Kimberella

Taxobox

---

image_caption = Cast of a partial "Kimberella" fossil. The bulges at the top of the image are the sides of the organism, and surrounded the rim of the creature; these are further flanked with a line which may represent the extent of a muscular foot.
fossil_range = Ediacaran (558-5Ma)
regnum = Animalia
phylum = Mollusca?
genus = "Kimberella"
genus_authority = Glaessner & Wade, 1966The Rise and Fall of the Ediacaran Biota|New data on "Kimberella", the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications|Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.|157|179|12] [Originally the genus was called "Kimberia", but since this was already assigned to a turtle, it was later altered to Kimberella by Dennis-Bryan & Miles in "Zoological Journal of the Linnean Society", 1975 .]
species = "K. quadrata"
binomial = "Kimberella quadrata"
binomial_authority = Glaessner & Wade, 1966

"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 ma|555|558. 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 ma|555. 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

Graphical timeline
help=off

title="Kimberella" in context
align=left

plot-colour=#ffffff

from=-655
to=-490
scale-increment=10
width=6
height=25
annotations-width=4

legend1=Neoproterozoic
(last Era (geology)!era of the Precambrian)
legend1-colour=period color|neoproterozoic
bar1-from=-655
bar1-to=-542
bar1-left=.0
bar1-right=.1
bar1-colour=period color|neoproterozoic

legend2=Palæozoic
(first era of the Phanerozoic)
legend2-colour=period color|paleozoic
bar2-from=-542
bar2-to=-490
bar2-left=0
bar2-right=.1
bar2-colour=period color|palæozoic

bar3-text=Cryogenian
bar3-to=-635
bar3-left=.1
bar3-colour=period color|cryogenian

bar23-from=-635.3
bar23-to=-634.7
bar23-colour=black
bar23-left=0.1

bar4-from=-635
bar4-to=-542
bar4-left=.1
bar4-colour=period color|Ediacaran
bar4-border-width=.0
bar4-text=Ediacaran
bar4-nudge-down=5.2
bar4-nudge-right=1.3

bar24-from=-542.3
bar24-to=-541.7
bar24-colour=black

bar5-text=Cambrian
bar5-from=-542
bar5-to=-490
bar5-left=.1
bar5-colour=period color|cambrian
bar5-nudge-up=0.8

bar25-from=-490.3
bar25-to=-489.7
bar25-colour=black
bar25-left=0.09

bar6-from=-655
bar6-to=-490
bar6-left=.99
bar6-right=1
bar6-colour=#000000

bar17-from=-655
bar17-to=-490
bar17-left=0.09
bar17-right=0.1
bar17-colour=#000000

bar21-from=-635
bar21-to=-551
bar21-left=0.15
bar21-right=0.25
bar21-colour=#007777
note21=Doushantuo Formation
note21-at=-600
note21-nudge-left=4.5

bar22-from=-585
bar22-to=-542
bar22-left=0.30
bar22-right=.40
bar22-colour=#777700
note22=Ediacara biota
note22-at=-570
note22-colour=#777700
note22-nudge-left=3.6
note22-nudge-down=0.2

bar23-from=-558
bar23-to=-555

bar23-left=0.9

bar25-from=-530
bar25-to=-520
bar25-left=0.90
bar25-right=0.99
bar25-colour=#666666
note25=? Cambrian explosion, if it was sudden
note25-at=-525
note25-colour=#666666
note25-nudge-up=1.5

note7="Kimberella"
note7-at=-556.5

caption=Axis scale: millions of years ago.
References for dates:
Doushantuo Formation cite journal
title=U-Pb Ages from the Neoproterozoic Doushantuo Formation, China
journal=Science | date=1 April 2005 | issue=5718 | pages=95–98 | doi=10.1126/science.1107765
url=http://www.sciencemag.org/cgi/content/abstract/308/5718/95 | accessdate=2008-07-18
author=Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y.
year=2005
volume=308
format=abstract
pmid=15731406]
Ediacara biota cite journal
title=Decoding the Ediacaran Enigma | author=Brasier, M., and Antcliffe, J. | journal=Science
date=20 August 2004 | volume=305 | issue=5687 | pages-1115-1117 | doi=10.1126/science.1102673
url=http://www.sciencemag.org/cgi/content/summary/305/5687/1115?ck=nck | accessdate=2008-07-18 ]
Kimberella
Cambrian explosion

