| Kimberella | |
|---|---|
 | |
| Remarkably preservedKimberella atArkhangelsk Regional Museum,Russia. | |
 | |
| Cast of a partialKimberella fossil. | |
Scientific classification | |
| Kingdom: | Animalia |
| Clade: | Bilateria |
| Genus: | †Kimberella Wade, 1972[1] |
| Type species | |
| †Kimberella quadrata (Glaessner & Wade, 1966) | |
| Species | |
| Synonyms | |
Kimberella is an extinctgenus of marinebilaterian known only from rocks of theEdiacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to thegastropods,[3] although its affinity with this group is contentious.[4][5][6]
Specimens were first found in Australia'sEdiacara Hills, but recent research has concentrated on the numerous finds near theWhite Sea inRussia, which cover an interval of time from555 to 558 million years ago.[3] As with manyfossils from this time, its evolutionary relationships to other organisms are hotly debated. Paleontologists initially classifiedKimberella as a type ofCubozoan, but, since 1997, features of its anatomy and its association with scratch marks resembling those made by aradula have been interpreted as signs that it may have been amollusc. Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least abilaterian.[7]
The classification ofKimberella is important for the scientific understanding of theCambrian explosion; if it was a mollusc, or at least aprotostome, this would mean that the two dominantNephrozoan lineages would have diverged significantly before555 million years ago, and if it was at least bilaterian, its age would indicate thatanimals were diversifying well before the start of theCambrian.[7]
The genus is named in honour of Mr. John Kimber, student, teacher, and collector; who lost his life during an expedition to Central Australia in 1964. Originally, the genus was given the nameKimberia,[8] however, the nameKimberia was already in use as a subgenus ofTurritella (Gastropoda), according to Dr. N. H. Ludbrook; and a new genus,Kimberella, was proposed byMary Wade in 1972.[1]
Kimberella has been found in theEdiacara Hills ofSouth Australia,[9] in theUst' Pinega Formation in theWhite Sea region of northwestRussia, the Kushk Series of centralIran, theMohyliv Formation inPodolia,Ukraine, and theKauriyala Formation in theLesser Himalayan Range region ofIndia. In 2014, Kimberella fossils were found in Brazil, but later studies showed fossils of other Silurian animals.[10][11][12] The White Sea fossils are often associated with theEdiacaran animalsTribrachidium andDickinsonia, as well as meanderingtrace fossil trails (possibly made byKimberella itself), andalgae. Beds in the White Sea succession have been dated to555.3 ± 0.3 million years ago and558 million years ago byradiometric dating, using uranium-lead ratios inzircons found in volcanic ash layers that are sandwiched between layers that containKimberella fossils.[13]Kimberella fossils are also known from beds both older and younger than this precisely dated range.[3] The fossils from the Ediacara Hills have not been dated precisely.[citation needed]
Over 1,000 specimens, representing organisms of all stages of maturity, have now been found in theWhite Sea area at the bottom of fine-grained sandstone layers.[3][14] 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 aboutKimberella's external form, internal anatomy, locomotion and feeding style.[3]
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction,[14] perhaps by as much as a factor of two. Like many other specimens found in the White Sea, the most common type ofsymmetry observed appears to be bilateral; with little to no sign of any of the kinds of radial symmetry found inCnidarians, the group that includesjellyfish,sea anemones andhydras. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry observed in the fossils. The White Sea fossils and the surrounding sediments also show thatKimberella lived on thesurface of the sea-floor.[15]
Kimberella had adorsalcovering that has been described as a (non-mineralized) "soft shell"; in larger specimens, this reached up to 15 cm in length, 5–7 cm in width, and 3–4 cm in height;[16] with a minimum length of 2–3 mm.[3]
The shell was stiff but flexible, and appears to not have beenmineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens.[3] The deformation observed in elongated and folded specimens illustrates that the shell was highly malleable; perhaps, rather than a singleintegument, it consisted of an aggregation of mineralised sclerites.[14] At the highest point was a hood-like structure, forming what is thought to be the front.[15][16] In some specimens, the inner surface of the shell bears stripes spanning the width of the creature; these may represent points of muscle attachment.[3] Similar stripes around the edge of the shell may have been connected to muscles that retracted the foot into the shell.[3]
The long axis of the organism is marked by a raised ridge; the middle axis is slightly humped. Kimberella's body had no visiblesegmentation 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 laterally across the underside of the body. The combination of these dorso-ventral and transverse ventral muscle bands enabled Kimberella to move by rippling its foot.[15][16]
The body also had a frilled fringe that may have been part of the animal's respiratory system, performing a function similar to that ofgills. The fact that the fringe extended well beyond the shell may indicate thatKimberella's "gills" were inefficient and needed a large area, or that there were no effective predators onKimberella and the shell's main function was to provide a platform for the muscles.[16]
Kimberella dwelt in shallow waters (up to tens of meters in depth), sharing the calm, well-oxygenated sea floor withphotosynthetic organisms andmicrobial mats.[3] Assemblages bearingKimberella often also bear fossils ofAndiva,Yorgia,Dickinsonia,Tribrachidium andCharniodiscus, suggesting that it lived alongside these organisms.[3]
