CREDIT Klaus Rassinger, Katie Ahlfeld, Jon Zander, Espen Rekdal

A 500-million-year-old fossil from Morocco, discovered by Natural History Museum scientists, is offering extraordinary new insights into one of evolution’s most puzzling transformations: how echinoderms, the group that includes starfish, sea cucumbers and sea urchins, evolved from ancestors which showed bilateral symmetry, like humans, to the unique fivefold symmetry we see today.

The fossil, Atlascystis acantha, is the oldest known echinoderm with a bilateral body plan. It bridges the evolutionary gap between the closest living relatives of echinoderms, all of which have bilateral symmetry, and the familiar pentaradial (five-armed) forms alive today.

“This fossil evidence allows us to piece together how the body plans of starfish and their relatives evolved step-by-step from ancestors that were much more similar in shape to other animals,” said Dr Imran Rahman, Principal Researcher at the Museum and co-lead author of the study.

Unearthed in the Anti-Atlas mountains of Morocco, Atlascystis has a flattened, spine-covered body with bilateral symmetry and a pair of specialised sets of skeletal plates similar to those used by living echinoderms to move and feed, known as ambulacra. These features are characteristic of modern echinoderms, yet until now their presence in early bilaterally symmetrical forms was unclear.

Rewriting Evolutionary History

Using 3D imaging, growth analyses and cutting-edge phylogenetic methods, the research team reconstructed the evolutionary trajectory of echinoderms. The study challenges previous suggestions that early bilateral echinoderms were simply offshoots of more derived forms. Instead, the researchers show these organisms lie at the base of the echinoderm evolutionary tree, and that five-rayed forms likely evolved later through the duplication of a single ambulacrum, enabled by the loss of a defined trunk region.

Dr Frances Dunn, senior researcher at the Oxford University Museum of Natural History said, “The fossil record remains our only direct insight into the evolutionary history of groups through time, and this discovery shines a light on the first steps of the evolution of one of the most recognisable body plans we find in animals today: the starfish.”

Dr Jeff Thompson, lecturer at the University of Southampton and co-author on the paper said, "We were able to determine how this animal grew when it was alive, which was the key to understanding its place in the tree of life."

The discovery reinforces the importance of the fossil record in solving evolutionary mysteries that genetics alone cannot untangle. With Atlascystis acantha, scientists now have a powerful new window into how major animal groups like echinoderms assembled their body plans in deep time.

The paper ‘A new Cambrian stem-group echinoderm reveals the evolution of the anteroposterior axis’ is available now in Current Biology.

This research is part of the Evolution of Life research theme that seeks to reveal the causes and consequences of evolutionary and environmental change which is central to understanding life on Earth.

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