Wiwaxia
Encyclopedia
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 3.4 millimetre (0.133858267716535 in) to a little over 50.8 millimeters (2 in) in length. The precise taxonomic affinities of the genus are a matter of ongoing debate amongst palaeontologists.
in 1911 as a result of one of his field trips to the nearby Burgess Shale
in the Canadian Rocky Mountains
, and he classified it as a member of the polychaete
group of annelid
worms.
In 1966 and 1967 a team led by Harry B. Whittington
revisited the Burgess Shale and found so many fossils that it took years to analyze them all, and Wiwaxia was one of the most difficult to analyze. Eventually in 1985 Simon Conway Morris
, then a member of Whittington's team, published a detailed description that concluded Wiwaxia was not a polychaete. All the known specimens came from in and around the Burgess Shale until 1991, when fragmentary fossils were reported from Australia's Georgina Basin. In 2004 additional finds which may represent two different species were reported from the same area.
, middle part of the Middle Cambrian with date , and fragmentary specimens in nearby strata slightly older than and younger than the Burgess Shale, in China
's lowermost Middle Cambrian beds of the Kaili Formation
, in the Middle Cambrian beds of the Tyrovice Member, Buchava Formation of the Czech Republic
, in the Lower Cambrian Mount Cap formation
(Mackenzie Mountains, Canada), in the Emu Bay Shale
of Kangaroo Island
, South Australia
, upper Botomian Stage of the Lower Cambrian, and in the Middle Botomian Sinsk Biota of the Siberia, Russia. These finds show that Wiwaxia and most of the other Burgess Shale type fauna
were very widespread. The Chinese material was originally considered to represent a separate species; like W. corrugata, it possessed spines and regions of sclerites (although it is only known from disarticulated remains), but the sclerites bear a higher density of ribs, and there are two distinct thicknesses of rib (i.e. larger and smaller). However, at a microscopic level, the sclerites do not differ from Burgess Shale or Mount Cap sclerites. The possibility remains that knob-bearing sclerites from all three localities belong to a different species.
Isolates spines are more common than sclerites in localities with a poor preservation potential, suggesting that the spines were more recalcitrant; however, in well-preserved sites such as the Phyllopod bed
, spine and sclerite abundance is comparable in disarticulated instances to the proportions on complete fossils.
This concentrates on the species Wiwaxia corrugata, found in the Burgess Shale, since the other specimens consist only of fragments, while the Burgess Shale has provided at least 138 complete ones.
Wiwaxia was a bilaterally symmetrical animal. Viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. The most complete fossils fall into two size ranges: 2 centimetre (0.78740157480315 in) to 5 centimetres (2 in) long, which are thought to be adults; and 3.4 millimetre (0.133858267716535 in) to 1.5 centimetre (0.590551181102362 in), which are thought to be juveniles. Estimating their height is difficult because specimens were compressed after death; a specimen of the average length, 3.4 centimetres (1.3 in), may have been 1 centimetre (0.393700787401575 in) high excluding the spines on their backs. The ratio of width to length does not appear to change as the animals grew.
The animal was covered in small ribbed armor plates called sclerites, that lay flat against the body, overlapped so that the rear of one covered the front of the one behind, and formed five main regions — the top, with 8-9 rows of sclerites; the upper part of the sides, with 11-12; the lower part of the sides, with 8; the front; and the area nearest the sea-floor, with 12-17 rows. Most of the sclerites were shaped like oval leaves, but the ventro-lateral ones, nearest the sea-floor, were crescent
-shaped, rather like flattened banana
s, and formed a single row with the tips pointing down. In addition there were two rows of ribbed spines running from to rear, one along each side of the top surface, and projecting out and slightly upwards, with a slight upwards curve near the tips. Specimens ranging from 11 millimetre (0.433070866141732 in) 52 millimetres (2 in) have about the same number of ventro-lateral sclerites just above the foot. On the other hand the number of spines seems to depend on the size of the specimen, up to about 12 per side. The number and spacing of the spines is asymmetrical in the specimens found, and this may have been natural rather than a result of events in the animal's life or after death. Although the spines in the middle of each row are usually the longest, up to 5 centimetres (2 in), a few specimens have rather short middle spines, perhaps because these were part-grown replacements. The smallest specimens may have lacked the long dorsal spines, which appear to have grown quickly in larger juveniles and then more slowly in adults.