Kimberella probably grazed on microbial mats, but a selective predatory habit cannot be ruled out.[3] Fossilized gut content seems to confirm it was grazing on bentic bacteria and algae, the latter having gone through their own ecological "big bang" 650 million years ago in the marine ecosystem, providing a more nutrient rich diet for early animals.[17][18] Fedonkin reckons that as it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity.[3] Conversely, Gehling et al. claim that it moved 'forwards'.[19] 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 two "teeth".[14] Gehlinget al. reconstructKimberella as having a long neck that operated like the arm of a digger, rotating about an axis perpendicular to the sea floor in order to produce the sweep of the fan, and rotating towards and away from the animal to scrape food from the substrate to the mouth. In one communityKimberella has been shown to be avoiding its grazing traces, demonstrating complex sensory behaviour.[20]
The lack of evidence of asexual reproduction suggests that the organisms reproduced sexually. Budding or fission has never been observed.[3]
The waters in whichKimberella dwelt were occasionally disturbed bysandy currents, caused when sediments were whipped up by storms ormeltwater 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.[3] 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.[3]
.jpg)
Kimberella fossils are generally preserved on top of clay-rich beds and beneath sandy beds.[14] 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.[3]
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.[3] It has been suggested that amucus trail produced by the organism may have assisted its preservation,[3] but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.[21]
All theKimberella fossils found so far are assigned to onespecies,K. quadrata. The first specimens were discovered in Australia in 1959. They were originally classified asjellyfish byMartin Glaessner andMary Wade in 1966,[8] and then asbox jellyfish by Wade in 1972,[1] a view that remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation.[3] Research on these specimens byMikhail A. Fedonkin, initially with Benjamin M. Waggoner in 1997,[15] led toKimberella being recognised as the oldest well-documentedtriploblasticbilaterian organism — not a jellyfish at all.[23]
So far,Kimberella fossils show no sign of aradula, the toothedchitinous "tongue" that is the diagnostic feature of modernmolluscs, excludingbivalves. Since radulae are very rarely preserved in fossil molluscs, its absence does not necessarily mean thatK. quadrata did not have one. The rocks in the immediate vicinity ofKimberella fossils bear scratch marks that have been compared to those made by the radulae of molluscs as they graze onmicrobial mats. Thesetraces, namedRadulichnus andKimberichnus, have been interpreted as circumstantial evidence for the presence of a radula. In conjunction with the univalve shell, this has been taken to indicateKimberella was a mollusc or very closely related to molluscs.[15] In 2001 and 2007, Fedonkin suggested that the feeding mechanism might be a retractableproboscis with hook-like organs at its end.[16]Kimberella's feeding apparatus appears to differ significantly from the typical mollusc radula, and this demonstrates thatKimberella is at best a stem-group mollusc.[24] Notably, the scratch marks indicate that the 'teeth' were dragged towards the organism, not pushed away as in molluscs, and that the maximum impact on the sediment was when the mouthpart was furthest from the organism.[25] The direction of grazing is also backwards, as opposed to forwards as in molluscs.[25] Furthermore, the constant width of grooves implies stereoglossy – a trait that is very derived in molluscs.[26] 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 a radula placesKimberella well outside the molluscan crown group.[14] Butterfield points out that plenty of other groups of organisms bear structures capable of making similar marks.[7][27]
Taken together, sceptics doubt that the available evidence is enough to reliably identifyKimberella as a mollusc or near-mollusc, and suggest that it is presumptuous to call it anything more than a "possible" mollusc,[13] or even just a "probable bilaterian".[7]
Possible known relative ofKimberella isSolza margarita from Russia. Some use grouping "Kimberellomorpha" for those two genera,[6] and some even consider thatSolza would be taphonomic variety ofKimberella.[2]
| Part of a series on |
| TheCambrian explosion |
|---|
 |
Fossil localities |
Evolutionary concepts |
TheCambrian explosion is an apparently rapid increase in the variety of basic body structures of animals in the EarlyCambrian period, starting after543 million years ago and finishing before518 million years ago.[28] A few of the Early Cambrian fossils were already known in the mid-19th century, andCharles Darwin saw the apparently sudden appearance anddiversification ofanimals as one of the main objections that could be made against his theory of evolution bynatural selection.[29]
The majority of animals more complex thanjellyfish and other cnidarians are split into two groups, theprotostomes anddeuterostomes.[23] The mollusc-like features ofKimberella strongly suggest that it was a member of the protostomes.[15][16] If so, this means that the protostome and deuterostome lineages must have split some time beforeKimberella appeared — at least558 million years ago, and hence well before the start of the Cambrian538.8 million years ago. Even if it is not a protostome, it is widely accepted as a member of the more inclusivebilaterian clade.[23][7] Since fossils of rather modern-looking cnidarians have been found in theDoushantuolagerstätte, the cnidarian and bilaterian lineages would have diverged well over580 million years ago.[23]
