; and for which the exact level in the rock sequence had not been recorded, making chronological analyses impossible. Both studies noted that the set of species in Walcott's Phyllopod Bed (Conway Morris, 1986) and its expanded version the Greater Phyllopod Bed (Caron and Jackson, 2008) was different from those found in other parts of the Burgess Shale; and Conway Morris commented that faunas at most other Burgess Shale sites resembled those of the Raymond Quarry, which is above and therefore more recent than the Greater Phyllopod Bed (abbreviated "GPB").
Conway Morris found that the shelly fossils in Walcott's Phyllopod Bed were about as abundant as in other shelly fossil deposits, but accounted for only 14% of the Phyllopod Bed fossils. Assuming that, as in modern marine ecosystem
s, about 70% of the species that lived in the Early Cambrian seas are unsuitable for fossilization, he estimated that the shelly fossils probably represent about 2% of the animals that were alive at the time. Since these shelly fossils are found in other parts of North America and, in many cases, over a much wider range, the Burgess Shale fossils, including the soft-bodied ones, probably show how much diversity
could be expected at other sites if Burgess Shale type preservation
were found there.
Caron and Jackson found that about 25% of the 172 known species were abundant and widespread throughout the time range of the GPB, while the majority of species were rare, and occurred in a small area for a short period of time. In most layers the five most abundant species accounted for 50% to 75% of individual animals. The species that had wide ranges in time and space may have been generalists, while the rest were specialists in particular types of environment. Alternatively some wide-ranging species may have been opportunists that were quick to recolonize the area after each burial event. The 6 species that appeared in all layers were very probably generalists.
In each burial event layer the commonest species generally has several times as many individuals as the second most common, and accounts for 15% to 30% of individual fossil animals. The more common a species is in one layer, the greater the number of other layers it appears in. These "recurrent" species account for 88% of the individual specimens, but only 27% of the number of species. This suggests that the majority of species were in existence for much shorter periods than the "recurrent" ones. Species that cover shorter periods of time occur mainly in the higher, younger layers. The GPB shows an overall trend of increasing diversity as time progresses.
In almost all layers arthropod
s are the most abundant and diverse group of fossils in the GPB, followed by sponges. 69.2% of GPB individuals and 63.9% of species lived on the surface of the sea bed; within this group, mobile deposit feeders that extracted food particles from the sediment accounted for 38.2% of the total number of individuals and 16.8% of the total species; the smallest sub-group was mobile hunters and scavengers; and the rest were sessile
suspension feeders. Animals that lived in the sediment made up 12.7% of the species and 7.4% of the individuals; the largest sub-group was mobile mobile hunters and scavengers. Bottom-dwelling animals capable of swimming comprised 12.7% of species and 7.4% of individuals. Organisms which spent their whole life swimming were very rare, accounting for only 1.5% of individuals and 8.3% of species.
These patterns – a few common species and many rare ones; the dominance of arthropods and sponges; and the percentage frequencies of different life-styles – seem to apply to all of the Burgess Shale. However the identity of the dominant species differs between sites. For example Marrella splendens is often credited as the commonest animal in the Burgess Shale, because of its abundance among the specimens collected by Walcott, but is only the third-most abundant organism in the Greater Phyllopod Bed, and very rare at other localities.
The overall community and ecology is very similar to that of other Cambrian localities, suggesting a global pool of species which repopulated localities after calamitous burial events occurred.
Caron and Jackson used computer software to simulate the numbers of species that would be found if smaller numbers of specimens were included, and found that the number of species "discovered" kept increasing as the number of specimens increased, rather than reaching a plateau. This suggests that Burgess Shale probably still contains as-yet undiscovered species, although probably very rare ones. Some recently discovered species, known in 2008 only by nicknames like "woolly bear" and "Siamese lantern" are familiar to the collecting teams, but have yet to be formally described and named. The team also nicknamed another discovery as "Creeposaurus", and in 2010 this animal was described and formally named Herpetogaster
.
Marrella was the first Burgess Shale fossil that Whittington
re-examined, and gave the first indication that surprises were on the way. Although clearly an arthropod
that walked on the sea-floor, Marella was very different from the known marine arthropod groups (trilobites, crustaceans and chelicerate
s) in the structure of its legs and gills, and above all in the number and positions of the appendage
s on its head, which are the main feature used to classify arthropods. A fossil of Marrella from the Burgess Shale has also provided the earliest clear evidence of molting.
Whittington’s first presentation about Opabinia
made the audience laugh. The reconstruction showed a soft-bodied animal with a slim, segmented body; a pair of flap-like appendages on each segment with gills above the flaps; five stalked eyes; a backward-facing mouth under the head; and a long, flexible, hose-like proboscis
which extended from under the front of the head and ended in a "claw" fringed with spines. Whittington concluded that it did not fit into any phylum
known in the mid-1970s. Opabinia was one of the main reasons why Stephen Jay Gould
in his book Wonderful Life
considered that Early Cambrian life was much more diverse and "experimental" than any later set of animals, and that the Cambrian explosion
was a truly dramatic event, possibly driven by unusual evolution
ary mechanisms. He regarded Opabinia as so important to understanding this phenomenon that he originally wanted call his book Homage to Opabinia. However subsequent research has concluded that Opabinia is closely related to the arthropods and possibly even closer to ancestors of the arthropods.
The discovery of Anomalocaris
("abnormal claw") was a comedy of errors
. The name was initially given to a fossil that looked like the rear end of a shrimp-like crustacean
. Walcott classified a ring-like fossil he called Peytoia as a kind of jellyfish, and another poorly preserved fossil he called Laggania as a holothurian (sea cucumber). After many plot twists, Derek Briggs started dissecting another ill-defined fossil in very thin slices and found a pair of "Anomalocaris"-like structures on one end of a specimen of "Laggania", which also had a specimen of "Peytoia" attached just behind those of "Anomalocaris". After dissecting more specimens and finding similar configurations, Briggs and Whittington concluded that the whole assemblage represented a single animal, which was named Anomalocaris because that was the earliest name assigned to any of its parts. This animal's body was fragile and usually disintegrated before it could be fossilized. But the complete animal had tough grasping appendages ("Anomalocaris"), a tough, ring-like mouth with teeth on the inner edge ("Peytoia") and a long, segmented body ("Laggania") with flaps on the sides that enabled it to swim with a Mexican wave motion, and perhaps to turn quickly by putting the flaps on one side into reverse. This monster was over 2 foot (0.6096 m) long when other animals were only a few inches at most. Nedin suggested in 1999 that the animal was capable of taking heavily armored trilobites apart, possibly by grabbing one end of their prey in their jaws while using their appendages to quickly rock the other end of the animal back and forth, causing the prey's exoskeleton to rupture and allowing the predator to access its innards. In 2009 Hagadorn found that anomalocarid mouthparts showed little wear, which suggests they did not come into regular contact with mineralised trilobite shells. Computer modeling of the Anomalocaris mouthparts suggests they were in fact better suited to sucking on smaller, soft-bodied organisms. Although Whittington and Briggs concluded that Anomalocaris did not fit into any known phylum, research since the 1990s has concluded that it was closely related to Opabinia and to the ancestors of arthropods. In 2009 a fossil named Schinderhannes bartelsi
, an apparent relative of Anomalocaris, was found in the Early Devonian
period, about 100 million years later than the Burgess Shale.