Each sclerite was rooted separately in the body; the roots of body sclerites are 40% of the external length or a little less, while the roots of the spines are a little over 25% of the external length; all were rooted in pockets in the skin, rather like the follicle
s of mammal
ian hair
. The roots of the body sclerites were significantly narrower than the sclerites, but the spines had roots about as wide as their bases; both types of root were made of fairly soft tissue. They bore protrusive, presumably structural, ribs on their upper and (seemingly) lower surfaces. The sclerites and spines were not mineralized, and the frayed appearance of some broken ones suggests a fibrous structure. The way they were preserved suggests they were not made of chitin
, from which insect
s' exoskeletons are formed. They may have been made of tanned
proteins or of collagen
, which is the main component of cartilage
s and tendon
s in humans.
Since the body sclerites had bases that were narrower than the hard external parts, it is hard to see how they grew. They may have enclosed soft tissue that could have secreted the hard walls, but there is no convincing evidence for this. Butterfield (1990) examined some sclerites under both optical
and scanning electron microscope
s and concluded that they were not hollow, and that the bases split and spread to form the blades, a pattern that is also seen in monocot leaves.
Wiwaxia’s flat underside was soft and unarmored. Little is known of the internal anatomy, although the gut apparently ran straight and all the way from the front to the rear. At the front end of the gut, about 5 millimetre (0.196850393700787 in) from the animal's front in an average specimen about 2.5 centimetre (0.984251968503937 in) long, there was a feeding apparatus that consisted of two (or in rare large specimens three) rows of backward-pointing conical teeth. The feeding apparatus was tough enough to be frequently preserved, but unmineralized and fairly flexible, as it folded and retracted when not in use. It would have had to be pushed forward out of the mouth in order to feed. Even the smallest specimens have this type of apparatus, with two rows containing the same number of teeth as in larger ones. This indicates that Wiwaxia’s feeding habits remained the same throughout its life after the larva
l stage. The feeding apparatus may have acted as a rasp
to scrape bacteria
off the top of the microbial mat
that covered the sea-floor, or as a rake
to gather food particles that lay on the sea-floor.
Since there is no sign of eyes or tentacles, Wiwaxia may have relied mainly on chemical senses such as smell and taste. Its respiratory system is also unknown.
The long dorsal spines may have been a defense against predators, and finds of broken spines suggest that Wiwaxia was attacked. The animal appears to have crawled on the surface of the sea-floor feeding on particles that fell from higher levels of the sea. Wiwaxia shows no signs of legs and was probably too large to move on cilia, so it probably moved by muscular contraction that made its foot ripple. Juveniles may have burrowed into the sea-floor. In one specimen a small brachiopod
, Diraphora
bellicostata, appears to be attached to one of the ventro-lateral sclerites. This suggests that adult Wiwaxia did not burrow or even plough much into the sea-floor as they moved. Two other specimens of Diraphora
bellicostata have been found attached to dorsal sclerites. Wiwaxia appears to have been solitary rather than gregarious.
One juvenile specimen appears to be preserved while molting and not yet completely detached from its discarded armor. Its new set of spines seem less rigid than the old ones and slightly underdeveloped, as if the next stages were going to be inflated by body fluids and then hardening. The new armor may have had an internal volume 50% to 70% larger than the old one. Molting appears to have occurred all at once, as adult specimens shows no signs of interruptions in the sclerite armor that would indicate molting of parts of the armor or of individual sclerites. Since the bases of the body sclerites are relatively narrow and these is no sign of sclerites splitting during molting, withdrawing soft tissue from the old sclerites would probably have required the tissues to be broken down in to a more fluid form, as happens in the claws of lobsters and crabs when they molt. The skin must also have been shed, since the discarded armor appears as a complete unit rather than scattered sclerites. In the juvenile that was apparently molting when it died, the feeding apparatus also appears to have been shed, as half of one tooth row is pointing forwards.
