''Kimberella'' is an extinct
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
bilaterian
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
known only from rocks of the
Ediacaran
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the
gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
s,
[ although its affinity with this group is contentious.
Specimens were first found in Australia's ]Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide. The hills are known for being the location of significant fossils, and have given their nam ...
, but recent research has concentrated on the numerous finds near the White Sea
The White Sea (russian: Белое море, ''Béloye móre''; Karelian and fi, Vienanmeri, lit. Dvina Sea; yrk, Сэрако ямʼ, ''Serako yam'') is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is su ...
in Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
, which cover an interval of time from . 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
Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their box-like (i.e. cube-shaped) body. Some species of box jellyfish produce potent venom delivered by contact with their tentacles. Stings from some species, including '' ...
, 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
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
. 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 the scientific understanding of the Cambrian explosion
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
; if it was a mollusc, or at least a protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...
, this would mean that the two dominant Nephrozoan lineages would have diverged significantly before , and if it was at least bilaterian, its age would indicate that animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s were diversifying well before the start of the Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
.
Etymology
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'', however, the name ''Kimberia'' was already in use as a subgenus of ''Turritella
''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www.m ...
'' (Gastropoda), according to Dr. N. H. Ludbrook; and a new genus, ''Kimberella'', was proposed by Mary Wade in 1972.
Occurrence
''Kimberella'' has been found in the Ediacara Hills
Ediacara Hills are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide. The hills are known for being the location of significant fossils, and have given their nam ...
of South Australia
South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
, in the Ust’ Pinega Formation in the White Sea
The White Sea (russian: Белое море, ''Béloye móre''; Karelian and fi, Vienanmeri, lit. Dvina Sea; yrk, Сэрако ямʼ, ''Serako yam'') is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is su ...
region of Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
and the Kushk Series of central Iran. The White Sea fossils are often associated with the Ediacaran "animals" ''Tribrachidium
''Tribrachidium heraldicum'' is a tri-radially symmetric fossil animal that lived in the late Ediacaran (Vendian) seas. In life, it was hemispherical in form. ''T. heraldicum'' is the best known member of the extinct group Trilobozoa.
Etymol ...
'' and '' Dickinsonia,'' meandering trace fossil
A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...
trails, possibly made by ''Kimberella''; and algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
. Beds in the White Sea succession have been dated to and by radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
, using uranium-lead ratios in zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
s found in volcanic ash layers that are sandwiched between layers that contain ''Kimberella'' fossils. ''Kimberella'' fossils are also known from beds both older and younger than this precisely dated range.[ The fossils from the Ediacara Hills have not been dated precisely.
]
Description
Over 1000 specimens, representing organisms of all stages of maturity, have now been found in the White Sea
The White Sea (russian: Белое море, ''Béloye móre''; Karelian and fi, Vienanmeri, lit. Dvina Sea; yrk, Сэрако ямʼ, ''Serako yam'') is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is su ...
area at the bottom of fine-grained sandstone layers.[ 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.][
]
All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction, perhaps by as much as a factor of two. Like many other specimens found in the White Sea, the most common type of symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...
observed appears to be bilateral; with little to no sign of any of the kinds of radial symmetry found in Cnidarians
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
, the group that includes jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
, sea anemone
Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...
s 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.[
''Kimberella'' had a ]dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal c ...
covering 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; with a minimum length of 2–3 mm.[
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.][ The deformation observed in elongated and folded specimens illustrates that the shell was highly malleable; perhaps, rather than a single ]integument
In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, shell, germ or rind.
Etymology
The term is derived from ''integumentum'', which is Latin for "a covering". In a transferred, or ...
, it consisted of an aggregation of mineralised sclerites.[ At the 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 points of muscle attachment.][ Similar stripes around the edge of the shell may have been connected to muscles that retracted the 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
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
". 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.[
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
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
. 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
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
organisms and microbial mat
A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts ...
s.[ Assemblages bearing ''Kimberella'' often also bear fossils of '']Andiva
''Andiva ivantsovi'' is a Vendian fossil, identified to be a bilaterian triploblastic animal in the Ediacaran phylum Proarticulata, known from the Winter Coast, White Sea, Russia. It was first discovered in 1977, and described as a new species ...
'', ''Yorgia
''Yorgia waggoneri'' is a discoid Ediacaran organism. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of . It is classified within the extinct animal phylum Proarti ...
