How the shark forgot his skeleton

VERTEBRATES—THOSE animals with a backbone—are a diverse bunch, encompassing everything from tuna and budgerigars to snakes, chinchillas and human beings. One way biologists divide them up concerns the composition of their skeletons. Most vertebrates sport hard, calcified bones, and are dubbed the osteichthyans. A second, much smaller category is the chondrichthyans, whose members include sharks, rays and skate. Unlike their hard-boned cousins, chondrichthyans make do with structural parts made of soft, tough cartilage.

Palaeontologists had long assumed that cartilage was the more primitive arrangement. Osteichthyan fetuses, after all, begin life with a cartilaginous skeleton that is gradually replaced by harder, more durable bone as they grow. But a paper just published in Nature Ecology & Evolution suggests that view may be mistaken. In it a team led by Martin Brazeau of Imperial College London describe the discovery of a 410m-year-old fossil in Turgen, a district of Mongolia close to the Russian border. The fossil is the partial skull of a new species of placoderm, a type of armoured fish, which Dr Brazeau and his colleagues have dubbed Minjinia turgenensis.

Placoderms are of interest to palaeontologists because they are an immediate common ancestor of both the...