Kimberella is an extinct 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 gastropods,[2] although its affinity with this group is contentious.[3]
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 555to558 million years ago.[2] As with many fossils from this time, its evolutionary relationships to other organisms are hotly debated. Paleontologists initially classified Kimberella as a type of Cubozoan, 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.[4]
The classification of Kimberella is important for the scientific understanding of the Cambrian explosion; if it was a mollusc, or at least a protostome, this would mean that the two dominant Nephrozoan lineages would have diverged significantly before 555million years ago, and if it was at least bilaterian, its age would indicate that animals were diversifying well before the start of the Cambrian.[4]
Etymology
Reconstruction of Kimberella quadrata
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 name Kimberia,[5] however, the name Kimberia was already in use as a subgenus of Turritella (Gastropoda), according to Dr. N. H. Ludbrook; and a new genus, Kimberella, was proposed by Mary Wade in 1972.[1]
Occurrence
Kimberella has been found in the Ediacara Hills of South Australia,[6] in the Ust' Pinega Formation in the White Sea region of Russia and the Kushk Series of central Iran.[7] 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 555.3±0.3million years ago and 558million 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.[8]Kimberella fossils are also known from beds both older and younger than this precisely dated range.[2] The fossils from the Ediacara Hills have not been dated precisely.[citation needed]
Description
History of Kimberella quadrata reconstruction: * M. Wade (1972) – Cubozoa. * M.A. Fedonkin, B. Waggoner (1997) M.A. Fedonkin (2001) – mollusc-like organism with soft shell and big foot. * M.A. Fedonkin, A. Simonetta, A.Y. Ivantsov (2007) – mollusc-like organism with soft shell and proboscis carrying two hook-like teeth at its end. * R.J.F. Jenkins (1992) – hypothetical trilobite-like arthropod as a maker of feeding traces of Kimberella * A.Y. Ivantsov (2009) – no complete consolidated shell, but with mineral sclerites and several teeth in its mouth.
Over 1,000specimens, representing organisms of all stages of maturity, have now been found in the White Sea area at the bottom of fine-grained sandstone layers.[2][11] 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.[2]
Current life restoration of Kimberella based on A.Y. Ivantsov's 2009 interpretation of the animal.A detailed diorama of K.quadrata (pink animals on the right) along with simple traces of animal movement (lowered lines on the left).
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction,[11] perhaps by as much as a factor of two. Like many other specimens found in the White Sea, the most common type of symmetry observed appears to be bilateral; with little to no sign of any of the kinds of radial symmetry found in Cnidarians, 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 observed in the fossils. The White Sea fossils and the surrounding sediments also show that Kimberella lived on the surface of the sea-floor.[9]
Kimberella had a dorsalcovering that has been described as a (non-mineralized) "soft shell"; in larger specimens, this reached up to 15cm in length, 5–7cm in width, and 3–4cm in height;[12] with a minimum length of 2–3mm.[2]
The shell was stiff but flexible, and appears to not have been mineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens.[2] 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 mineralised sclerites.[11] At the highest point was a hood-like structure, forming what is thought to be the front.[9][12] In some specimens, the inner surface of the shell bears stripes spanning the width of the creature; these may represent points of muscle attachment.[2] Similar stripes around the edge of the shell may have been connected to muscles that retracted the foot into the shell.[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 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.[9][12]
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.[12]
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.[2] Assemblages bearing Kimberella often also bear fossils of Andiva, Yorgia, Dickinsonia, Tribrachidium and Charniodiscus, suggesting that it lived alongside these organisms.[2]
Kimberella probably grazed on microbial mats, but a selective predatory habit cannot be ruled out.[2] 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.[13][14] Fedonkin reckons that as it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity.[2] Conversely, Gehling et al. claim that it moved 'forwards'.[15] 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".[11] Gehling et al. reconstruct Kimberella 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 community Kimberella has been shown to be avoiding its grazing traces, demonstrating complex sensory behaviour.[16]
The lack of evidence of asexual reproduction suggests that the organisms reproduced sexually. Budding or fission has never been observed.[2]
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.[2] 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.[2]
Preservation
A fossil of Kimberella which can be hardly distinguished by the sediments it was preserved in.
Kimberella fossils are generally preserved on top of clay-rich beds and beneath sandy beds.[11] 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.[2]
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.[2] It has been suggested that a mucus trail produced by the organism may have assisted its preservation,[2] but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.[17]
Kimberichnus teruzzii, grazing traces left by Kimberella while it fed.