recognised today were beginning to diverge. Consequently, many lineages (that would later become extinct) appear intermediate to two or more modern groups, or lack features common to all modern members of a group, and hence fall into the "stem group" of a modern taxon
. Debate is ongoing as to whether Wiwaxia can be placed within a modern crown group and, if it cannot, in which group's stem it falls. When Walcott first described Wiwaxia, he regarded it as a polychaete
annelid
worm, and its sclerite
s as similar to the elytra ("scales") of annelids. More recently the debate has been intense, and proposed classifications include: a member of an extinct phylum
distantly related to the molluscs; a crown-group polychaete; a stem-group annelid; a problematic bilaterian; a stem- or possibly primitive crown-group mollusc.
In 1985 Simon Conway Morris
agreed that there were similarities to polychaetes, but considered that Wiwaxia’s sclerites were different in construction from annelids' elytra. He was more impressed by the similarities between Wiwaxia’s feeding apparatus and a molluscan radula
, and assigned the animal to a new taxon Molluscata, which he proposed should also contain the molluscs and hyolithid
s. When he later described the first fairly complete specimens of Halkieria
, he suggested that these were closely related to Wiwaxia.
Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by Stephen Jay Gould
's lectures, agreed that the sclerites were not like elytra, which are relatively fleshy and soft. However, since the sclerites were solid, he concluded that Wiwaxia could not be a member of the "Coeloscleritophora", a taxon
that had been proposed in order to unite organisms with hollow sclerites, and could not be closely related to the halkieriids, which have hollow sclerites. Instead he thought that they were very similar in several ways to the
chitin
ous bristles (setae) that project from the bodies of modern annelids and in some genera
form leaf-like scales that cover the back like roof tiles – in composition, in detailed structure, in how they were attached to the body via "follicle
s" and in overall appearance. Some modern annelids also develop on each side rows of longer bristles, which both Walcott and Butterfield considered similar to Wiwaxia’s dorsal spines. including the halkieriids.
Butterfield also contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychates. He went so far as to classify Wiwaxia as a member of a modern order
, Phyllodocida
, and pointed out that Wiwaxia’s lack of obvious segmentation
is no barrier to this, as some modern polychaetes also show no segmentation except during development. He later noted that Wiwaxia lack some polychate features which he would expect to be easily preserved in fossils, and therefore a stem-group annelid
, in other words an evolutionary "aunt" of modern annelids.
Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes, and said Butterfield had informed them that the microscopic structure of Wiwaxia’s sclerites was identical to that of the bristles of two Burgess Shale polychaetes Burgessochaeta and Canadia. Conway Morris and Peel also wrote that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia’s feeding apparatus was much more like a molluscan radula
. They also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller and more numerous; they also said that in 1994 Butterfield had found Wiwaxia sclerites that were clearly hollow. They presented a large cladogram
according to which:
Marine biologist Amélie H. Scheltema et al. (2003) argued that Wiwaxia’s feeding apparatus is very similar to the radula
s of some modern shell-less aplacophora
n molluscs, and that the sclerites of the two groups are very similar. They concluded that Wiwaxia was a member of a clade
that includes molluscs. Scheltema has also highlighted similarities between Wiwaxia and the larvae of certain solenogaster molluscs, which bear iterated calcareous sclerites arranged into three symmetrical lateral zones.
Danish zoologist Danny Eibye-Jacobsen argued in 2004 that Wiwaxia lacks any characters that would firmly place it as a polychaete
or annelid
. Eibye-Jacobsen regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if Wiwaxia’s sclerites closely resembled bristles, which he doubted, this would not prove that Wiwaxia’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of Wiwaxia’s body do not correspond to any reasonable pattern of segmentation; while Eibye-Jacobsen did not think that this alone would prevent classification of Wiwaxia as a polychaete, he thought it was a serious objection given the lack of other clearly polychaete features. In his opinion there were 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.