Conway Morris gave Hallucigenia
its name because in his reconstruction it looked bizarre – a worm-like animal that walked on long, rigid spines and had a row of tentacles along its back. Science fiction
author Greg Bear
says the Jarts in his The Way
stories were scaled-up versions of this reconstruction. However in the late 1980s Lars Ramsköld literally turned it over, so that the "tentacles", which he found were paired, became legs and the spines were defensive equipment on its back. Ramsköld classified it as one of the Onychophora, a phylum of "worms with legs" that is considered closely related to arthropods. Another current view is that Hallucigenia was an armored lobopod, in other words more closely related to arthropods than the onychophorans are, but less closely related to arthropods than Opabinia or Anomalocaris are.
Most fossils of Wiwaxia
are just a jumble of armor plates and spines, but after examining dozens of them Conway Morris reconstructed them as slug
-like animals covered in rows of overlapping armor plates, with two rows of longer spines projecting upwards. Since 1990 there has been an intense debate about whether Wiwaxia
was more closely related to molluscs or to polychaete
annelid
s. Supporters of a close relationship with molluscs maintained that a pair of bars, running across the mouth and armed with backward-pointing teeth, were a rudimentary form of the radula
, the toothed tongue that molluscs use to scrape up food and convey it back to the throat. Nicholas Butterfield, the one academic who has so far published articles placing Wiwaxia closer to polychaetes, stated that Wiwaxia′s two-row feeding apparatus could not have performed the sophisticated functions of the multi-row, conveyor-belt radula, suggesting instead that Wiwaxia′s apparatus was like the side-by-side pair of toothed bars found in some polychaetes. Later he found some fragmentary fossils, 5 to 10 million years before the Burgess Shale, that he regarded as a much more convincing early radula. Butterfield has also described Wiwaxia′s armor plates and spines as similar in internal structure to the chetae ("hairs") of polychaetes. Supporters of the link with molluscs have stated that Wiwaxia
shows no signs of segmentation, appendages in front of the mouth, or "legs" – all of which are typical polychaete features. One writer adopted a neutral position, saying he saw no strong grounds for classifying Wiwaxia as a proto-annelid or a proto-mollusc, although he thought the objections against classification as a proto-annelid were the stronger.
For many years Odontogriphus
("toothed riddle") was known from only one specimen, an almost featureless oval smear on a slab, with hints of tiny conical teeth. However, 189 new finds in the years immediately preceding 2006 made a detailed description possible. It had a pair of slightly V-shaped tooth-rows just ahead of the mouth, very like Wiwaxia′s. This pitched Odontogriphus into the middle of the debate about whether Wiwaxia was closer to the mollusc or the annelid lineage, resulting in a frank exchange of views.
Orthrozanclus
reburrus ("Dawn scythe with bristling hair") was discovered in 2006 and formally described in 2007. This animal had a soft, unarmored underside, but the upward-facing surfaces were armored by: a small shell, near the front end; three zones of armor plates, which fitted close to the body and one of which ran all the way round the animal; 16 to 20 long, upwards-curving spines on each side of the body. The arrangement of Orthrozanclus′ armor plates is very similar to that of its Burgess Shale contemporary Wiwaxia
. Its shell is very similar to one of the two Burgess Shale shell types labelled Oikozetetes
; the forward shell of halkieriids
, most fossils of which are dated to the Early Cambrian
; and those of other Early Cambrian
fossils such as Ocruranus and Eohalobia. These similarities suggest that Orthrozanclus was an intermediate form between Wiwaxia and the Halkieriids and that all three of these taxa
formed a clade
, in other words a group that consists of a common ancestor and all of its descendants. So Orthrozanclus was also drawn into the complex debate about whether Wiwaxia is more closely related to molluscs or to polychaete
worms.
For many years only one fossil Nectocaris
was known, poorly preserved and without a counterpart
. This fossil was a puzzle, as its head looked rather like that of an arthropod
but its body, with what seemed to be fins along its back and belly, looked rather like of a chordate
's. In 2010 Smith and Caron described another 91 specimens, some very good, and reconstructed it as a cephalopod
, and the earliest one as of 2010. Unlike later cephalopods it had only two arms, and its eyes seem mounted on stalks. But it had a soft funnel, similar to the ones used for propulsion by modern cephalopods.
Canadia
has always been classified as a polychaete
worm. Recent microscopic examination has indicated that the surfaces of the many bristles on its "legs" were diffraction grating
s that made the animal iridescent.
Fossils of chordate
s, the phylum to which humans belong, are very rare in Cambrian sediments. Conway Morris classified the Burgess Shale fossil Pikaia
as a chordate because it had a rudimentary notochord
, the rod of cartilage
that evolved into the backbone
of vertebrates. Doubts have been raised about this, because most of the important features are not quite like those of chordates: it has repeated blocks of muscle along its sides but they are not chevron-shaped; there is no clear evidence of anything like gill
s; and its throat appears to be in the upper part of its body rather than the lower. It also has "tentacles" on the front of its head, unlike living chordates. At best it may be a stem group chordate, in other words an evolutionary "aunt" of living chordates. Metaspriggina
, also found in the Burgess Shale but even rarer, may be a chordate, if the repeated chevron-shaped structures along its sides represent muscle blocks. While Pikaia was celebrated in the mid-1970s as the earliest known chordate, three jawless fish
have since been found among the Chengjiang fossils
, which are about 17 million years older than the Burgess Shale.
(1784–1856) realised that a dramatic change in the fossil record occurred around the start of the Cambrian period, . The earliest Cambrian trilobite
fossils are about 530 million years old, but were already both diverse and widespread, suggesting that the group had a long, hidden history. The earliest fossils widely accepted as echinoderm
s appeared at about the same time Because Darwin's contemporaries had too little information by which to establish the relative dates of Cambrian rocks, the impression was that animals appeared instantaneously. Charles Darwin regarded this sudden appearance of many animal groups with few or no antecedents as a major objection to his theory of evolution, and in 1859 devoted a chapter of The Origin of Species
to this problem. He speculated that the phenomenon, now known as the Cambrian explosion
, was a product of gaps in the sequence of fossil-bearing rocks and in contemporary knowledge of those rocks.
While some geological evidence was presented to suggest that earlier fossils did exist, for a long time this evidence was widely rejected. Fossils from the Ediacaran
period, immediately preceding the Cambrian, were first found in 1868, but scientists at that time assumed there was no Precambrian life and therefore dismissed them as products of physical processes. Between 1883 and 1909 Walcott
discovered other Precambrian fossils, which were accepted at the time. However in 1931 Albert Charles Seward
dismissed all claims to have found Precambrian fossils. In 1946, Reg Sprigg
noticed "jellyfishes" in rocks from Australia's Ediacara Hills
. However, while these are now recognized as coming from the Ediacaran
period, they were thought at the time to have been formed in the Cambrian. From 1872 onwards small shelly fossils, none more than a few millimeters in size, were found in very Early Cambrian rocks, and later also found in rocks dating to the end of the preceding Ediacaran period, but scientists only started in the 1960s to recognize that these were left by a wide range of animals, some of which are now recognized as molluscs.