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,[5] and then as box 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.[2] Research on these specimens by Mikhail A. Fedonkin, initially with Benjamin M. Waggoner in 1997,[9] led to Kimberella being recognised as the oldest well-documented triploblasticbilaterian organism — not a jellyfish at all.[19]
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 have been compared to those made by the radulae of molluscs as they graze on microbial mats. These traces, named Radulichnus and Kimberichnus, 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.[9] In 2001 and 2007, Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end.[12]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.[20] 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.[21] The direction of grazing is also backwards, as opposed to forwards as in molluscs.[21] Furthermore, the constant width of grooves implies stereoglossy – a trait that is very derived in molluscs.[22] 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 places Kimberella well outside the molluscan crown group.[11] Butterfield points out that plenty of other groups of organisms bear structures capable of making similar marks.[4][23]
Taken together, sceptics doubt that the available evidence is enough to reliably identify Kimberella as a mollusc or near-mollusc, and suggest that it is presumptuous to call it anything more than a "possible" mollusc,[8] or even just a "probable bilaterian".[4]
The Cambrian explosion is an apparently rapid increase in the variety of basic body structures of animals in the Early Cambrian period, starting after 543million years ago and finishing before 518million years ago.[24] 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.[25]
The majority of animals more complex than jellyfish and other cnidarians are split into two groups, the protostomes and deuterostomes.[19] The mollusc-like features of Kimberella strongly suggest that it was a member of the protostomes.[9][12] If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least 558million years ago, and hence well before the start of the Cambrian 538.8million years ago. Even if it is not a protostome, it is widely accepted as a member of the more inclusive bilaterian clade.[19][4] Since fossils of rather modern-looking cnidarians have been found in the Doushantuolagerstätte, the cnidarian and bilaterian lineages would have diverged well over 580million years ago.[19]
The Ediacaran is a geological period of the Neoproterozoic era that spans 96 million years from the end of the Cryogenian period at 635 Mya to the beginning of the Cambrian period at 538.8 Mya. It is the last period of the Proterozoic eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian period marks the start of the Phanerozoic eon, where recognizable fossil evidence of life becomes common.
Dickinsonia is a genus of extinct organism, most likely an animal, that lived during the late Ediacaran period in what is now Australia, China, Russia and Ukraine. It is one of the best known members of the Ediacaran biota. The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth has been considered consistent with a stem-group bilaterian affinity, though various other affinities have been proposed. The discovery of cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal, though these results have been questioned.
A trace fossil, also known as an ichnofossil, is a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization. The study of such trace fossils is ichnology and is the work of ichnologists.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
Cephalonega stepanovi is a fossil organism from Ediacaran deposits of the Arkhangelsk Region, Russia. It was described by Mikhail A. Fedonkin in 1976
The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.
Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization.
Trilobozoa is a phylum of extinct, sessile animals that were originally classified into the Cnidaria. The basic body plan of trilobozoans is often a tri-radial or radial sphere-shaped form with lobes radiating from its centre. Fossils of trilobozoans are restricted to marine strata of the Late Ediacaran period.
Praecambridium sigillum is an extinct organism that superficially resembles a segmented trilobite-like arthropod. It was originally described as being a trilobite-like arthropod, though the majority of experts now place it within the Proarticulata as a close relative of the much larger Yorgia. It is from the Late Ediacaran deposit of Ediacara Hills, Australia, about 555 million years ago. On average, P. sigillum had at least 5 pairs of segments, with each unit becoming progressively larger as they approach the cephalon-like head.
The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period.
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
The Cambrian explosion is an interval of time approximately 538.8 million years ago in the Cambrian period of the early Paleozoic when there was a sudden radiation of complex life, and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.
The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.
Ediacaran type preservation relates to the dominant preservational mode in the Ediacaran period, where Ediacaran organisms were preserved as casts on the surface of microbial mats.
Kimberichnus is an ichnofossil associated with the early bilaterian Kimberella. It is known mostly from shallow marine Ediacaran sediments, often occurring alongside its producer. Kimberichnus often occurs in Russia and South Australia, where it is most abundant in the shape of multiple arcuate sets of ridges with fan-shaped arrangements.
Temnoxa molluscula is a small creature approximately 8 mm wide found in the Ediacaran period in Russia. The Temnoxa has a resemblance to a vertically cut penny bun mushroom. Due to the lack of information regarding the fossils of this organism, researchers are unable to place Temnoxa molluscula into any known phylum. The genus was originally discovered by Russian paleontologist Andrey Yu. Ivantsov in 2004.
↑ Jenkins, R.J.F. (1992). "Functional and ecological aspects of Ediacaran assemblages". In Lipps, J.; Signor, P.W. (eds.). Origin and Early Evolution of the Metazoa. New York, NY: Springer. pp.131–176. ISBN0-306-44067-9.
↑ Gehling, J. G.; Runnegar, B. N.; Droser, M. L. (2014). "Scratch Traces of Large Ediacara Bilaterian Animals". Journal of Paleontology. 88 (2): 284–298. doi:10.1666/13-054. S2CID140559034.
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