Butterfield returned to the debate in 2006, repeating the arguments he presented in 1990 for regarding Wiwaxia as an early polychaete and adding that, while bristles are a feature of several groups, they appear as a covering over the back only in polychaetes.
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...
type Lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from 3.4 millimetre (0.133858267716535 in) to a little over 50.8 millimeters (2 in) in length. The precise taxonomic affinities of the genus are a matter of ongoing debate amongst palaeontologists.
History of discovery
Wiwaxia was originally described by G.F. Matthew in 1899, from an isolated spine that had been found earlier in the Ogyopsis Shale, and classified as a hyolithid. Further specimens were found by American paleontologist Charles Doolittle WalcottCharles 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:...
in 1911 as a result of one of his field trips to the nearby Burgess Shale
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...
in the Canadian 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...
, and he classified it as a member of the 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...
group of 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...
worms.
In 1966 and 1967 a team led by Harry B. 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...
revisited the Burgess Shale and found so many fossils that it took years to analyze them all, and Wiwaxia was one of the most difficult to analyze. Eventually in 1985 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...
, then a member of Whittington's team, published a detailed description that concluded Wiwaxia was not a polychaete. All the known specimens came from in and around the Burgess Shale until 1991, when fragmentary fossils were reported from Australia's Georgina Basin. In 2004 additional finds which may represent two different species were reported from the same area.
Occurrence
Reasonably complete specimens have been found in the Burgess ShaleBurgess 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...
, middle part of the Middle Cambrian with date , and fragmentary specimens in nearby strata slightly older than and younger than the Burgess Shale, in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
's lowermost Middle Cambrian beds of the Kaili Formation
Kaili Formation
The Kaili Formation named for nearby city of Kaili in the Guizhou province of southwest China, the Kaili formation is more than 200 meters thick with boundaries dated from the Lower–Middle Cambrian boundary to the early Middle Cambrian...
, in the Middle Cambrian beds of the Tyrovice Member, Buchava Formation of the Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
, in the Lower Cambrian Mount Cap formation
Mount Cap formation
The Mount Cap formation is a geological unit exposed in the Mackenzie Mountains, northern Canada. It was deposited in a shallow shelf setting in the late Early Cambrian, and contains an array of Burgess Shale-type microfossils that have ben recovered by acid maceration.The formation is one to...
(Mackenzie Mountains, Canada), in the Emu Bay Shale
Emu Bay shale
The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätten . It is one of two in the world containing Redlichiidan trilobites...
of Kangaroo Island
Kangaroo Island
Kangaroo Island is Australia's third-largest island after Tasmania and Melville Island. It is southwest of Adelaide at the entrance of Gulf St Vincent. Its closest point to the mainland is off Cape Jervis, on the tip of the Fleurieu Peninsula in the state of South Australia. The island is long...
, South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
, upper Botomian Stage of the Lower Cambrian, and in the Middle Botomian Sinsk Biota of the Siberia, Russia. These finds show that Wiwaxia and most of the other Burgess Shale type fauna
Burgess shale type fauna
A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are also preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale type fauna" covers assemblages based on taxonomic criteria only.-Extent:The fauna of the middle...
were very widespread. The Chinese material was originally considered to represent a separate species; like W. corrugata, it possessed spines and regions of sclerites (although it is only known from disarticulated remains), but the sclerites bear a higher density of ribs, and there are two distinct thicknesses of rib (i.e. larger and smaller). However, at a microscopic level, the sclerites do not differ from Burgess Shale or Mount Cap sclerites. The possibility remains that knob-bearing sclerites from all three localities belong to a different species.
Isolates spines are more common than sclerites in localities with a poor preservation potential, suggesting that the spines were more recalcitrant; however, in well-preserved sites such as the 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...
, spine and sclerite abundance is comparable in disarticulated instances to the proportions on complete fossils.
Description
This concentrates on the species Wiwaxia corrugata, found in the Burgess Shale, since the other specimens consist only of fragments, while the Burgess Shale has provided at least 138 complete ones.