Darwin's view – that gaps in the fossil record accounted for the apparently sudden appearance of diverse life forms – still had scientific support over a century later. In the early 1970s Wyatt Durham and Martin Glaessner
both argued that the animal kingdom had a long Proterozoic
history that was hidden by the lack of fossils. However, Preston Cloud
held a different view about the origins of complex life, writing in 1948 and 1968 that the evolution of animals in the Early Cambrian was "explosive". This "explosive" view was supported by the hypothesis of punctuated equilibrium
, which Eldredge
and Gould
developed in the early 1970s, and which views evolution as comprising long intervals of near-stasis "punctuated" by short periods of rapid change.
The fossils of the Burgess Shale were hidden in store rooms until the 1960s. When Whittington and his colleagues first began to publish their Burgess finds in the early 1970s, the fossils became central to the debate about how quickly animals arose, and were interpreted as evidence that all the living bilaterian phyla
had appeared in the Early Cambrian, along with many other phyla that had become extinct by the end of the Cambrian.
However, at this time cladistics
, which appeared in the 1950s, was starting to change scientists' approach to biological classification
. Unlike previous methods, cladistics attempts to consider all the characteristics of an organism, rather than those which are subjectively chosen as most important. As a result it gives less significance to unique or bizarre characteristics than to those that are shared, since only the latter can demonstrate relationships. Cladistics also emphasises the concept of a monophyletic group, in other words one that consists only of a common ancestor and all its descendants – for example it regards the traditional term "reptile" as useless, since mammal
s and bird
s are descendants of different groups of "reptiles", but are not considered "reptiles". The concepts of crown group
s and stem groups, first presented in English in 1979, are consequences of this approach. A crown group is a monophyletic group of living organisms, and a stem group is a non-monophyletic set of organisms that do not have all the shared features of the crown group but have enough to distinguish them clearly from close relatives of other crown groups – in very simple terms, they are "evolutionary aunts" of the organisms in the crown group. Phyla are crown groups, and the fact that some of their characteristics are considered defining features is simply a consequence of the fact that their ancestors survived while closely related lineages became extinct.
including Hallucigenia
and Microdictyon
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Simplified summary of Budd's "broad-scale" cladogram (1996)Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
itself, formed around in the Mid Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period. They were discovered in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
in 1886, and Charles Doolittle Walcott
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
collected over 60,000 specimens in a series of field trips up from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to discover new species, and statistical analysis suggests discoveries will continue for the foreseeable future. Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
's book Wonderful Life
Wonderful Life (book)
Wonderful Life: The Burgess Shale and the Nature of History is a book on the evolution of Cambrian fauna by Harvard paleontologist Stephen Jay Gould...
describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
is largely superseded.
The fossil beds are in a series of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
layers, averaging 30 millimetres (1.2 in) and totalling about 160 metres (524.9 ft) in thickness. These layers were deposited against the face of a high undersea limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
cliff. All these features were later raised up 2500 metres (8,202.1 ft) above current sea level during the creation of the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
.
These fossils have been preserved in a distinctive style known as Burgess shale type preservation
Burgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
, in which fairly tough tissues such as cuticle are preserved as thin films, very soft tissues are preserved as solid shapes and so quickly that decay had too little time to destroy them, while moderately soft tissues such as muscles are lost. Scientists are still unsure about the processes that created these fossils. While there is little doubt that the animals were buried under catastrophic flows of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
, it is uncertain whether they were transported by the flows from other locations, or lived in the area where they were buried, or were a mixture of local and transported specimens. This issue is closely related to whether conditions around the burial sites were anoxic or had a moderate supply of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Anoxic conditions are generally thought the most favourable for fossilization, but imply that the animals could not have lived where they were buried.
In the 1970s and early 1980s the Burgess fossils were largely regarded as evidence that the familiar phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
of animals appeared very rapidly in the Early Cambrian, in what is often called the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. This view was already known to Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
, who regarded it as one of the greatest difficulties for the theory of evolution he presented in The Origin of Species
The Origin of Species
Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
in 1859. However, from the early 1980s the cladistics
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
method of analysing "evolutionary family trees" has persuaded most researchers that many of the Burgess Shale's "weird wonders", such as Opabinia
Opabinia
Opabinia is an animal genus found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed,...
and Hallucinogenia, were evolutionary "aunts and cousins" of present-day types of animal rather than a rapid proliferation of separate phyla, some of which were short-lived. Nevertheless there is still debate, sometimes vigorous, about the relationships between some groups of animals.
Discovery, collection, and re-examinations
The first Burgess Shale fossils were found on Mount StephenMount Stephen
Mount Stephen is a mountain located in the Kicking Horse River Valley of Yoho National Park, ½ km east of Field. The mountain was named in 1886 for George Stephen, the first president of the Canadian Pacific Railway....
in Canada's Rocky Mountains
Canadian Rockies
The Canadian Rockies comprise the Canadian segment of the North American Rocky Mountains range. They are the eastern part of the Canadian Cordillera, extending from the Interior Plains of Alberta to the Rocky Mountain Trench of British Columbia. The southern end borders Idaho and Montana of the USA...
by a construction worker, whose reports of them reached Richard McConnell of the Geological Survey of Canada. McConnell found trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
beds there in 1886, and some unusual fossils that he reported to his superior. These were misdiagnosed as headless shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s with unjointed appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s, and were named Anomalocaris
Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
because of their unusual appendages – but turned out to be pieces of a puzzle that took 90 years to solve.
Mount Field (British Columbia)
Mount Field is a mountain located about east of the town of Field in Yoho National Park, Canada. The mountain was named in 1883 after Cyrus West Field, a guest of the CPR who were building the national railway....
, another part of the Stephen formation
Stephen Formation
The Stephen Formation is a middle Cambrian unit exposed in the Canadian Rockies of British Columbia. It is famous for the exceptional preservation of soft-bodied fossils: the Burgess Shale biota...
. These may have been why Charles Doolittle Walcott
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
visited Mount Field in 1909. While taking photographs there Walcott found a slab of fossils that he described as "Phyllopod crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s". From late August to early September 1909 his team, including his family, collected fossils there, and in 1910 Walcott opened a quarry which he and his colleagues re-visited in 1911, 1912, 1913, 1917 and 1924, bringing back over 60,000 specimens in total. Walcott was Secretary of the Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...
from 1907 to his death in 1927, and this kept him so busy that he was still trying to make time for analyzing his finds two years before his death. Although he drew attention to the exceptional detail of the specimens, which were the first known fossils of soft-bodied animals from the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, he also had other research interests: the Early Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of the Canadian Rockies, which took up the great majority of his time there; and Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
fossils of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, to which he assigned as much importance as to the fossils of animals. He managed to publish four "preliminary" papers on the fossil animals in 1911 and 1912, and further articles in 1918, 1919 and 1920. Four years after Walcott's death his associate Charles Resser produced a package of additional descriptions from Walcott's notes. Walcott's classification
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
s of most of the fossils are now rejected, but were supported at the time, and he accepted a change for one of the few where his conclusion was disputed. Many of the later comments were made with the benefits of hindsight, and of techniques and concepts unknown in Walcott's time.
