Bilateral symmetry in the fossil record

Fossils of the bilaterian Ikaria wariootia found in South Australia represent one of the oldest examples of an organism with anterior and posterior differentiation, according to a study. During the Ediacaran period, 571-539 million years ago, the first organisms with bilateral symmetry and a gut that opens at each end appeared. Evidence for these early fossils is rare, with most studies relying on trace fossils, or the tracks they left behind, rather than preservation of the small, soft-bodied organisms. With the aid of 3D laser scanning, Scott Evans and colleagues searched for the progenitor of Helminthoidichnites, trace fossils that represent curved burrows with evidence of sand displacement. The authors identified 108 distinct tube-like organisms in former seafloor strata from the Ediacara at the National Heritage Nilpena site in South Australia. The fossils exhibited a consistent ratio of length to width and matched predictions for the type of organism responsible for the fossilized burrows: appropriate diameter, existence of head and tail ends, and modular body divisions. According to the authors, based on its position in the strata, I. wariootia may represent the oldest example of bilateral symmetry in South Australia and provide a critical link between Ediacaran and Cambrian animals.

Article #20-01045: ” Discovery of the oldest bilaterian from the Ediacaran of South Australia,” by Scott D. Evans, Ian V. Hughes, James G. Gehling, and Mary L. Droser.

MEDIA CONTACT: Scott D. Evans, National Museum of Natural History, Washington, DC; e-mail:

[email protected]

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