Oldest ichthyosaur fossil hints they evolved before mass extinction

The oldest fossils of an ichthyosaur ever found indicate that these fish-like reptiles evolved earlier than we thought – perhaps even before the world’s worst mass extinction, which hit 252 million years ago.

Ichthyosaurs were among the reptile lineages that proliferated in the wake of the end-Permian mass extinction, along with the dinosaurs. It was thought that they evolved from land-dwelling ancestors after the mass extinction.

The new fossils are 11 vertebrae and 15 bone fragments found in Spitsbergen, a Norwegian island in the Arctic. Benjamin Kear at Uppsala University in Sweden and his colleagues believe they belonged to an ichthyopterygian – a group of eel-like reptiles that lived in water and were ancestral to the shark-shaped ichthyosaurs.

The team carried out a series of analyses ranging from rock chemistry to microscopic bone structure. “The vertebrae turned out to be from a highly advanced, fast-growing, probably warm-blooded and fully oceanic ichthyosaur,” says Kear.

The fossils were encased in a rock layer that dates to about 2 million years after the end-Permian mass extinction, making them the earliest ichthyopterygian fossils known to date.

The fact that the animal from Spitsbergen was already aquatic suggests that the first amphibious ichthyosaur ancestors must be even older, says Kear.

This hints that these animals originated prior to the mass extinction, although more fossils will be needed to confirm whether ichthyopterigians really were swimming in the seas before the ecological disaster struck.

Neil Kelley at the Vanderbilt University in Nashville, Tennessee, who wasn’t involved in the new study, says the idea that ichthyosaur ancestors evolved in the Permian is reasonable, but it is also possible that this group rapidly evolved in the 2 million years after the mass extinction as life recovered. Previous research on giant Triassic ichthyosaurs indicate that these reptiles ballooned to monstrous proportions within about 2.5 million years, for example.

Until we find some Permian-aged fossils of ichthyosaurs or their close ancestors, it will be difficult to say when these aquatic reptiles took the plunge, says Kelley.