Although in 1931 Percy Raymond opened and briefly excavated another quarry about 20 metres (65.6 ft) above Walcott's "Phyllopod bed
Phyllopod bed
The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess shale fossil lagerstatte. It was quarried by Charles Walcott from 1911–1917, and was the source of 95% of the fossils he collected during this time;tens of thousands of...
", there was very little interest in the Burgess Shale fossils from the 1930s to the early 1960s, and most of those collected by Walcott were stored on high shelving in back rooms at the Smithsonian Institution. Between 1962 and the mid-1970s Alberto Simonetta re-examined some of Walcott's collection and suggested some new interpretations. Beginning in the early 1970s Harry Whittington
Harry B. Whittington
Harry Blackmore Whittington FRS was a British paleontologist based at the Department of Earth Sciences, Cambridge, and was affiliated to Sidney Sussex College. He attended Handsworth Grammar School in Birmingham, followed by a degree and Ph.D in geology from the University of Birmingham...
, his associates David Bruton and Christopher Hughes, and his graduate students Derek Briggs
Derek Briggs
Derek Ernest Gilmor Briggs is an Irish paleontologist and taphonomist based at Yale University. Briggs is one of three paleontologists who were key in the reinterpretation of the fossils of the Burgess Shale.-Professional achievements:...
and Simon Conway Morris
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
began a thorough re-examination of Walcott's collection. Although they assigned groups of fossils to each member of the team, they all decided for themselves which fossils to investigate and in what order. Their publications and Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
s' popularization of their work in his book Wonderful Life
Wonderful Life (book)
Wonderful Life: The Burgess Shale and the Nature of History is a book on the evolution of Cambrian fauna by Harvard paleontologist Stephen Jay Gould...
aroused enduring scientific interest and some public interest in the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, the apparently rapid appearance of moderately complex bilaterian animals in the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
.
The continuing search for Burgess Shale fossils since the mid-1970s has led to the description in the 1980s of an arthropod Sanctacaris
Sanctacaris
Sanctacaris is a Middle Cambrian arthropod from the Burgess Shale of British Columbia. It was most famously regarded as a primitive chelicerate, a group which includes spiders and scorpions, although subsequent phylogenetic studies have not always supported this conclusion; it is best accommodated...
and in 2007 of Orthrozanclus
Orthrozanclus
Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body...
, which looked like a slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
with a small shell at the front, chain mail
Chain Mail
"Chain Mail" is a single by Mancunian band James, released in March 1986 by Sire Records, the first after the band defected from Factory Records. The record was released in two different versions, as 7" single and 12" EP, with different artworks by John Carroll and, confusingly, under different...
over the back and long, curved spines round the edges. Recent digs have discovered species which have yet to be formally described and named. They have also unearthed more and sometimes better fossils of animals that were discovered earlier, for example Odontogriphus
Odontogriphus
Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to...
was for many years known from just one poorly preserved specimen, but the discovery of a further 189 formed the basis for a detailed description and analysis in 2006. Re-examination of Walcott's collection also continues, and has led to the reconstruction of the large marine animal Hurdia
Hurdia
Hurdia victoria is an extinct species of anomalocaridid that lived 500 million years ago during the Cambrian era. It is part of the ancestral lineage that led to Arthropods and is related to Anomalocaris.-Description:...
in 2009.
Geology
The Burgess Shale is a series of sedimentSediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
deposits spread over a vertical distance of hundreds of metres, extending laterally for at least 50 kilometres (31.1 mi). The deposits were originally laid down on the floor of a shallow sea; during the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
Laramide orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
, mountain-building processes squeezed the sediments upwards to their current position at around 2500 metres (8,202.1 ft) elevation in the Rocky Mountains.
The rocks containing the fossils are on the border between two partially overlapping bands of rock that run along the western face of the Canadian Rockies. On the eastern side of this border is the Cathedral Formation, a platform of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
formed by algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
. The western surface of the Cathedral Formation is steep and consists of the resistant rock dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, which was originally part of the limestone platform, but between the Mid Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and Late Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
was transformed by hydrothermal flows of brine at up to 200 °C (392 °F), which replaced
Dolomitization
Dolomitization is a process by which dolomite is formed when magnesium ions replace calcium ions in calcite. It is common for this alteration into dolomite to take place due to evaporation of water in the sabkha area. Dolomitization involves substantial amount of recrystallization...
much of the limestone's calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
with magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
. A layer of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
lies partly on top of and partly to the west of the Cathedral Formation. This shale layer used to be called the "thin" Stephen Formation where it lies above the Cathedral Formation and the "thick" Stephen Formation where it lies to the west; but the "thick" Stephen Formation is now generally known as the Burgess Formation.
The shale is made of alternating fine-grained layers of siliceous mudstone (compressed, hardened mud originally made of ground-up silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
rock) and calcisiltite
Calcisiltite
Calcisiltite is a type of limestone that is composed predominately, more than 50 percent, of detrital silt-size carbonate grains. These grains consist either of the silt-size particles of ooids, fragments of fossil shells, fragments of older limestones and dolomites, intraclasts, pellets, other...
originally animal shells). The calcisiltite layers contain relatively unremarkable shells and occasional non-biomineralized fossils (such as priapulid tubes). The soft-bodied organisms for which the Burgess Shale is famous are fossilized in the mudstone layers, which are between 2 and 170 mm (0.078740157480315 and 6.7 in) thick, averaging 30 millimetres (1.2 in), and have well-defined bases. Opinions vary about how the mudstone layers were produced: perhaps by mudslides from the top of the "Cathedral" limestone platform, when its edge collapsed occasionally; or possibly by storms which created back-currents that abruptly washed large volumes of mud off the platform. Each mudstone layer is the result of one such catastrophe. The Greater Phyllopod Bed, a 7 metres (23 ft) thick sequence consisting of Walcott
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
's famous "Phyllopod Bed" plus the 5 metres (16.4 ft) below that, contains at least 36 layers, deposited over 10 to 100 thousand years, during which the environment was essentially stable.
Fossil Preservation
The processes responsible for the exceptional preservational quality of the Burgess Shale fossils are far from clear. The interpretation of what is preserved depends partly on two issues that are interlinked: whether the animals were buried where they lived, or washed long distances by sediment flows; and whether the water at the burial sites was anoxic or provided enough oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to sustain animals. The traditional view is that soft bodies and organs could only be preserved in anoxic conditions, otherwise oxygen-breathing bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
would have made decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
too rapid for fossilization. This would imply that the sea-floor organisms could not have lived there. However, in 2006 Caron and Jackson concluded that the sea-floor animals were buried where they lived. One of their main reasons was that many fossils represented partially decayed soft-bodied animals such as polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
s, which had already died shortly before the burial event, and would have been fragmented if they had been transported any significant distance by a storm of swirling sediment. Other evidence for burial where the animals had lived includes the presence of tubes and burrows, and of assemblies of animals preserved while they fed – such as a group of carnivorous priapulids clustered round a freshly moult
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
ed arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
whose new cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
would not yet have hardened. Fossilized swimming organisms were also buried immediately below where they lived.
