Kimberella
Encyclopedia
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 Australia's Ediacara Hills
, but recent research has concentrated on the numerous finds near the White Sea
in Russia
, which cover an interval of time from . As with many fossils from this time
, its evolutionary relationships to other organisms are hotly debated. Paleontologists initially classified Kimberella as a type of jellyfish
, but since 1997 features of its anatomy and its association with scratch marks resembling those made by a radula
have been interpreted as signs that it may have been a mollusc. Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least a bilateria
n.
The classification of Kimberella is important for scientific understanding of the Cambrian explosion
: if it was a mollusc or at least a protostome
, the protostome and deuterostome lineages must have diverged significantly before . Even if it was a bilaterian but not a mollusc, its age would indicate that animal
s were diversifying well before the start of the Cambrian
.
of South Australia
and in the Ust’ Pinega Formation
in the White Sea
region of Russia
. The White Sea fossils are often associated with the Ediacaran "animals" Tribrachidium
and Dickinsonia
; meandering trace fossil
trails, possibly made by Kimberella; and algae
. Beds in the White Sea succession have been dated to and by radiometric dating
, using uranium-lead ratios in zircon
s found in volcanic ash layers that are sandwiched between layers that contain Kimberella fossils. Kimberella fossils are also known from beds older and younger than this precisely dated range. The fossils from the Ediacara Hills have not been dated precisely.
Over 1000 specimens, representing organisms of all stages of maturity, have now been found in the White Sea
area at the bottom of fine-grained sandstone layers. The large number of specimens, the small grain-size of the sediments and the variety of circumstances in which specimens were preserved provide detailed information about Kimberella′s external form, internal anatomy, locomotion and feeding style.
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction, perhaps by as much as a factor of two. The only type of symmetry
visible in the White Sea specimens is bilateral; there is no sign of any of the kinds of radial symmetry that are normal in the Cnidaria
, the group that includes jellyfish
, sea anemone
s and hydra
s. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry in the fossils. The White Sea fossils and the surrounding sediments also show that Kimberella lived on the surface of the sea-floor
.
Kimberella had a dorsal integument that has been described as a non-mineralized "shell"; in the larger specimens this reached up to 15 cm in length, 5 to 7 cm in width, and was 3 to 4 cm high; the smallest specimens are only about 2–3 mm long. The shell was stiff but flexible, and appears to have been non-mineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens. The deformation observed in elongated and folded specimens illustrates that the shell was highly malleable; perhaps, rather than a single integument, it consisted of an aggregation of (mineralized?) sclerites. At its highest point was a hood-like structure, forming what is thought to be the front. In some specimens, the inner surface of the shell bears stripes spanning the width of the creature; these may represent the attachment sites of muscles. Similar stripes around the edge of the shell may have been connected to muscles involved in retracting the muscular foot into the shell.
The long axis of the organism is marked by a raised ridge; the middle axis is slightly humped. Kimberella′s body had no visible segmentation
but had a series of repeated "modules
". Each module included a well-developed band of dorso-ventral muscles running from the top to the single, broad, muscular "foot", and smaller transverse ventral muscles from side to side on the underside of the body. The combination of the bands of dorso-ventral and transverse ventral muscles enabled Kimberella to move by making the foot ripple.
The body also had a frilled fringe that may have been part of the animal's respiratory system, performing a function similar to that of gills. The fact that the fringe extended well beyond the shell may indicate that Kimberella′s "gills" were inefficient and needed a large area, or that there were no effective predators on Kimberella and the shell's main function was to provide a platform for the muscles.
s. Assemblages bearing Kimberella often also bear fossils of Yorgia
, Dickinsonia
, Tribrachidium
and Charniodiscus
, suggesting that it lived alongside these organisms.
Kimberella probably grazed on microbial mats, but a selective predatory habit cannot be ruled out. As it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity. Fans of grooves are often found radiating from the "head" end of the organism; these indicate that the organism stayed in one place, and raked the surface of the microbial mat towards it by extension of its head, which bore a number of regularly spaced "teeth".
The lack of evidence to the contrary suggests that the organisms reproduced sexually.
The waters in which Kimberella dwelt were occasionally disturbed by sandy currents, caused when sediments were whipped up by storms or meltwater
discharge, and washed over the creatures. In response to this stress, the organisms appear to have retracted their soft parts into their shells; apparently they could not move fast enough to outrun the currents. Some organisms survived the current, and attempted to burrow out of the sand that had been deposited above them; some unsuccessful attempts can be seen where juveniles were fossilised at the end of a burrow a few centimetres long.
