Credit: JOHN CUNNINGHAM

Internal structures of embryonic jellyfish-like organisms have been found in limestone that formed more than 530 million years ago.

Basic animal body plans were established over half a billion years ago, and their origins are often murky. Differing hypotheses have tied one relatively common specimen, Olivooides, to three disparate phyla, of which penis worms, sea stars and jellyfish are modern examples. A team led by Philip Donoghue at the University of Bristol, UK, found three specimens in which the internal anatomy (pictured) of late-stage Olivooides embryos had been preserved, revealing details that place it with Cnidaria, which includes jellyfish.

The authors argue that assigning ancient organisms to the correct group is crucial to our understanding of how changes in embryonic development led to the formation of current animal phyla.

Proc. R. Soc. B 280, 20130071 (2013)