Five rays twisting down from the top of a fossil hint at how creatures such as starfish gained their unusual symmetry.

Credit: R. SOC.

Starfish, sea urchins and all other known living echinoderms have a symmetry that allows them to be sliced into five identical parts, but some of their counterparts in the Cambrian period, which began about 540 million years ago, were asymmetric or had bilateral symmetry.

Andrew Smith at the Natural History Museum in London and Samuel Zamora at the Smithsonian Institution in Washington DC discovered Cambrian fossils in Morocco that show what stages intermediate to the body plan of living echinoderms might have looked like.

Helicocystis moroccoensis (pictured) is the oldest known echinoderm with five-part symmetry; it resembled an egg with its tapered end planted in the sea floor. Its mouth opened upward and its body spiralled down.

Proc. R. Soc. B 280, 20131197 (2013)