Oldest ichthyosaur fossils hint that they evolved before mass extinction

The oldest ichthyosaur fossils ever found indicate that these fish-like reptiles evolved earlier than we thought – perhaps even before the mass extinction at the end of the Permian.

The ichthyosaurs were thought to be one of the lineages of reptiles that bred after the world’s worst biological disaster 252 million years ago, along with the dinosaurs.

The new fossils are 11 vertebrae and 15 bone fragments found on Svalbard, a Norwegian island in the Arctic. Benjamin Kier of Uppsala University in Sweden and colleagues believe they belonged to the Ichthyopterygians, a group of eel-like reptiles that lived in the water and were the ancestors of the shark-like ichthyosaurs.

The team ran a series of analyzes ranging from rock chemistry to microscopic bone structure. “The vertebrae came from a highly advanced, fast growing, probably warm-blooded and completely oceanic ichthyosaur,” Cyrus says.

The fossils were encased in a rock layer that dates back to about 2 million years after the mass extinction at the end of the Permian period. This makes them the earliest ichthyopterygian fossils known to date and hints that these animals predate the mass extinction.

Given that ichthyosaurs evolved from terrestrial ancestors, the fact that the Svalbard animal was already aquatic hints that the very first amphibian ancestors of ichthyosaurs must be even older, Kier says.

But more fossils will be needed to confirm whether Ichthyopterygians were indeed swimming in the seas before the mass extinction occurred.

Neil Kelly of Vanderbilt University in Nashville, Tennessee, who was not involved in the new study, says the idea that ichthyosaur ancestors evolved in the Permian is reasonable, but it’s also possible that the group evolved rapidly within 2 million years of the mass extinction as life is restored. Previous studies of giant Triassic ichthyosaurs show that these reptiles swelled to monstrous sizes, for example, in about 2.5 million years.

“Until we find fossils of ichthyosaurs or their close Permian ancestors, it will be difficult to say when these aquatic reptiles took the plunge,” Kelly says.