Fossil tracks
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
are rare and no burrows under the sea-floor have so far been found in the Burgess Shale. These absences have been used to support the idea that the water near the sea-floor was anoxic. However it is possible that the water just above the sea-floor was oxygenated while the water in the sediment below it was anoxic, and also possible that there simply were no deep-burrowing animals in the Burgess Shale. Some fossils, such as Marrella, are almost always the right way up, which suggests they were not transported far if at all. Others, such as Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
, are often at odd angles, and some fossils of animals with shelly or toughened components very rarely contain remains of soft tissues. This suggests that the distances over which corpses were transported may have varied between genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, although most were buried where they had lived.
Girvanella
Girvanella is thought to represent the calcified sheath of a filamentous cyanobacterium known from the Burgess Shale and other Cambrian fossil deposits....
and Morania
Morania
Morania is a genus of cyanobacterium preserved as carbonaceous films in the Middle Cambrian Burgess Shale. it is present throughout the shale; 2580 specimens of Morania are known from the Greater Phyllopod bed, where they comprise 4.90% of the community. It is filamentous, forms sheets, and...
may represent members of microbial mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
communities. Morania appears on about a third of the slabs Caron and Jackson studied, and in some cases presents the wrinkled "elephant skin" texture typical of fossilized microbial mats. If such mats were present, they may have provided food for grazing animals and possibly helped to preserve soft bodies and organs, by creating oxygen-free zones under the mats and thus inhibiting the bacteria that cause decomposition.
The Burgess Shale animals were probably killed by changes in their environment either immediately preceding or during the mud-slides that buried them. Proposed killing mechanisms include: changes in salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
; poisoning by chemicals such as hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
or methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
; changes in the availability of oxygen; and changing consistency of the sea floor. The death event was not necessarily related to the burial, and there may have been multiple death events between burial events; but only organisms killed immediately before a burial event would stand any chance of being fossilised, instead of rotting or being eaten.
Burgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
is defined as the fossilization of non-biomineralized organisms as flattened carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
aceous films in marine shales. When the animals started to decompose, their tissues collapsed under the weight of the sediment that buried them. The typical flattened fossils are outlines of tougher parts such as cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
s and jaws, which resisted decomposition for long enough to be fossilized. Soft elements, such as muscles and gut contents, were sometimes squeezed out of the decomposing organism to produce dark stains on the fossils. Organisms that lack tougher structures, such as flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, nemerteans and shell-less molluscs, were not preserved by this process. Very soft but chemically active tissues may be preserved by different processes. For instance during decomposition, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
modify the chemically unusual mid-gut glands of some organisms into the durable mineral phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
. This change happens extremely quickly, before the corpse is compressed, and leaves a three-dimensional mold
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
of the tissues. Gills can also be preserved in something close to their original three-dimensional shape by this process. Both preservation mechanisms can appear in the same fossil. In Burgess-like shales, organisms and parts that are only quite soft, such as muscles, are generally lost, while those that are extremely soft and those that are fairly tough are preserved. The preservation of different body parts in different ways may sometimes help palaeontologists, by suggesting whether a body part was fairly tough like an arthropod limb (preserved as flat film) or very soft and chemically active, like a part of the gut (preserved as a solid piece of mineral). These differences may also help to identify fossils, by excluding from consideration organisms whose body parts do not match the combination of types of preservation found in a particular fossil bed.
It has often been suggested that this type of preservation was possible only when sediments were not disturbed
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
by burrowing animals or the anchors
Holdfast
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...
of plants. However a similar type of preservation has been found in fossils from the Late Riphean period, about , but in no known fossils between the end of that epoch and the start of the Cambrian. This suggests that such bioturbation has little to do with the appearance and disappearance of Burgess Shale type preservation
Burgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
. Such preservation may depend on the presence of clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
-like minerals that inhibit decomposition, and ocean chemistry may only have favoured the production of such minerals for limited periods of time. If so, it is impossible to be sure when the animals known as "Burgess Shale fauna" first appeared or when they became extinct. A few fossils of animals similar to those found in the Burgess Shale have been found in rocks from the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
periods, in other words up to 100 million years after the Burgess Shale.
Faunal composition
As of 2008 only two in-depth studies of the mix of fossils in any part of the Burgess Shale had been published, by Simon Conway MorrisSimon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
in 1986 and by Caron and Jackson in 2008. Caron and Jackson commented that Conway Morris had to rely on a set of specimens which may not have been representative, since their excavators discarded specimens they found uninteresting; and for which the exact level in the rock sequence had not been recorded, making chronological analyses impossible. Both studies noted that the set of species in Walcott's Phyllopod Bed (Conway Morris, 1986) and its expanded version the Greater Phyllopod Bed (Caron and Jackson, 2008) was different from those found in other parts of the Burgess Shale; and Conway Morris commented that faunas at most other Burgess Shale sites resembled those of the Raymond Quarry, which is above and therefore more recent than the Greater Phyllopod Bed (abbreviated "GPB").
Conway Morris found that the shelly fossils in Walcott's Phyllopod Bed were about as abundant as in other shelly fossil deposits, but accounted for only 14% of the Phyllopod Bed fossils. Assuming that, as in modern marine ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s, about 70% of the species that lived in the Early Cambrian seas are unsuitable for fossilization, he estimated that the shelly fossils probably represent about 2% of the animals that were alive at the time. Since these shelly fossils are found in other parts of North America and, in many cases, over a much wider range, the Burgess Shale fossils, including the soft-bodied ones, probably show how much diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
could be expected at other sites if Burgess Shale type preservation
Burgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
were found there.
Caron and Jackson found that about 25% of the 172 known species were abundant and widespread throughout the time range of the GPB, while the majority of species were rare, and occurred in a small area for a short period of time. In most layers the five most abundant species accounted for 50% to 75% of individual animals. The species that had wide ranges in time and space may have been generalists, while the rest were specialists in particular types of environment. Alternatively some wide-ranging species may have been opportunists that were quick to recolonize the area after each burial event. The 6 species that appeared in all layers were very probably generalists.
In each burial event layer the commonest species generally has several times as many individuals as the second most common, and accounts for 15% to 30% of individual fossil animals. The more common a species is in one layer, the greater the number of other layers it appears in. These "recurrent" species account for 88% of the individual specimens, but only 27% of the number of species. This suggests that the majority of species were in existence for much shorter periods than the "recurrent" ones. Species that cover shorter periods of time occur mainly in the higher, younger layers. The GPB shows an overall trend of increasing diversity as time progresses.
