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"Fish-Lizards" Turn Out to Be More Lizard than Fish

Recent Fossil Ichthyosaur Find Establishes Evolutionary Origin of These Dolphin-like Creatures

by Robert Sanders, Public Affairs
posted July 15, 1998

The origin of ichthyosaur, a creature that swam the warm Mesozoic seas millions of years ago, has puzzled paleontologists for more than a century. When the first skull was found in England in 1814, scientists speculated that this sea dweller with the long toothy snout, to which they gave a name meaning fish-lizard, might be related to the crocodile. Subsequent fossils showed more dolphin-like characteristics. Now the dilemma is solved, report Berkeley researchers and their Japanese colleagues in the May 21 issue of Nature.

Based on analysis of the earliest complete ichthyosaur fossil found to date, Berkeley postdoctoral fellow Ryosuke Motani and researchers at Hokkaido University in Japan find that ichthyosaur sits squarely within the diapsids or advanced reptiles, making them distant relatives of crocodiles, birds, lizards and snakes. But ichthyosaurs separated from the other diapsids before the evolution of reptiles such as snakes and lizards.

"Some people thought ich-thyosaurs were close to the common ancestor of the crocodiles and dinosaurs," said Motani, principal author of the report. "Our analysis shows that they branched off long before that, so they are not included within the Sauria with lizards, crocodiles, birds and dinosaurs."

Motani's analysis of ichthyosaur fossils included a 240 million-year-old fossil of a species called Utatsusaurus hataii,. found in 1982 in a slate quarry near Ogatsu, Japan, by Motani's co-author on the Nature paper, Nachio Minoura.

Until recently, the fossils were difficult to interpret because they had been distorted by shearing of the rock over millions of years. Motani developed a computerized technique to undistort the fossils.

After undistorting the skull, Motani added it to data already accumulated about other known ichthyosaur fossils and after re-analysis concluded that ichthy-

osaurs are diapsids, as many paleontologists had thought, though outside the group that includes all living reptiles. Nevertheless, he said, they are closer to living reptiles than are turtles.

"I had the cast of the skull in my office for three years before I realized what was going on, before I understood that I was looking at the orbit (eye) and where the medial axis of the skull was," Motani said.

As one of the most primitive known ichthyosaurs, Utatsusaurus exhibits features midway between the terrestrial animals from which it arose and the more evolved ichthyosaurs, such as those from Germany and England dating to the Early Jurassic period some 180 million years ago, Motani said.

Ichthyosaurs were difficult to categorize because they became so well adapted to their marine environment that they developed many features similar to marine organisms such as fish and dolphins. This obscured their real origin.

"When you find really primitive members of a group, they retain more of the group's general characteristics and thus tell us more about where they came from," said Motani's postdoctoral sponsor Kevin Padian, Berkeley professor of integrative biology and curator of reptiles at the Museum of Paleontology.

Motani's method of removing the distortion from fossils long buried in moving rock could have broad application in paleontology. Motani built on a method that has been used before in undistorting invertebrate fossils such as trilobites, but his method works with vertebrates, too.

"Motani did some clever things with undeforming that could be used with many fossil specimens," Padian said. "Once people find out how useful this is it will be picked up widely."

The work was supported by the Fujiwara Natural History Foundation in Tokyo, Berkeley's Miller Institute for Basic Research in Science and the Fukada Geological Institute.

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