Most brittle stars have round bodies with long, flexible arms. But the brittle star genus Astrophiura has arms that are fused together, similar to the appearance of certain sea stars. Astrophiura is thought to have evolved around 175–200 million years ago, but scientists based this estimate on data from modern-day animals because not a single fossil of it had ever been found—until now. According to a study published November 13 in PeerJ, researchers have discovered the first fossil from this genus, along with another brittle star fossil, that suggest this pentagon appearance evolved in separate lineages of these animals more than once.
A team led by Ben Thuy, a paleontologist at the Luxembourg National Museum of Natural History, excavated brittle star fossils in Germany and the United Kingdom. They found lateral arm plates, spiny bones on brittle star skeletons. By comparing the appearance of the fossils to that of modern-day Astrophiura species, the researchers determined that the ancient specimens were from an animal in the same genus and named it A. markbeneckei. This extinct species is from the late Cretaceous Period, 100–66 million years ago.
The team also found fossilized lateral arm plates that more closely resemble those of a different brittle star family, and named the species Astrosombra rammsteinensis. Although this genus evolved separately from Astrophiura, it also has fused arms, showing that this type of brittle star appearance evolved multiple times independently.
B. Thuy et al., “Brittle stars looking like starfish: the first fossil record of the Astrophiuridae and a remarkable case of convergent evolution,” PeerJ, doi:10.7717/peerj.8008, 2019.
Emily Makowski is an intern at The Scientist. Email her at email@example.com.