Discovery of the oldest bilaterian from the Ediacaran of South Australia

Abstract

Analysis of modern animals and Ediacaran trace fossils predicts that the oldest bilaterians were simple and small. Such organisms would be difficult to recognize in the fossil record, but should have been part of the Ediacara Biota, the earliest preserved macroscopic, complex animal communities. Here, we describeIkaria wariootiagen. et sp. nov. from the Ediacara Member, South Australia, a small, simple organism with anterior/posterior differentiation. We find that the size and morphology ofIkariamatch predictions for the progenitor of the trace fossilHelminthoidichnites—indicative of mobility and sediment displacement. In the Ediacara Member,Helminthoidichnitesoccurs stratigraphically below classic Ediacara body fossils. Together, these suggest thatIkariarepresents one of the oldest total group bilaterians identified from South Australia, with little deviation from the characters predicted for their last common ancestor. Further, these trace fossils persist into the Phanerozoic, providing a critical link between Ediacaran and Cambrian animals.

Intravital damage to the body ofDickinsonia(Metazoa of the late Ediacaran)

“…Several species of the genus Dickinsonia , along with the genus Yorgia , closely related to it, destroyed the upper layer of microbial mats, leaving deep and wide depressions arranged in chains (Ivantsov and Malakhovskaya, 2002; Gehling et al, 2005; Ivantsov, 2011; Evans et al, 2019a, b). The small organisms that left Helminthoidichnites traces apparently fed on the substance of buried mats and dead bodies of macroorganisms, including those belonging to such relatively highly organized creatures as Dickinsonia and Spriggina (Gehling and Droser, 2018; Evans et al, 2020). The unique bilateral animal Kimberella had sharp “teeth,” by which it left scratches on the microbial mat and tore out large pieces of it (Ivantsov, 2013; Gehling et al, 2014).…”

Developmental processes in Ediacara macrofossils

“…Mobility in Kimberella, Ikaria and Dickinsonia has been attributed to muscular activity [ 52 , 55 , 56 , 67 ]. Muscles are composite tissues consisting of multiple cell types [ 65 ].…”

“…Muscles are composite tissues consisting of multiple cell types [ 65 ]. Feeding in Kimberella [ 48 , 49 ] and Ikaria [ 52 , 53 ] suggests the presence of a mouth and gut, potentially a through-gut, although such structures are not preserved. It is unclear whether these represent true organs, but, if present, a gut likely required multiple tissue layers, including muscles for particle transport.…”

These bizarre ancient species are rewriting animal evolution

“…But earlier this year, scientists detailed what might be the oldest-known animal that had both. Called Ikaria wariootia, it lived at roughly the same time as the Dickinsonia specimens that Bobrovskiy's team studied, or perhaps earlier 7 .…”

“…Droser and her colleagues used the scanner to image hundreds of tiny blobs found near the twisting burrows. The team's high-resolution 3D reconstructions show that the blobs were, in fact, organisms 7 . They were smaller than grains of rice, but they had left-right symmetry and both a front and back end, and features of the burrows suggest that the creatures could control where they moved.…”