Preservation of most specimens was made possible by the fast sedimentation that quickly cut the organism off from seawater; it may also have been enhanced by the decay products of the rotting organism, which could have helped the overlying sediment to mineralise and harden. It has been suggested that a mucus
trail produced by the organism may have assisted its preservation, but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.
, K. quadrata. The first specimens were discovered in Australia in 1959. They were originally classified as jellyfish
by Martin Glaessner
and Mary Wade
in 1966, and then as box jellyfish by Wade in 1972, a view that remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation. Research on these specimens by Mikhail A. Fedonkin
, initially with Benjamin M. Waggoner in 1997, led to Kimberella being recognised as the oldest well-documented triploblastic bilateria
n organism — not a jellyfish at all.
So far Kimberella fossils show no sign of a radula
, the toothed chitinous "tongue" that is the diagnostic feature of modern molluscs, excluding bivalves. Since radulae are very rarely preserved in fossil molluscs, its absence does not necessarily mean that K. quadrata did not have one. The rocks in the immediate vicinity of Kimberella fossils bear scratch marks that are very similar those made by the radulae of molluscs as they graze on microbial mat
s. These trace
s, named Radulichnus
, have been interpreted as circumstantial evidence for the presence of a radula. In conjunction with the univalve shell, this has been taken to indicate Kimberella was a mollusc or very closely related to molluscs. In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis
with hook-like organs at its end. Kimberella′s feeding apparatus appears to differ significantly from the typical mollusc radula, and this demonstrates that Kimberella is at best a stem-group mollusc.
However, sceptics feel that the available evidence is not enough to reliably identify Kimberella as a mollusc or near-mollusc, considering it presumptuous to call it anything more than a "possible" mollusc, or even just a "probable bilaterian". Nicholas J. Butterfield argues that Kimberellas association with Radulichnus marks is not strong evidence that it was a mollusc, as other groups of organisms bear structures capable of making similar marks.
Indeed it has been argued that the shape of the feeding traces is incompatible with a radula, and that despite the molluscan body form, the lack of this synapomorphy
places it well outside the molluscan crown group.
is an apparently rapid increase in the variety of basic body structures of animals in the Early Cambrian
period, starting after and finishing before . A few of the Early Cambrian fossils were already known in the mid-19th century, and Charles Darwin
saw the apparently sudden appearance and diversification
of animal
s as one of the main objections that could be made against his theory of evolution by natural selection
.
The majority of animals more complex than jellyfish
and other Cnidarians are split into two groups, the protostome
s and deuterostome
s. The mollusc-like features of Kimberella strongly suggest that it was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least , and hence well before the start of the Cambrian . Even if it is not a protostome, it is widely accepted as a member of the more inclusive bilaterian clade. Since fossils of rather modern-looking Cnidaria
ns have been found in the Doushantuo
lagerstätte
, the Cnidarian and bilaterian lineages would have diverged well over .
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...
of bilaterian known only from rocks of 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. 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 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...
, but recent research has concentrated on the numerous finds near the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
in Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, which cover an interval of time from . As with many fossils from this time
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...
, its evolutionary relationships to other organisms are hotly debated. Paleontologists initially classified Kimberella as a type of jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, but since 1997 features of its anatomy and its association with scratch marks resembling those made by a 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...
have been interpreted as signs that it may have been a mollusc. Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least a bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
n.
The classification of Kimberella is important for scientific understanding of 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...
: if it was a mollusc or at least a protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
, the protostome and deuterostome lineages must have diverged significantly before . Even if it was a bilaterian but not a mollusc, its age would indicate that animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s were diversifying well before the start of 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...
.
Etymology
The genus is named after Mr. John Kimber, student, teacher, and collector who lost his life during an expedition to Central Australia in 1964. Originally it was described with the name Kimberia. Dr. N. H. Ludbrook drew attention to the fact that the name Kimberia is preoccupied by Kimberia Cotton and Woods, a turtle subgenus. Accordingly the new name Kimberella was proposed by Mary Wade in 1972.Occurrence
Kimberella has been found both in the Ediacara HillsEdiacara 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...
of 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...
and in the Ust’ Pinega Formation
Ust’ Pinega Formation
The Ust’ Pinega Formation is a geological formation exposed along the banks of the Onega river in Russia, famed for the vast quantities of fossils of the Ediacara biota preserved in its ash beds....
in the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
region of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. The White Sea fossils are often associated with the Ediacaran "animals" Tribrachidium
Tribrachidium
Tribrachidium heraldicum was an early Ediacaran organism famous for its unusual tri-radial symmetry. It was named and first described from South Australia by Martin Glaessner and Brian Daily in 1959....
and Dickinsonia
Dickinsonia
Dickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A...
