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. 2020 Apr 7;117(14):7845-7850.
doi: 10.1073/pnas.2001045117. Epub 2020 Mar 23.

Discovery of the oldest bilaterian from the Ediacaran of South Australia

Scott D Evans  1 , Ian V Hughes  2 , James G Gehling  3 , Mary L Droser  4
Affiliations

Discovery of the oldest bilaterian from the Ediacaran of South Australia

Scott D Evans et al. Proc Natl Acad Sci U S A. .

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 describe Ikaria wariootia gen. et sp. nov. from the Ediacara Member, South Australia, a small, simple organism with anterior/posterior differentiation. We find that the size and morphology of Ikaria match predictions for the progenitor of the trace fossil Helminthoidichnites-indicative of mobility and sediment displacement. In the Ediacara Member, Helminthoidichnites occurs stratigraphically below classic Ediacara body fossils. Together, these suggest that Ikaria represents 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.

Keywords: Ediacara Biota; Ediacaran; bilaterian; phylogenetics; trace fossil.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Type specimen of I. wariootia from Nilpena, including (A) photograph; and (B–D) 3D laser scans. Notice distinct bilateral symmetry (wider end identified by white star in C and deeper end by black star in D). P57685. (Scale bars, 1 mm.)
Fig. 2.
Fig. 2.
Photographs (A and B) and 3D laser scans (C–J) of I. wariootia. (A) Specimen (white arrow) associated with Helminthoidchnites. (B–E) Associated specimens; black boxes in B and C are the same specimen shown in close up in negative hyporelief (D) and inverted (E). (F and J) Bent specimens. (G and H) N bedding plane (G) and profile (H) of the same specimen. (I) Profile demonstrating variable relief. Notice correlation between broader, wider end (white stars) in the bedding-pane view and more significant relief end (black stars) in the profile. (A) P57686. (B–E) 1T-A 001 to 003. (F) 1T-A 004. (G and H) 1T-A 005. (I) 1T-A 006. (J) 1T-A 007. (Scale bars, 1 mm.)
Fig. 3.
Fig. 3.
Reconstruction of Ikaria in life position forming a Helminthoidichnites-type trail.
See this image and copyright information in PMC

References

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