Wiwaxia was a bilaterally symmetrical animal. Viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. The most complete fossils fall into two size ranges: 2 centimetre (0.78740157480315 in) to 5 centimetres (2 in) long, which are thought to be adults; and 3.4 millimetre (0.133858267716535 in) to 1.5 centimetre (0.590551181102362 in), which are thought to be juveniles. Estimating their height is difficult because specimens were compressed after death; a specimen of the average length, 3.4 centimetres (1.3 in), may have been 1 centimetre (0.393700787401575 in) high excluding the spines on their backs. The ratio of width to length does not appear to change as the animals grew.
The animal was covered in small ribbed armor plates called sclerites, that lay flat against the body, overlapped so that the rear of one covered the front of the one behind, and formed five main regions — the top, with 8-9 rows of sclerites; the upper part of the sides, with 11-12; the lower part of the sides, with 8; the front; and the area nearest the sea-floor, with 12-17 rows. Most of the sclerites were shaped like oval leaves, but the ventro-lateral ones, nearest the sea-floor, were crescent
Crescent
In art and symbolism, a crescent is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points .In astronomy, a crescent...
-shaped, rather like flattened banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....
s, and formed a single row with the tips pointing down. In addition there were two rows of ribbed spines running from to rear, one along each side of the top surface, and projecting out and slightly upwards, with a slight upwards curve near the tips. Specimens ranging from 11 millimetre (0.433070866141732 in) 52 millimetres (2 in) have about the same number of ventro-lateral sclerites just above the foot. On the other hand the number of spines seems to depend on the size of the specimen, up to about 12 per side. The number and spacing of the spines is asymmetrical in the specimens found, and this may have been natural rather than a result of events in the animal's life or after death. Although the spines in the middle of each row are usually the longest, up to 5 centimetres (2 in), a few specimens have rather short middle spines, perhaps because these were part-grown replacements. The smallest specimens may have lacked the long dorsal spines, which appear to have grown quickly in larger juveniles and then more slowly in adults.
Each sclerite was rooted separately in the body; the roots of body sclerites are 40% of the external length or a little less, while the roots of the spines are a little over 25% of the external length; all were rooted in pockets in the skin, rather like the follicle
Hair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s of 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...
ian hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
. The roots of the body sclerites were significantly narrower than the sclerites, but the spines had roots about as wide as their bases; both types of root were made of fairly soft tissue. They bore protrusive, presumably structural, ribs on their upper and (seemingly) lower surfaces. The sclerites and spines were not mineralized, and the frayed appearance of some broken ones suggests a fibrous structure. The way they were preserved suggests they were not made of chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
, from which insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s' exoskeletons are formed. They may have been made of tanned
Tanning
Tanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...
proteins or of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, which is the main component 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...
s and tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
s in humans.
Since the body sclerites had bases that were narrower than the hard external parts, it is hard to see how they grew. They may have enclosed soft tissue that could have secreted the hard walls, but there is no convincing evidence for this. Butterfield (1990) examined some sclerites under both optical
Optical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
and scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
s and concluded that they were not hollow, and that the bases split and spread to form the blades, a pattern that is also seen in monocot leaves.
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l stage. The feeding apparatus may have acted as a rasp
Rasp
A rasp is a tool used for shaping wood or other material. It consists of a point or the tip, then a long steel bar or the belly, then the heel or bottom, then the tang. The tang is joined to a handle, usually made of plastic or wood. The bar has sharp teeth...
to scrape 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...
off the top of the 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...
that covered the sea-floor, or as a rake
Rake (tool)
A rake is a broom for outside; an horticultural implement consisting of a toothed bar fixed transversely to a handle, and used to collect leaves, hay, grass, etc., and, in gardening, for loosening the soil, light weeding and levelling, removing dead grass from...
to gather food particles that lay on the sea-floor.