In almost all layers arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s are the most abundant and diverse group of fossils in the GPB, followed by sponges. 69.2% of GPB individuals and 63.9% of species lived on the surface of the sea bed; within this group, mobile deposit feeders that extracted food particles from the sediment accounted for 38.2% of the total number of individuals and 16.8% of the total species; the smallest sub-group was mobile hunters and scavengers; and the rest were sessile
Sessility (limnology)
In limnology, sessility is that quality of an organism which rests unsupported directly on a base, either attached or unattached to a substrate. It is a characteristic of vegetation which is anchored to the benthic environment. There are two families of sessile rotifers: Flosculariidae and...
suspension feeders. Animals that lived in the sediment made up 12.7% of the species and 7.4% of the individuals; the largest sub-group was mobile mobile hunters and scavengers. Bottom-dwelling animals capable of swimming comprised 12.7% of species and 7.4% of individuals. Organisms which spent their whole life swimming were very rare, accounting for only 1.5% of individuals and 8.3% of species.
These patterns – a few common species and many rare ones; the dominance of arthropods and sponges; and the percentage frequencies of different life-styles – seem to apply to all of the Burgess Shale. However the identity of the dominant species differs between sites. For example Marrella splendens is often credited as the commonest animal in the Burgess Shale, because of its abundance among the specimens collected by Walcott, but is only the third-most abundant organism in the Greater Phyllopod Bed, and very rare at other localities.
The overall community and ecology is very similar to that of other Cambrian localities, suggesting a global pool of species which repopulated localities after calamitous burial events occurred.
Caron and Jackson used computer software to simulate the numbers of species that would be found if smaller numbers of specimens were included, and found that the number of species "discovered" kept increasing as the number of specimens increased, rather than reaching a plateau. This suggests that Burgess Shale probably still contains as-yet undiscovered species, although probably very rare ones. Some recently discovered species, known in 2008 only by nicknames like "woolly bear" and "Siamese lantern" are familiar to the collecting teams, but have yet to be formally described and named. The team also nicknamed another discovery as "Creeposaurus", and in 2010 this animal was described and formally named Herpetogaster
Herpetogaster
Herpetogaster is an extinct monotypic genus of animal from the Middle Cambrian Burgess Shale with a single species Herpetogaster collinsi. Six specimens of Herpetogaster are known from the Greater Phyllopod bed, where they comprise less than 0.01% of the community.-See also:* at the Fossil Wiki....
.
Soft-bodied fossils
Caron and Jackson's survey covered 172 species found in the Greater Phyllopod Bed. The list below concentrates on recent discoveries and on species that have been central to major scientific debates.Marrella was the first Burgess Shale fossil that Whittington
Harry B. Whittington
Harry Blackmore Whittington FRS was a British paleontologist based at the Department of Earth Sciences, Cambridge, and was affiliated to Sidney Sussex College. He attended Handsworth Grammar School in Birmingham, followed by a degree and Ph.D in geology from the University of Birmingham...
re-examined, and gave the first indication that surprises were on the way. Although clearly an arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
that walked on the sea-floor, Marella was very different from the known marine arthropod groups (trilobites, crustaceans and chelicerate
Chelicerata
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
s) in the structure of its legs and gills, and above all in the number and positions of the appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s on its head, which are the main feature used to classify arthropods. A fossil of Marrella from the Burgess Shale has also provided the earliest clear evidence of molting.
Opabinia
Opabinia is an animal genus found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed,...
made the audience laugh. The reconstruction showed a soft-bodied animal with a slim, segmented body; a pair of flap-like appendages on each segment with gills above the flaps; five stalked eyes; a backward-facing mouth under the head; and a long, flexible, hose-like proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
which extended from under the front of the head and ended in a "claw" fringed with spines. Whittington concluded that it did not fit into any phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
known in the mid-1970s. Opabinia was one of the main reasons why Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
in his book Wonderful Life
Wonderful Life (book)
Wonderful Life: The Burgess Shale and the Nature of History is a book on the evolution of Cambrian fauna by Harvard paleontologist Stephen Jay Gould...
considered that Early Cambrian life was much more diverse and "experimental" than any later set of animals, and that the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
was a truly dramatic event, possibly driven by unusual evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary mechanisms. He regarded Opabinia as so important to understanding this phenomenon that he originally wanted call his book Homage to Opabinia. However subsequent research has concluded that Opabinia is closely related to the arthropods and possibly even closer to ancestors of the arthropods.
Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
("abnormal claw") was a comedy of errors
Comedy of errors
A comedy of errors is a narrative work that is light and often humorous or satirical in tone, in which the action usually features a series of comic instances of mistaken identity, and which typically culminates in a happy resolution of the thematic conflict.-Satire and farce:A slight variation of...
. The name was initially given to a fossil that looked like the rear end of a shrimp-like crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
. Walcott classified a ring-like fossil he called Peytoia as a kind of jellyfish, and another poorly preserved fossil he called Laggania as a holothurian (sea cucumber). After many plot twists, Derek Briggs started dissecting another ill-defined fossil in very thin slices and found a pair of "Anomalocaris"-like structures on one end of a specimen of "Laggania", which also had a specimen of "Peytoia" attached just behind those of "Anomalocaris". After dissecting more specimens and finding similar configurations, Briggs and Whittington concluded that the whole assemblage represented a single animal, which was named Anomalocaris because that was the earliest name assigned to any of its parts. This animal's body was fragile and usually disintegrated before it could be fossilized. But the complete animal had tough grasping appendages ("Anomalocaris"), a tough, ring-like mouth with teeth on the inner edge ("Peytoia") and a long, segmented body ("Laggania") with flaps on the sides that enabled it to swim with a Mexican wave motion, and perhaps to turn quickly by putting the flaps on one side into reverse. This monster was over 2 foot (0.6096 m) long when other animals were only a few inches at most. Nedin suggested in 1999 that the animal was capable of taking heavily armored trilobites apart, possibly by grabbing one end of their prey in their jaws while using their appendages to quickly rock the other end of the animal back and forth, causing the prey's exoskeleton to rupture and allowing the predator to access its innards. In 2009 Hagadorn found that anomalocarid mouthparts showed little wear, which suggests they did not come into regular contact with mineralised trilobite shells. Computer modeling of the Anomalocaris mouthparts suggests they were in fact better suited to sucking on smaller, soft-bodied organisms. Although Whittington and Briggs concluded that Anomalocaris did not fit into any known phylum, research since the 1990s has concluded that it was closely related to Opabinia and to the ancestors of arthropods. In 2009 a fossil named Schinderhannes bartelsi
Schinderhannes bartelsi
Schinderhannes bartelsi is an anomalocarid known from one specimen from the lower Devonian Hunsrück Slates. Its discovery was astonishing because previously, anomalocaridids had only been known from exceptionally preserved fossil beds from the Cambrian, 100 million years earlier.Anomalocaridids...
, an apparent relative of Anomalocaris, was found in the Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period, about 100 million years later than the Burgess Shale.
