''Ikaria wariootia'' is an early example of a wormlike, bilaterian organism. Its fossils are found in rocks of the Ediacara Member 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 ...

that are estimated to be between 560 and 555 million years old. A representative of the Ediacaran biota, ''Ikaria'' lived during the

Ediacaran period 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 ...

, roughly 15 million years before the Cambrian, when the Cambrian explosion occurred and where widespread fossil evidence of modern bilaterian taxa appear in the fossil record.

Discovery

Scott D. Evans, Ian V. Hughes, James G. Gehling, and Mary L. Droser published a paper in the ''

Proceedings of the National Academy of Sciences of the United States of America ''Proceedings of the National Academy of Sciences of the United States of America'' (often abbreviated ''PNAS'' or ''PNAS USA'') is a peer-reviewed multidisciplinary scientific journal. It is the official journal of the National Academy of Sc ...

'' on 23 March 2020, describing the finding and identification of ''I. wariootia''.

Age

The age of Ediacara Member strata are not well-defined through radiometric dating, and are primarily estimated comparatively with other Ediacaran Biota assemblages, likely ranging between approximately 562 Ma and 542 Ma. Brazilian trace fossils associated with later bilaterians, found 30-40m above a bed radiometrically dated to 555 Ma, are thought to be younger than ''Ikaria''. It is possible that ''Ikaria'' evolved prior to 560 Ma.

Etymology

The generic name is taken from the

Adnyamathanha The Adnyamathanha (Pronounced: ) are a contemporary Aboriginal Australian people of the northern Flinders Ranges, South Australia, formed as an aggregate of several distinct peoples. Strictly speaking the ethnonym Adnyamathanha was an alternativ ...

word for "meeting place" (, also the name for nearby

Wilpena Pound Wilpena Pound – also known by its Adnyamathanha language, Adnyamathanha name of Ikara, meaning "meeting place" – is a natural amphitheatre of mountains located north of Adelaide, South Australia, Australia in the heart of the Ikara-Flinders ...

) in recognition of the local indigenous people who originally lived in the region where the fossils were collected. The specific name refers to Warioota Creek, the type locality.

Features

Over 100 ''Ikaria'' fossils have been found. These are simple imprints resembling a small grain of

rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...

(from 1.9 to 6.7 mm in length), slightly thickening to one end. The "anterior"/"posterior" differentiation may indicate that ''Ikaria'' was a bilaterally symmetrical animal. No other details of ''Ikaria'' anatomy were found on its fossils. On the same sandstone bed there are numerous

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, ...

s of the type '' Helminthoidichnites''. The animal that produced such traces moved or burrowed through thin layers of well-oxygenated

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

on the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

as it sought sustenance and appeared to show sensory and seeking behaviour, turning as it moved. It is thought to have moved by

peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...

, constricting muscles against the animal's

hydrostatic skeleton A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refe ...

, and may have possessed a coelom, mouth, anus, and through- gut, in a similar way to a worm. The authors of the ''Ikaria'' description find that the size and

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

of ''Ikaria'' match predictions for the producer of the trace fossil ''Helminthoidichnites''. At least one of the fossils of ''Ikaria'' identified in the study was found in close proximity to ''Helminthoidichnites'', which the discoverers attribute to vertical motion of the organism through sediment before its death - noting that due to differing preservation methods it is unlikely that both trace and body fossil could otherwise form simultaneously. However, this does not entirely remove the possibility that the association of ''Ikaria'' with ''Helminthoidichnites'' is erroneous.

Significance

This discovery is notable because while it has been long suspected that bilaterians evolved in the Ediacaran, for example '' Temnoxa'' and ''

Kimberella ''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, although its affinity with t ...

'', yet the vast majority of Ediacaran biota fossils are very different from the animals that came to dominate the life on Earth in the Cambrian and until present day.

References

External links

{{Taxonbar, from=Q88471230 Nephrozoa Ediacaran life Ediacaran first appearances Fossils of Australia Fossil taxa described in 2020