; meandering trace fossil
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...
trails, possibly made by Kimberella; and 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...
. Beds in the White Sea succession have been dated to and by radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
, using uranium-lead ratios in zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s found in volcanic ash layers that are sandwiched between layers that contain Kimberella fossils. Kimberella fossils are also known from beds older and younger than this precisely dated range. The fossils from the Ediacara Hills have not been dated precisely.
Description
Over 1000 specimens, representing organisms of all stages of maturity, have now been found in the White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
area at the bottom of fine-grained sandstone layers. The large number of specimens, the small grain-size of the sediments and the variety of circumstances in which specimens were preserved provide detailed information about Kimberella′s external form, internal anatomy, locomotion and feeding style.
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction, perhaps by as much as a factor of two. The only type of symmetry
Symmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...
visible in the White Sea specimens is bilateral; there is no sign of any of the kinds of radial symmetry that are normal in the Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, the group that includes jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s and hydra
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
s. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry in the fossils. The White Sea fossils and the surrounding sediments also show that Kimberella lived on the surface of the sea-floor
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
.
Kimberella had a dorsal integument that has been described as a non-mineralized "shell"; in the larger specimens this reached up to 15 cm in length, 5 to 7 cm in width, and was 3 to 4 cm high; the smallest specimens are only about 2–3 mm long. The shell was stiff but flexible, and appears to have been non-mineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens. The deformation observed in elongated and folded specimens illustrates that the shell was highly malleable; perhaps, rather than a single integument, it consisted of an aggregation of (mineralized?) sclerites. At its highest point was a hood-like structure, forming what is thought to be the front. In some specimens, the inner surface of the shell bears stripes spanning the width of the creature; these may represent the attachment sites of muscles. Similar stripes around the edge of the shell may have been connected to muscles involved in retracting the muscular foot into the shell.
The long axis of the organism is marked by a raised ridge; the middle axis is slightly humped. Kimberella′s body had no visible 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,...
but had a series of repeated "modules
Modularity (biology)
Many organisms consist of modules, both anatomically and in their metabolism. Anatomical modules are usually segments or organs. When we look at illustrations of metabolic reactions, we find that they, too, are modular: we can clearly identify, for instance, the citric acid cycle as a complex...
". Each module included a well-developed band of dorso-ventral muscles running from the top to the single, broad, muscular "foot", and smaller transverse ventral muscles from side to side on the underside of the body. The combination of the bands of dorso-ventral and transverse ventral muscles enabled Kimberella to move by making the foot ripple.
The body also had a frilled fringe that may have been part of the animal's respiratory system, performing a function similar to that of gills. The fact that the fringe extended well beyond the shell may indicate that Kimberella′s "gills" were inefficient and needed a large area, or that there were no effective predators on Kimberella and the shell's main function was to provide a platform for the muscles.
Ecology
Kimberella dwelt in shallow waters (up to tens of meters in depth), sharing the calm, well-oxygenated sea floor with photosynthetic organisms and microbial matMicrobial 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...
s. Assemblages bearing Kimberella often also bear fossils of Yorgia
Yorgia
Yorgia waggoneri is a member of the Ediacara biota, and resembles a cross between the organisms Dickinsonia and Spriggina. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of...
, Dickinsonia
Dickinsonia
Dickinsonia is an iconic fossil of the Ediacaran biota. It resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; most interpretations consider it to be an animal, although others suggest it may be fungal, or a member of an "extinct kingdom".-Species variety:A...
, Tribrachidium
Tribrachidium
Tribrachidium heraldicum was an early Ediacaran organism famous for its unusual tri-radial symmetry. It was named and first described from South Australia by Martin Glaessner and Brian Daily in 1959....
and Charniodiscus
Charniodiscus
Charniodiscus is an Ediacaran fossil that was probably a stationary filter feeder that lived anchored to a sandy sea bed. The organism had a holdfast, stalk and frond. The holdfast was bulbous shaped, and the stalk was flexible. The frond was segmented and had a pointed tip...
, suggesting that it lived alongside these organisms.
Kimberella probably grazed on microbial mats, but a selective predatory habit cannot be ruled out. As it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity. Fans of grooves are often found radiating from the "head" end of the organism; these indicate that the organism stayed in one place, and raked the surface of the microbial mat towards it by extension of its head, which bore a number of regularly spaced "teeth".
The lack of evidence to the contrary suggests that the organisms reproduced sexually.