"Kimberella" has been found both in the Ediacara Hills of South Australia cite journal | author=Glaessner, M., and Daily, B. | date=1959 | url=http://www.samuseum.sa.gov.au/Journals/RSAM/RSAM_v013/rsam_v013_p369p402.pdf | title=The Geology and Late Precambrian Fauna of the Ediacara Fossil Reserve | journal=Records of the South Australian Museum | volume=13 |pages=369–401 | accessdate=2008-07-16 ] 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 Ma|555.3|error=0.3 and Ma|558 by radiometric dating, using uranium-lead ratios in zircons found in volcanic ash layers that are sandwiched between layers that contain "Kimberella" fossils.cite journal
author = Martin, M.W.
coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
year = 2000
date = 2000-05-05
title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
journal = Science
volume = 288
issue = 5467
pages = 841
doi = 10.1126/science.288.5467.841
accessdate = 2007-05-10
format = abstract
pmid = 10797002] "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 800 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 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. cite journal | title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications | author=Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y. | journal=Geological Society, London, Special Publications | date=2007 | volume-286 | pages=157-179 | doi=10.1144/SP286.12 | url=http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | accessdate=2008-07-10 ]

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.

"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; 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. 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 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.

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. 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. 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

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.

Preservation of most specimens was made possible by the fast sedimentation which 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.cite journal
author = Getty, P.R. | | year = 2006 | title = Producing And Preserving "Climactichnites"
journal = 2006 Philadelphia Annual Meeting
url = http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_111068.htm
accessdate = 2008-06-02]

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,cite journal
author = Glaessner, M.F.
coauthors = Wade, M.
year = 1966
title = The late Precambrian fossils from Ediacara, South Australia
journal = Palaeontology
url = http://palaeontology.palass-pubs.org/pdf/Vol%209/Pages%20599-628.pdf
format = Free full text
volume = 9
issue = 4
pages = 599] and then as box jellyfish by Wade in 1972,cite journal | author=Wade, M. | title=Hydrozoa and Scyphozoa and other medusoids from the Precambrian Ediacara fauna, South Australia | journal=Palaeontology | volume=15 | pages=197–225 | date=1972 ] a view which 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 bilaterian organism - not a jellyfish at all.cite journal|author=Erwin, Douglas H.; Eric H. Davidson|year=2002|title=The last common bilaterian ancestor|journal=Development|volume=129|pages=3021–3032|url=http://dev.biologists.org/cgi/content/full/129/13/3021|pmid=12070079]

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.cite journal
author = Fedonkin, M.A.
coauthors = Waggoner, B.M.
year = 1997
title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal = Nature
volume = 388
issue = 6645
pages = 868
doi = 10.1038/42242] In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end. 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".cite journal
author = Butterfield, N.J.
year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays
volume = 28
issue = 12
pages = 1161–6
doi = 10.1002/bies.20507
accessdate = 2007-05-21] 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.cite journal | author = Butterfield | year = 2008 | url = http://jpaleontol.geoscienceworld.org/cgi/content/full/82/3/543
doi = 10.1666/07-066.1
title = An Early Cambrian Radula
journal = Journal of Paleontology
volume = 82
pages = 543]

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 ma|543 and finishing before ma|518. cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science | edition=3rd | page=p. 63 | date=2000 | isbn=0-632-04444-6 ] 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.cite book
title=On the Origin of Species by Natural Selection
author=Darwin, C
authorlink=Charles Darwin
year=1859
pages=315–316
publisher=Murray, London, United Kingdom]

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes. 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 ma|558, and hence well before the start of the Cambrian ma|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 Cnidarians have been found in the Doushantuo lagerstätte, the Cnidarian and bilaterian lineages would have diverged well over Ma|580.

References

External links

* [http://www.ucmp.berkeley.edu/vendian/kimberella2.html image from UCMP]