The long dorsal spines may have been a defense against predators, and finds of broken spines suggest that Wiwaxia was attacked. The animal appears to have crawled on the surface of the sea-floor feeding on particles that fell from higher levels of the sea. Wiwaxia shows no signs of legs and was probably too large to move on cilia, so it probably moved by muscular contraction that made its foot ripple. Juveniles may have burrowed into the sea-floor. In one specimen a small brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
, Diraphora
Diraphora
Diraphora is an extinct genus of brachiopod that lived in the Cambrian. Its remains have been found in Australia and North America. - References :* at the Field Museum's Evolving Planet-External links:* in the Paleobiology Database...
bellicostata, appears to be attached to one of the ventro-lateral sclerites. This suggests that adult Wiwaxia did not burrow or even plough much into the sea-floor as they moved. Two other specimens of Diraphora
Diraphora
Diraphora is an extinct genus of brachiopod that lived in the Cambrian. Its remains have been found in Australia and North America. - References :* at the Field Museum's Evolving Planet-External links:* in the Paleobiology Database...
bellicostata have been found attached to dorsal sclerites. Wiwaxia appears to have been solitary rather than gregarious.
Ontogeny
Wiwaxia grew by expansion rather than addition; that is to say it appears to have maintained a constant complement of scales as it grew, with each sclerite growing larger (by an unspecified means, probably by molting and replacement) and changing slightly in shape, but no new sclerites being inserted. 16–18 ventro-lateral sclerites are present on each side of a specimen, and the dorsal sclerites are arranged in 7–9 rows; the variation in number may be genuine but could reflect partial preservation. Its length:width ratio was constant through growth (although spineless "juveniles" may have been slightly flatter).One juvenile specimen appears to be preserved while molting and not yet completely detached from its discarded armor. Its new set of spines seem less rigid than the old ones and slightly underdeveloped, as if the next stages were going to be inflated by body fluids and then hardening. The new armor may have had an internal volume 50% to 70% larger than the old one. Molting appears to have occurred all at once, as adult specimens shows no signs of interruptions in the sclerite armor that would indicate molting of parts of the armor or of individual sclerites. Since the bases of the body sclerites are relatively narrow and these is no sign of sclerites splitting during molting, withdrawing soft tissue from the old sclerites would probably have required the tissues to be broken down in to a more fluid form, as happens in the claws of lobsters and crabs when they molt. The skin must also have been shed, since the discarded armor appears as a complete unit rather than scattered sclerites. In the juvenile that was apparently molting when it died, the feeding apparatus also appears to have been shed, as half of one tooth row is pointing forwards.
Classification
During the Cambrian, most of the main groupings of animalsPhylum
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....
recognised today were beginning to diverge. Consequently, many lineages (that would later become extinct) appear intermediate to two or more modern groups, or lack features common to all modern members of a group, and hence fall into the "stem group" of a modern taxon
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...
. Debate is ongoing as to whether Wiwaxia can be placed within a modern crown group and, if it cannot, in which group's stem it falls. When Walcott first described Wiwaxia, he regarded it 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...
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...
worm, and its sclerite
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
s as similar to the elytra ("scales") of annelids. More recently the debate has been intense, and proposed classifications include: a member of an extinct 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....
distantly related to the molluscs; a crown-group polychaete; a stem-group annelid; a problematic bilaterian; a stem- or possibly primitive crown-group mollusc.
In 1985 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...
agreed that there were similarities to polychaetes, but considered that Wiwaxia’s sclerites were different in construction from annelids' elytra. He was more impressed by the similarities between Wiwaxia’s feeding apparatus and a molluscan 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...
, and assigned the animal to a new taxon Molluscata, which he proposed should also contain the molluscs and hyolithid
Hyolitha
Hyolitha are enigmatic animals with small conical shells known from the Palaeozoic Era.-Shell morphology:The calcareous shells have a cover and two curved supports known as helens. Most are one to four centimeters in length and are triangular or elliptical in cross section...
s. When he later described the first fairly complete specimens of Halkieria
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...
, he suggested that these were closely related to Wiwaxia.
Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by 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 lectures, agreed that the sclerites were not like elytra, which are relatively fleshy and soft. However, since the sclerites were solid, he concluded that Wiwaxia could not be a member of the "Coeloscleritophora", a taxon
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...
that had been proposed in order to unite organisms with hollow sclerites, and could not be closely related to the halkieriids, which have hollow sclerites. Instead he thought that they were very similar in several ways to the
chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous bristles (setae) that project from the bodies of modern annelids and in some 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...
form leaf-like scales that cover the back like roof tiles – in composition, in detailed structure, in how they were attached to the body via "follicle
Hair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s" and in overall appearance. Some modern annelids also develop on each side rows of longer bristles, which both Walcott and Butterfield considered similar to Wiwaxia’s dorsal spines. including the halkieriids.
Butterfield also contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychates. He went so far as to classify Wiwaxia as a member of a modern order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, Phyllodocida
Phyllodocida
Phyllodocida is an order of polychaete worms in the subclass Aciculata. These worms are mostly marine though some are found in brackish water. Most are active benthic creatures, moving over the surface or burrowing in sediments, or living in cracks and crevices in bedrock. A few construct tubes in...
, and pointed out that Wiwaxia’s lack of obvious segmentation
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
is no barrier to this, as some modern polychaetes also show no segmentation except during development. He later noted that Wiwaxia lack some polychate features which he would expect to be easily preserved in fossils, and therefore a stem-group 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...
, in other words an evolutionary "aunt" of modern annelids.
Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes, and said Butterfield had informed them that the microscopic structure of Wiwaxia’s sclerites was identical to that of the bristles of two Burgess Shale polychaetes Burgessochaeta and Canadia. Conway Morris and Peel also wrote that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia’s feeding apparatus was much more like a molluscan 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...
. They also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller and more numerous; they also said that in 1994 Butterfield had found Wiwaxia sclerites that were clearly hollow. They presented a large cladogram
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...
according to which:
- The earliest halkieriids were a "sister" group to the molluscs, in other words descendants of a fairly closely related common ancestor.
- The halkieriids which Conway Morris had found in Greenland's Sirius PassetSirius PassetSirius 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....
lagerstätteLagerstätteA Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
were a "sister" group to brachiopods, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophoreLophophoreThe lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
. - Another halkieriid genusGenusIn 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...
, Thambetolepis, was a "great aunt" of annelids and Wiwaxia was an "aunt" of annelids.
Marine biologist Amélie H. Scheltema et al. (2003) argued that Wiwaxia’s feeding apparatus is very similar to 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...
s of some modern shell-less aplacophora
Aplacophora
Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...
n molluscs, and that the sclerites of the two groups are very similar. They concluded that Wiwaxia was a member of 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...
that includes molluscs. Scheltema has also highlighted similarities between Wiwaxia and the larvae of certain solenogaster molluscs, which bear iterated calcareous sclerites arranged into three symmetrical lateral zones.
Danish zoologist Danny Eibye-Jacobsen argued in 2004 that Wiwaxia lacks any characters that would firmly place it 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...
or 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...
. Eibye-Jacobsen regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if Wiwaxia’s sclerites closely resembled bristles, which he doubted, this would not prove that Wiwaxia’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of Wiwaxia’s body do not correspond to any reasonable pattern of segmentation; while Eibye-Jacobsen did not think that this alone would prevent classification of Wiwaxia as a polychaete, he thought it was a serious objection given the lack of other clearly polychaete features. In his opinion there were 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.
Butterfield returned to the debate in 2006, repeating the arguments he presented in 1990 for regarding Wiwaxia as an early polychaete and adding that, while bristles are a feature of several groups, they appear as a covering over the back only in polychaetes.
External links
- PharyngulaPharyngula (blog)Pharyngula is a blog on FreeThoughtBlogs and ScienceBlogs run by PZ Myers. In 2006, the science journal Nature listed it as the top-ranked blog written by a scientist. Pharyngula also won the 2005 Koufax Award for Best Expert Blog. The blog topics are eclectic, delving into the non-scientific as...
entry on Orthrozanclus reburrus