For 25 years, Andrew Smith, a palaeontologist at the Natural History Museum in London, has been troubled by a question of symmetry. It began while he was working on a PhD thesis about the modern history of echinoderms — marine organisms such as starfish that are defined by radial symmetry, much like the symmetry seen in the spokes of a bicycle wheel. During subsequent research, he came across a debate over the provenance of asymmetric organisms called Stylophora.

Palaeontologists have long considered these extinct organisms to be primitive echinoderms — even though they had only one mobile structure that could have been an ‘arm’, rather than the multiple arm-like structures seen in echinoderms today. Nevertheless two opposing views arose, which put the organisms at different points along the evolutionary tree.

One view held that Stylophora were primitive chordates — organisms characterized by a hollow nerve tube — but with an echinoderm skeleton; the other that they were primitive echinoderms that predated forms with radial symmetry. More recently, a third view emerged that the organisms weren't so primitive after all, but were highly evolved echinoderms.

“These arguments have been going for some time,” Smith says. “What was needed was some hard data in place of speculative reconstruction of soft tissues.”

Putting Stylophora into the right category depends largely on determining the use of the ‘arm’, as each camp has ascribed it different functions. But pinning down its use has been difficult, because this would mean analysing soft tissue, such as muscle, which isn't present in fossils. Smith's PhD had shown him that it is possible to reconstruct the nature of soft tissues lost during fossilization — provided you have samples of the skeleton that preserved their original microscopic structure.

But such samples had eluded Smith and the rest of the field. Then last October, Smith received an e-mail from Sébastien Clausen at the University of Science and Technology in Lille, France, who was doing field work in the Moroccan desert. Clausen had uncovered a large bed of sediment in which there were pieces of Stylophora that showed remarkable preservation.

The two scientists had never met — Clausen had found Smith's name by doing a literature search — but they quickly agreed to collaborate. Smith's anticipation was justified when he got to handle the specimens this March. “The preservation of the fine details of the plates was absolutely spectacular,” he says. “Just as good as modern specimens.”

The preservation was good enough for Smith to reconstruct the soft tissue. His results, shown on page 351 of this issue, put the Stylophora solidly in the ‘primitive echinoderm’ camp. They also offer some insights into how these asymmetrical organisms evolved into symmetrical creatures such as starfish and sea urchins. “What we get from studying these early forms of echinoderms is how they worked towards symmetry,” Smith says.

Although Smith thinks that the evidence he and Clausen have produced is solid, he is unsure if it will end the debate. “Are controversies ever put to rest?” he asks.