Hallucigenia
Hallucigenia is an extinct genus of animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada, represented by the species H. sparsa, and in the Lower Cambrian Maotianshan shale of China, represented by the species H. fortis...
its name because in his reconstruction it looked bizarre – a worm-like animal that walked on long, rigid spines and had a row of tentacles along its back. Science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
author Greg Bear
Greg Bear
Gregory Dale Bear is an American science fiction and mainstream author. His work has covered themes of galactic conflict , artificial universes , consciousness and cultural practices , and accelerated evolution...
says the Jarts in his The Way
The Way (Greg Bear)
The Way fictional universe is a trilogy of science fiction novels and one short story by Greg Bear. The first novel was Eon , followed by a sequel, Eternity and a prequel, Legacy...
stories were scaled-up versions of this reconstruction. However in the late 1980s Lars Ramsköld literally turned it over, so that the "tentacles", which he found were paired, became legs and the spines were defensive equipment on its back. Ramsköld classified it as one of the Onychophora, a phylum of "worms with legs" that is considered closely related to arthropods. Another current view is that Hallucigenia was an armored lobopod, in other words more closely related to arthropods than the onychophorans are, but less closely related to arthropods than Opabinia or Anomalocaris are.
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
are just a jumble of armor plates and spines, but after examining dozens of them Conway Morris reconstructed them as slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
-like animals covered in rows of overlapping armor plates, with two rows of longer spines projecting upwards. Since 1990 there has been an intense debate about whether Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
was more closely related to molluscs or to polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s. Supporters of a close relationship with molluscs maintained that a pair of bars, running across the mouth and armed with backward-pointing teeth, were a rudimentary form of the radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
, the toothed tongue that molluscs use to scrape up food and convey it back to the throat. Nicholas Butterfield, the one academic who has so far published articles placing Wiwaxia closer to polychaetes, stated that Wiwaxia′s two-row feeding apparatus could not have performed the sophisticated functions of the multi-row, conveyor-belt radula, suggesting instead that Wiwaxia′s apparatus was like the side-by-side pair of toothed bars found in some polychaetes. Later he found some fragmentary fossils, 5 to 10 million years before the Burgess Shale, that he regarded as a much more convincing early radula. Butterfield has also described Wiwaxia′s armor plates and spines as similar in internal structure to the chetae ("hairs") of polychaetes. Supporters of the link with molluscs have stated that Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
shows no signs of segmentation, appendages in front of the mouth, or "legs" – all of which are typical polychaete features. One writer adopted a neutral position, saying he saw no strong grounds for classifying Wiwaxia as a proto-annelid or a proto-mollusc, although he thought the objections against classification as a proto-annelid were the stronger.
Odontogriphus
Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to...
("toothed riddle") was known from only one specimen, an almost featureless oval smear on a slab, with hints of tiny conical teeth. However, 189 new finds in the years immediately preceding 2006 made a detailed description possible. It had a pair of slightly V-shaped tooth-rows just ahead of the mouth, very like Wiwaxia′s. This pitched Odontogriphus into the middle of the debate about whether Wiwaxia was closer to the mollusc or the annelid lineage, resulting in a frank exchange of views.
Orthrozanclus
Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body...
reburrus ("Dawn scythe with bristling hair") was discovered in 2006 and formally described in 2007. This animal had a soft, unarmored underside, but the upward-facing surfaces were armored by: a small shell, near the front end; three zones of armor plates, which fitted close to the body and one of which ran all the way round the animal; 16 to 20 long, upwards-curving spines on each side of the body. The arrangement of Orthrozanclus′ armor plates is very similar to that of its Burgess Shale contemporary Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
. Its shell is very similar to one of the two Burgess Shale shell types labelled Oikozetetes
Oikozetetes
Oikozetetes is a scleritome-bearing Cambrian organism which may be related to the halkieriids.They bore two morphologically different shells, which were probably calcareous while the organism was alive...
; the forward shell of halkieriids
Halkieria
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
, most fossils of which are dated to the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
; and those of other Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
fossils such as Ocruranus and Eohalobia. These similarities suggest that Orthrozanclus was an intermediate form between Wiwaxia and the Halkieriids and that all three of these taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
formed a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, in other words a group that consists of a common ancestor and all of its descendants. So Orthrozanclus was also drawn into the complex debate about whether Wiwaxia is more closely related to molluscs or to polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms.
For many years only one fossil Nectocaris
Nectocaris
Nectocaris pteryx is a species of possible cephalopod or arthropod affinity, known from the Middle Cambrian Burgess Shale.Nectocaris was a free-swimming, predatory or scavenging organism, possibly occupying a niche similar to the arrow worms...
was known, poorly preserved and without a counterpart
Counter slab
Counter slab and slab are the matching halves of a fossil-bearing matrix formed in sedimentary deposits. When excavated the matrix may be split along the natural grain or cleavage of the rock. A fossil embedded in the sediment may then also split down the middle, with fossil remains sticking to...
. This fossil was a puzzle, as its head looked rather like that of an arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
but its body, with what seemed to be fins along its back and belly, looked rather like of a chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
's. In 2010 Smith and Caron described another 91 specimens, some very good, and reconstructed it as a cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
, and the earliest one as of 2010. Unlike later cephalopods it had only two arms, and its eyes seem mounted on stalks. But it had a soft funnel, similar to the ones used for propulsion by modern cephalopods.
Canadia
Canadia (genus)
Canadia is a genus of extinct annelid worm present in Burgess Shale type Konservat-Lagerstätte. It is found in strata dating back to the Delamaran stage of the Middle Cambrian around 505 million years ago, during the time of the Cambrian explosion. It was about 3 centimeters in length...
has always been classified as a polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worm. Recent microscopic examination has indicated that the surfaces of the many bristles on its "legs" were diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s that made the animal iridescent.
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s, the phylum to which humans belong, are very rare in Cambrian sediments. Conway Morris classified the Burgess Shale fossil Pikaia
Pikaia
Pikaia gracilens is an extinct animal known from the Middle Cambrian Burgess Shale of British Columbia. -Discovery:It was discovered by Charles Walcott and was first described by him in 1911. It was named after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation...
as a chordate because it had a rudimentary notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
, the rod of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
that evolved into the backbone
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
of vertebrates. Doubts have been raised about this, because most of the important features are not quite like those of chordates: it has repeated blocks of muscle along its sides but they are not chevron-shaped; there is no clear evidence of anything like gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s; and its throat appears to be in the upper part of its body rather than the lower. It also has "tentacles" on the front of its head, unlike living chordates. At best it may be a stem group chordate, in other words an evolutionary "aunt" of living chordates. Metaspriggina
Metaspriggina
Metaspriggina is a genus of chordate known from only two specimens in the middle Cambrian Burgess shale.Whilst named after the Ediacaran organism Spriggina, later work has shown the two to be unrelated. Evidence for cranial structures are present in one of the specimens....
, also found in the Burgess Shale but even rarer, may be a chordate, if the repeated chevron-shaped structures along its sides represent muscle blocks. While Pikaia was celebrated in the mid-1970s as the earliest known chordate, three jawless fish
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
have since been found among the Chengjiang fossils
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
, which are about 17 million years older than the Burgess Shale.
