Kimberella
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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 have been dated to Vorlage:Ma. 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 acepted 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 Vorlage:Ma. 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 and in the Ust’ Pinega Formation in the White Sea region of Russia. In the White Sea area, the fossils are often associated with: the Ediacaran "animals" Tribrachidium and Dickinsonia; meandering trace fossil trails, possibly made by Kimberella itself; and algae. The White Sea strata have been dated to Vorlage:Ma by radiometric dating, using uranium-lead ratios in zircons found in volcanic ash layers that are sandwiched between layers that contain Kimberella fossils.[1] The fossils from the Ediacara Hills have not been dated precisely.
Description
Over 700 specimens 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 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.[2]
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.[3]
Kimberella had a single, dorsal shell; in the larger specimens this reached up to 15 cm in length, 5 to 7cm in width, and was 3 to 4 cm high;[2] the smallest specimens were only about 3 mm long.[3] The shell was stiff but flexible, and appears to have been non-mineralized in the early stages of growth but mineralized, possibly with aragonite, in more mature specimens. Its highest point was a hood-like structure at what is thought to be the front.[3][2]
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.[3][2]
The body also had a frilled fringe which 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. [2]
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,[4] and then as box jellyfish by Wade in 1972.[5] As a result of research on the White Sea specimens by Mikhail A. Fedonkin, initially with Benjamin M. Waggoner in 1997,[3] Kimberella is now recognised as the oldest well-documented triploblastic bilaterian organism and not a jellyfish at all.[6]
So far Kimberella fossils show no sign of a radula, the toothed chitinous "tongue" which 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 which 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.[3] In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end.[2]
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,[1] or even just a "probable bilaterian".[7] 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.[7][8]
Theoretical importance
The Cambrian explosion is an apparent rapid increase in the variety of basic body structures of animals in the Early Cambrian period, starting after Vorlage:Ma and finishing before Vorlage:Ma.[9] 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.[10]
If Kimberella was a mollusc-like protostome,[3][2] the protostome and deuterostome lineages must have split significantly before Vorlage:Ma.[6] Even if it is not a protostome, it is widely accepted as a bilaterian.[6][7] Since fossils of rather modern-looking Cnidarians have been found in the Doushantuo lagerstätte, the Cnidarian and bilaterian lineages would have diverged well over Vorlage:Ma.[6] If Kimberella is very similar to a mollusc, animals were diversifying long before the start of the Cambrian. This would cast doubt on the idea that the main groups of animals appeared rather abruptly in the early Cambrian.
References
External links
- Kimberella quadrata at Peripatus
- image from UCMP
Vorlage:Creatures of the slime
- ↑ a b Martin, M.W., Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.: Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution. In: Science. 288. Jahrgang, Nr. 5467, 5. Mai 2000, S. 841, doi:10.1126/science.288.5467.841, PMID 10797002.
- ↑ a b c d e f g Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y.: New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications. In: Geological Society, London, Special Publications. 2007, S. 157–179, doi:10.1144/SP286.12 (edu.au [PDF; abgerufen am 10. Juli 2008]).
- ↑ a b c d e f g Fedonkin, M.A., Waggoner, B.M.: The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. In: Nature. 388. Jahrgang, Nr. 6645, 1997, S. 868, doi:10.1038/42242.
- ↑ Glaessner, M.F., Wade, M.: The late Precambrian fossils from Ediacara, South Australia. In: Palaeontology. 9. Jahrgang, Nr. 4, 1966, S. 599.
- ↑ Wade, M.: Hydrozoa and Scyphozoa and other medusoids from the Precambrian Ediacara fauna, South Australia. In: Palaeontology. 15. Jahrgang, 1972, S. 197–225.
- ↑ a b c d Erwin, Douglas H.; Eric H. Davidson: The last common bilaterian ancestor. In: Development. 129. Jahrgang, 2002, S. 3021–3032, PMID 12070079 (biologists.org).
- ↑ a b c Butterfield, N.J.: Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale. In: Bioessays. 28. Jahrgang, Nr. 12, 2006, S. 1161–6, doi:10.1002/bies.20507.
- ↑ Butterfield: An Early Cambrian Radula. In: Journal of Paleontology. 82. Jahrgang, 2008, S. 543, doi:10.1666/07-066.1 (geoscienceworld.org).
- ↑ Cowen, R.: History of Life. 3rd Auflage. Blackwell Science, 2000, ISBN 0-632-04444-6, S. 63.
- ↑ Darwin, C: On the Origin of Species by Natural Selection. Murray, London, United Kingdom, 1859, S. 315–316.