| Ikaria wariootia | |
|---|---|
 | |
| Artist's restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Clade: | Bilateria |
| Genus: | †Ikaria Evans et al, 2020 |
| Species: | †I. wariootia |
| Binomial name | |
| †Ikaria wariootia Evans et al, 2020 | |
Ikaria wariootia is an early example of a wormlike, 2–7 mm-long (0.1–0.3 in)bilaterian organism. Its fossils are found in rocks of the Ediacara Member ofSouth Australia that are estimated to be between 560 and 555 million years old.[1] A representative of theEdiacaran biota,Ikaria lived during theEdiacaran period, roughly 15 million years before theCambrian, when theCambrian explosion occurred and where widespread fossil evidence of modern bilaterian taxa appear in the fossil record.[1][2][3]
Scott D. Evans, Ian V. Hughes, James G. Gehling, andMary L. Droser published a paper in theProceedings of the National Academy of Sciences of the United States of America on 23 March 2020, describing the finding and identification ofI. wariootia.[1]
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.[4] Brazilian trace fossils associated with later bilaterians, found 30-40m above a bed radiometrically dated to 555 Ma, are thought to be younger thanIkaria.[1] It remains possible thatIkaria evolved prior to 560 Ma.[1]
The generic name is taken from theAdnyamathanha word for "meeting place" (Ikara, also the name for nearbyWilpena Pound) 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.[1]
Over 100Ikaria fossils have been found.[2] These are simple imprints resembling a small grain ofrice (from 1.9 to 6.7 mm in length), slightly thickening to one end. The "anterior"/"posterior" differentiation may indicate thatIkaria was abilaterally symmetrical animal. No other details ofIkaria anatomy were found on its fossils.[1]
On the same sandstone bed there are numeroustrace fossils of the typeHelminthoidichnites. The animal that produced such traces moved or burrowed through thin layers of well-oxygenatedsand on theocean floor[3] as it soughtsustenance and appeared to show sensory and seeking behaviour, turning as it moved. It is thought to have moved byperistalsis, constricting muscles against the animal'shydrostatic skeleton, and may have possessed acoelom, mouth, anus, and through-gut, in a similar way to a worm.[1]
The authors of theIkaria description find that the size andmorphology ofIkaria match predictions for the producer of the trace fossilHelminthoidichnites.[2][3][1] At least one of the fossils ofIkaria identified in the study was found in close proximity toHelminthoidichnites, 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.[1] However, this does not entirely remove the possibility that the association ofIkaria withHelminthoidichnites is erroneous.
This discovery is notable because while it has been long suspected that bilaterians evolved in the Ediacaran, for exampleTemnoxa andKimberella, yet the vast majority ofEdiacaran biota fossils are very different from the animals that came to dominate the life on Earth in the Cambrian and until present day.