Trace fossils
Although trace fossils are rare in the Burgess Shale, arthropod trackways have been recovered.Significance
Analysis of the Burgess Shale fossils has been important to the interpretation of the Precambrian and Cambrian fossil records, and thus to scientific understanding of the nature of early evolution. English geologist and palaeontologist William BucklandWilliam Buckland
The Very Rev. Dr William Buckland DD FRS was an English geologist, palaeontologist and Dean of Westminster, who wrote the first full account of a fossil dinosaur, which he named Megalosaurus...
(1784–1856) realised that a dramatic change in the fossil record occurred around the start of the Cambrian period, . The earliest Cambrian trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
fossils are about 530 million years old, but were already both diverse and widespread, suggesting that the group had a long, hidden history. The earliest fossils widely accepted as echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s appeared at about the same time Because Darwin's contemporaries had too little information by which to establish the relative dates of Cambrian rocks, the impression was that animals appeared instantaneously. Charles Darwin regarded this sudden appearance of many animal groups with few or no antecedents as a major objection to his theory of evolution, and in 1859 devoted a chapter of The Origin of Species
The Origin of Species
Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
to this problem. He speculated that the phenomenon, now known as the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, was a product of gaps in the sequence of fossil-bearing rocks and in contemporary knowledge of those rocks.
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period, immediately preceding the Cambrian, were first found in 1868, but scientists at that time assumed there was no Precambrian life and therefore dismissed them as products of physical processes. Between 1883 and 1909 Walcott
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
discovered other Precambrian fossils, which were accepted at the time. However in 1931 Albert Charles Seward
Albert Charles Seward
Albert Charles Seward FRS was a British botanist and geologist.-Life:His first education was at Lancaster Grammar School and then on to St. John's College at Cambridge following his parents' wish to dedicate his life to the Church...
dismissed all claims to have found Precambrian fossils. In 1946, Reg Sprigg
Reg Sprigg
Reginald Claude Sprigg, AO, HonDSc ANU, HonDSc Flinders, MSc Adelaide, FTSE was an Australian geologist and conservationist. At age 17 he became the youngest Fellow of the Royal Society of South Australia. In 1946, in the Ediacara Hills, South Australia he discovered the Ediacara biota, an...
noticed "jellyfishes" in rocks from Australia's Ediacara Hills
Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around 650 km north of Adelaide. The area has many old copper and silver mines from mining activity in the late 19th century...
. However, while these are now recognized as coming from the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period, they were thought at the time to have been formed in the Cambrian. From 1872 onwards small shelly fossils, none more than a few millimeters in size, were found in very Early Cambrian rocks, and later also found in rocks dating to the end of the preceding Ediacaran period, but scientists only started in the 1960s to recognize that these were left by a wide range of animals, some of which are now recognized as molluscs.
Darwin's view – that gaps in the fossil record accounted for the apparently sudden appearance of diverse life forms – still had scientific support over a century later. In the early 1970s Wyatt Durham and Martin Glaessner
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
both argued that the animal kingdom had a long Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
history that was hidden by the lack of fossils. However, Preston Cloud
Preston Cloud
Preston Ercelle Cloud, Jr. was an American paleontologist, geographer, and professor. He was best-known for his work on the geologic time scale and the origin of life on Earth.-Early life:...
held a different view about the origins of complex life, writing in 1948 and 1968 that the evolution of animals in the Early Cambrian was "explosive". This "explosive" view was supported by the hypothesis of punctuated equilibrium
Punctuated equilibrium
Punctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...
, which Eldredge
Niles Eldredge
Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...
and Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
developed in the early 1970s, and which views evolution as comprising long intervals of near-stasis "punctuated" by short periods of rapid change.
The fossils of the Burgess Shale were hidden in store rooms until the 1960s. When Whittington and his colleagues first began to publish their Burgess finds in the early 1970s, the fossils became central to the debate about how quickly animals arose, and were interpreted as evidence that all the living bilaterian phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
had appeared in the Early Cambrian, along with many other phyla that had become extinct by the end of the Cambrian.
However, at this time cladistics
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
, which appeared in the 1950s, was starting to change scientists' approach to biological classification
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
. Unlike previous methods, cladistics attempts to consider all the characteristics of an organism, rather than those which are subjectively chosen as most important. As a result it gives less significance to unique or bizarre characteristics than to those that are shared, since only the latter can demonstrate relationships. Cladistics also emphasises the concept of a monophyletic group, in other words one that consists only of a common ancestor and all its descendants – for example it regards the traditional term "reptile" as useless, since mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s are descendants of different groups of "reptiles", but are not considered "reptiles". The concepts of crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
s and stem groups, first presented in English in 1979, are consequences of this approach. A crown group is a monophyletic group of living organisms, and a stem group is a non-monophyletic set of organisms that do not have all the shared features of the crown group but have enough to distinguish them clearly from close relatives of other crown groups – in very simple terms, they are "evolutionary aunts" of the organisms in the crown group. Phyla are crown groups, and the fact that some of their characteristics are considered defining features is simply a consequence of the fact that their ancestors survived while closely related lineages became extinct.
including Hallucigenia
Hallucigenia
Hallucigenia is an extinct genus of animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada, represented by the species H. sparsa, and in the Lower Cambrian Maotianshan shale of China, represented by the species H. fortis...
and Microdictyon
Microdictyon
Microdictyon is an extinct "armored worm" coated with dot-likescleritic scales, known from the Early CambrianMaotianshan shale of Yunnan China. Microdictyon is sometimes included...
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of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and their closest relatives
Briggs and Whittington started experimenting with cladistics in 1980 to 1981 and the results, while full of uncertainties, convinced them that cladistics offered reasonable prospects of making sense of the Burgess Shale animals. Other fossil beds discovered since 1980 – some rather small and others rivalling the Burgess Shale – have also produced similar collections of fossils, and show that the types of animals they represent lived in seas all over the world. It appears that most of the major animal lineages had arisen before the time of the Burgess Shale, and before that of the Chengjiang
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
and Sirius Passet
Sirius Passet
Sirius Passet is a Cambrian Lagerstätte in Greenland. The Sirius Passet Lagerstätte was named after the Sirius sledge patrol that operates in North Greenland. It comprises six localities located on the eastern shore of J.P. Koch Fjord in the far north of Greenland. It was discovered in 1984 by A....
lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
n about 15 million years earlier, in which very similar fossils are found, and that the Cambrian explosion was complete by then. In the 1990s it was suggested that some Ediacaran fossils
Ediacara biota
The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...
from , just before the start of the Cambrian, may have been primitive bilaterians, and one, Kimberella
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
, may have been a primitive mollusc. By 1996, with new fossil discoveries filling in some of the gaps in the "family tree", some Burgess Shale "weird wonders" such as Hallucinogenia and Opabinia
Opabinia
Opabinia is an animal genus found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed,...
were seen as stem members of a total group that included arthropods and some other living phyla.