'', '' Dickinsonia'', ''Tribrachidium
''Tribrachidium heraldicum'' is a tri-radially symmetric fossil animal that lived in the late Ediacaran (Vendian) seas. In life, it was hemispherical in form. ''T. heraldicum'' is the best known member of the extinct group Trilobozoa.
Etymol ...
'' and ''Charniodiscus
''Charniodiscus'' is an Ediacaran fossil that in life was probably a stationary filter feeder that lived anchored to a sandy sea bed. The organism had a holdfast, stalk and frond. The holdfast was bulbous shaped, and the stalk was flexible. Th ...
'', suggesting that it lived alongside these organisms.[
''Kimberella'' probably grazed on microbial mats, but a selective predatory habit cannot be ruled out.][ Fedonkin reckons that as it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity.][ Conversely, Gehling et al. claim that it moved 'forwards'. 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".][ 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.]
The lack of evidence of asexual reproduction suggests that the organisms reproduced sexually. Budding or fission has never been observed.[
The waters in which ''Kimberella'' dwelt were occasionally disturbed by sandy currents, caused when sediments were whipped up by storms or ]meltwater
Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...
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
''Kimberella'' fossils are generally preserved on top of clay-rich beds and beneath sandy beds.[ 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.][
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.][ It has been suggested that a ]mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
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.]
Classification
All the ''Kimberella'' fossils found so far are assigned to one species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, ''K. quadrata''. The first specimens were discovered in Australia in 1959. They were originally classified as jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
by Martin Glaessner
Martin Fritz Glaessner AM (25 December 1906 – 23 November 1989) was a geologist and palaeontologist. Born and educated in the Austro-Hungarian Empire, he spent the majority of his life in working for geoscientific institutes in Austria, Russia ...
and Mary Wade in 1966, and then as box jellyfish
Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their box-like (i.e. cube-shaped) body. Some species of box jellyfish produce potent venom delivered by contact with their tentacles. Stings from some species, including '' ...
by Wade in 1972, a view that 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
Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organo ...
bilaterian organism — not a jellyfish at all.
So far ''Kimberella'' fossils show no sign of a radula, the toothed chitinous
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
"tongue" that is the diagnostic feature of modern molluscs
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
, 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 mat
A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts ...
s. These trace
Trace may refer to:
Arts and entertainment Music
* ''Trace'' (Son Volt album), 1995
* ''Trace'' (Died Pretty album), 1993
* Trace (band), a Dutch progressive rock band
* ''The Trace'' (album)
Other uses in arts and entertainment
* ''Trace'' ...
s, named ''Radulichnus
''Radulichnus'' is an ichnogenus of trace fossil which resembles the marks produced by the action of a mollusc's radula on sediment. As an ichnogenus, its classification is based solely on ''appearance'', and does not necessarily imply anythin ...
'' and ''Kimberichnus
''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 Australi ...
'', 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. In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis
A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...
with hook-like organs at its end. ''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. 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. The direction of grazing is also backwards, as opposed to forwards as in molluscs. Furthermore, the constant width of grooves implies stereoglossy – a trait that is very derived in molluscs. 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.[ Butterfield points out that plenty of other groups of organisms bear structures capable of making similar marks.]
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,[ or even just a "probable bilaterian".]
Theoretical importance
The Cambrian explosion
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
is an apparently rapid increase in the variety of basic body structures of animals in the Early Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period, starting after and finishing before . A few of the Early Cambrian fossils were already known in the mid-19th century, and Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
saw the apparently sudden appearance and diversification
Diversification may refer to:
Biology and agriculture
* Genetic divergence, emergence of subpopulations that have accumulated independent genetic changes
* Agricultural diversification involves the re-allocation of some of a farm's resources to ...
of animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s as one of the main objections that could be made against his theory of evolution by natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
.
The majority of animals more complex than jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
and other cnidarians are split into two groups, the protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...
s 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 , and hence well before the start of the Cambrian . Even if it is not a protostome, it is widely accepted as a member of the more inclusive bilaterian
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
clade. Since fossils of rather modern-looking cnidarians have been found in the Doushantuo lagerstätte
A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...
, the cnidarian and bilaterian lineages would have diverged well over .
See also
* List of Ediacaran genera
This is a list of all described Ediacaran genera, including the Ediacaran biota. It contains 227 genera.
References
{{reflist, 30em
*
Ediacaran
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end ...
References
External links
image from UCMP
{{Taxonbar, from=Q135367
Enigmatic prehistoric animal genera
Ediacaran life
White Sea fossils
Fossil taxa described in 1972
Ediacaran