The waters in which Kimberella dwelt were occasionally disturbed by sandy currents, caused when sediments were whipped up by storms or meltwater
Meltwater
Meltwater is the water released by the melting of snow or ice, including glacial ice and ice shelfs over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing...
discharge, and washed over the creatures. In response to this stress, the organisms appear to have retracted their soft parts into their shells; apparently they could not move fast enough to outrun the currents. Some organisms survived the current, and attempted to burrow out of the sand that had been deposited above them; some unsuccessful attempts can be seen where juveniles were fossilised at the end of a burrow a few centimetres long.
Preservation
Kimberella fossils are generally preserved on top of a clay-rich bed and beneath a sandy bed. All fossils are preserved as depressions in the bases of beds, implying that the organism, although not mineralised, was firm enough to resist being crushed as sediment accumulated above it; as the soft parts of the organism decayed, the soft muds underneath would be squeezed up into the shell, preserving the shape of the organism.Preservation of most specimens was made possible by the fast sedimentation that quickly cut the organism off from seawater; it may also have been enhanced by the decay products of the rotting organism, which could have helped the overlying sediment to mineralise and harden. It has been suggested that a mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
trail produced by the organism may have assisted its preservation, but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.
Classification
All the Kimberella fossils found so far are assigned to one speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, K. quadrata. The first specimens were discovered in Australia in 1959. They were originally classified as jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
by 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...
and Mary Wade
Mary Wade (Paleontologist)
Dr Mary Julia Wade was an Australian paleontologist, perhaps best known for her work on the late Precambrian Ediacaran biota in South Australia....
in 1966, and then as box jellyfish by Wade in 1972, a view that remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation. Research on these specimens by Mikhail A. Fedonkin
Mikhail A. Fedonkin
Dr. Mikhail Aleksandrovich Fedonkin is an awarding winning paleontologist specializing in documentation of the earliest animals' body fossils, tracks, and trails. He was the first to describe several fossils including Hiemalora, Onega stepanovi, and Nimbia occlusa.Fedonkin is fluent in English and...
, initially with Benjamin M. Waggoner in 1997, led to Kimberella being recognised as the oldest well-documented triploblastic bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
n organism — not a jellyfish at all.
So far Kimberella fossils show no sign of a 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 chitinous "tongue" that is the diagnostic feature of modern molluscs, excluding bivalves. Since radulae are very rarely preserved in fossil molluscs, its absence does not necessarily mean that K. quadrata did not have one. The rocks in the immediate vicinity of Kimberella fossils bear scratch marks that are very similar those made by the radulae of molluscs as they graze on 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...
s. These trace
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...
s, named Radulichnus
Radulichnus
Radulichnus is an ichnogenus of trace fossil which resembles the marks produced by the action of a mollusc's radula on sediment. As an ichnogenus, its classification is based solely on appearance, and does not necessarily imply anything of the affinity of the organism which produced the...
, have been interpreted as circumstantial evidence for the presence of a radula. In conjunction with the univalve shell, this has been taken to indicate Kimberella was a mollusc or very closely related to molluscs. In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable 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:...
with hook-like organs at its end. Kimberella′s feeding apparatus appears to differ significantly from the typical mollusc radula, and this demonstrates that Kimberella is at best a stem-group mollusc.
However, sceptics feel that the available evidence is not enough to reliably identify Kimberella as a mollusc or near-mollusc, considering it presumptuous to call it anything more than a "possible" mollusc, or even just a "probable bilaterian". Nicholas J. Butterfield argues that Kimberellas association with Radulichnus marks is not strong evidence that it was a mollusc, as other groups of organisms bear structures capable of making similar marks.
Indeed it has been argued that the shape of the feeding traces is incompatible with a radula, and that despite the molluscan body form, the lack of this synapomorphy
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
places it well outside the molluscan crown group.
Theoretical importance
The Cambrian explosionCambrian 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...
is an apparently rapid increase in the variety of basic body structures of 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...
period, starting after and finishing before . A few of the Early Cambrian fossils were already known in the mid-19th century, and 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...
saw the apparently sudden appearance and diversification
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...
of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s as one of the main objections that could be made against his theory of evolution by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
.
The majority of animals more complex than jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
and other Cnidarians are split into two groups, the protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s. The mollusc-like features of Kimberella strongly suggest that it was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least , and hence well before the start of the Cambrian . Even if it is not a protostome, it is widely accepted as a member of the more inclusive bilaterian clade. Since fossils of rather modern-looking Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns have been found in the Doushantuo
Doushantuo Formation
The Doushantuo Formation is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils...
lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
, the Cnidarian and bilaterian lineages would have diverged well over .