Ichthyosaurs Linked to Crocodiles and Birds

The ancient fish-lizards called ichthyosaurs that swam the warm Mesozoic seas for more than 100 million years were sisters to present-day crocodiles, birds and snakes and to the dinosaurs, according to a new fossil analysis.

All had a common terrestrial ancestor, said Ryosuke Motani of UC Berkeley.

He analyzed two remarkably preserved fossil specimens of Utatsusaurus hataii from a slate quarry in Japan--one about 10 feet long and the other about eight feet long. He found the answer to their family histories in the bones at the back of the skull and the attachment of their hipbones to their ribs and spines.

The characteristics that Motani found put the ichthyosaurs squarely amid a group of advanced reptiles called diapsids and confirm that the water-loving creatures were descended from lizard-like animals that once roamed on land. At some point, however, they branched off from the crocodiles, dinosaurs, lizards and birds.

The entire group of ichthyosaurs lived from 240 million years ago to about 90 million years ago.

Utatsusaurus, the oldest of the group, had a dolphin-like snout with lots of teeth and swam with a tail fin like a tuna, although it was related to neither dolphins nor fish, said Motani, a postdoctoral fellow at the University of California Museum of Paleontology.

The work from Motani and colleagues at Hokkaido University in Japan appears in the May 21 issue of the journal Nature.