Kimberella

Kimberella Fossil range: Ediacaran 558–555 Ma PreЄ Є O S D C P T J K Pg N ↓
Fossil of Kimberella quadrata.
Cast of a partial Kimberella fossil.
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
(unranked):	Bilateria
Phylum:	Mollusca?
Genus:	Kimberella Glaessner & Wade, 1966[1][2]
Binomial name
Kimberella quadrata Glaessner & Wade, 1966[1]

Kimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized. 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 555 to 558 million years ago.^[1] As with many fossils from this time, its evolutionary relationships to other organisms is 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 bilaterian.

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 555 million years ago. Even if it was a bilaterian but not a mollusc, its age would indicate that animals were diversifying well before the start of the Cambrian.

Occurrence

Kimberella has been found both in the Ediacara Hills of South Australia^[3] 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 itself; and algae. Beds in the White Sea succession have been dated to 555.3 ± 0.3 million years ago and 558 million years ago by radiometric dating, using uranium-lead ratios in zircons found in volcanic ash layers that are sandwiched between layers that contain Kimberella fossils.^[4] Kimberella fossils are also known from beds older and younger than this precisely dated range.^[1] The fossils from the Ediacara Hills have not been dated precisely.

Description

History of Kimberella quadrata reconstruction:
* M. Wade, 1972^[5] - Cubozoa.
* M.A. Fedonkin, B. Waggoner, 1997^[6] - mollusc-like organism with soft shell and big foot.
* M.A. Fedonkin, A. Simonetta, A.Y. Ivantsov, 2007^[1] - mollusc-like organism with soft shell and proboscis carrying two hook-like teeth at its end.
* A.Y. Ivantsov, 2009^[7] - no complete consolidated shell, but she had the mineral sclerites and several teeth in her mouth.

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.^[1][7] The large number of specimens, the small grain 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.^[8]

All of the fossils are oval in outline, and larger specimens are more elongated. 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 anemones and hydras. 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.^[6]

Kimberella had a single, dorsal 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;^[8] the smallest specimens are only about 2–3 mm long.^[1] 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.^[1] At its highest point was a hood-like structure, forming what is thought to be the front.^[8][6] 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.^[1] Similar stripes around the edge of the shell may have been connected to muscles involved in retracting the muscular foot into the shell.^[1]

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.^[8][6]

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. ^[8]

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 mats.^[1] Assemblages bearing Kimberella often also bear fossils of Yorgia, Dickinsonia, Tribrachidium and Charniodiscus, suggesting that it lived alongside these organisms.^[1]

Kimberella probably grazed on microbial mats, but a selective predatory habit cannot be ruled out.^[1] As it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity.^[1] The lack of evidence to the contrary suggests that the organisms reproduced sexually.^[1]

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.^[1] 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.^[1]

Preservation

All Kimberella fossils were 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.^[1]

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.^[1] It has been suggested that a mucus trail produced by the organism may have assisted its preservation,^[1] but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.^[9]

Classification

All the Kimberella fossils found so far are assigned to one species, 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,^[13] and then as box jellyfish by Wade in 1972,^[5] a view that remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation.^[1] Research on these specimens by Mikhail A. Fedonkin, initially with Benjamin M. Waggoner in 1997,^[6] led to Kimberella being recognised as the oldest well-documented triploblastic bilaterian organism — not a jellyfish at all.^[14]

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 mats. These traces, 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.^[6] In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end.^[8]

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,^[4] or even just a "probable bilaterian".^[15] Nicholas J. Butterfield argues that Kimberella's 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.^[15][16]

Theoretical importance

The Cambrian explosion is an apparently rapid increase in the variety of basic body structures of animals in the Early Cambrian period, starting after 543 million years ago and finishing before 518 million years ago.^[12] 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 animals as one of the main objections that could be made against his theory of evolution by natural selection.^[17]

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes.^[14] The mollusc-like features of Kimberella strongly suggest that it was a member of the protostomes.^[8][6] If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 542 million years ago. Even if it is not a protostome, it is widely accepted as a member of the more inclusive bilaterian clade.^[14][15] Since fossils of rather modern-looking Cnidarians have been found in the Doushantuo lagerstätte, the Cnidarian and bilaterian lineages would have diverged well over 580 million years ago.^[14]

References

External links

• image from UCMP

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