Ikaria 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 word for "meeting place" (Ikara, also the name for nearby Wilpena 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.

Features

Over 100 Ikaria fossils have been found. These are simple imprints resembling a small grain of rice (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 fossils of the type Helminthoidichnites. The animal that produced such traces moved or burrowed through thin layers of well-oxygenated sand on the ocean floor as it sought sustenance and appeared to show sensory and seeking behaviour, turning as it moved. It is thought to have moved by peristalsis, constricting muscles against the animal's hydrostatic skeleton, 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 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, 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.