NEWS RELEASE, 05/20/98
UC Berkeley study establishes that extinct ichthyosaurs
are related more closely to other living reptiles than are turtles
By Robert Sanders, Public Affairs
BERKELEY -- A scientist at the University of California, Berkeley, has solved a problem that has puzzled paleontologists for more than 180 years - the origin of the dolphin-like ichthyosaurs that swam the warm Mesozoic seas as early as 250 million years ago.
The report by UC Berkeley postdoctoral fellow Ryosuke Motani and colleagues at Hokkaido University in Japan appears in the May 21 issue of the British journal Nature.
When the first ichthyosaur skull with its long toothy snout was found in England in 1814, naturalists thought it most likely was a relative of the crocodile.
Following subsequent finds in Germany, mostly of more recent 180 million-year-old ichthyosaurs, scientists noted their resemblance to dolphins but recognized that they were reptiles. Nevertheless, some people thought ichthyosaurs were related to early ancestors of mammals, though not to mammals themselves, while others even thought ichthyosaurs might have been amphibians.
And while one recent analysis of ichthyosaur fossils suggested that they were closely related to turtles, another placed them near the long-necked fish eaters called plesiosaurs, which also swam in the Mesozoic oceans.
The new study, based on analysis of the earliest complete ichthyosaur fossil found to date, places the group squarely within the diapsids or advanced reptiles, making them distant relatives of crocodiles, birds, lizards and snakes. But ichthyosaurs broke off from the other diapsids before the evolution of reptiles such as snakes and lizards.
"Some people thought ichthyosaurs 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 the most complete primitive ichthyosaur skeleton ever found, a 240 million-year-old fossil of a species called Utatsusaurus hataii. Three fossil skeletons of the species were found in 1982 in a slate quarry near Ogatsu, Miyagi Prefecture, Japan, the source of some of Japan's best and most expensive slate. The slate is used for ink palettes in the traditional art of calligraphy.
Co-author Nachio Minoura of the Department of Earth and Planetary Science at Hokkaido University, Sapporo, Japan, excavated the fossils and has spent the past 15 years cleaning them and making casts.
Until recently, however, the fossils - which included two nearly complete skeletons, one 2.5 meters and the other 3 meters long - 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, using digitized data on the entire skeleton obtained by Minoura's colleague Tatsuro Ando at Hokkaislide with other ichthyosaurs possible.
"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.
After undistorting the skull, Motani added it to data already accumulated about other known ichthyosaur fossils and after re-analysis concluded that ichthyosaurs 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.
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 known from Germany and England dating from the Early Jurassic period, some 180 million years ago, Motani said.
What has made it difficult to assign ichthyosaurs to their proper evolutionary spot is that they became so well adapted to their marine environment that they developed many of the same physical features as other marine organisms, including fish and dolphins. This obscured their real origin.
Then along came the nearly complete Utatsusaurus fossils, which had detailed skeletal features for nearly the entire skeleton, including previously unknown parts of the skull and pelvic girdle.
After careful study of the fossil, in particular the skull's eye holes and a hole at the temple, Motani was able to place the species relative to the first diapsid known, the land-dwelling Petrolacosaurus and more recent diapsids, like birds.
"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, professor of integrative biology at UC Berkeley and curator of reptiles at the Museum of Paleontology.
"This reinforces the importance of early fossils" in determining evolutionary relationships, Motani said.
A comparative anatomist as well as a paleontologist, Motani is trying to determine how animals adapt to a marine environment, which is why he is looking at "how diapsids evolved after first stepping into the water," he said.
One reason he came to UC Berkeley was its large collection of Middle to Late Triassic ichthyosaur fossils in the Museum of Paleontology, most dug up in California and Nevada by Berkeley paleontologist John C. Merriam between 1890 and 1910. Another UC Berkeley paleontologist, Charles Camp, found the largest known ichthyosaur, which is more than 15 meters long, in Nevada in the 1950s.
In a 1996 Nature paper, published while Motani was still at the University of Toronto, he showed how swimming behavior changed as ichthyosaurs evolved from the Early Triassic, the era of Utatsusaurus 240 million years ago, to the Late Cretaceous when they died out, about 90 to 100 million years ago.
Motani found that the earliest ichthyosaurs, including Utatsusaurus and the contemporaneous Chensaurus chaoxianensis, the oldest known ichthyosaur, swam with the undulating motion of eels. The later ones swam more like tuna, swishing their tails back and forth.
Motani concluded that the early ichthyosaurs are an evolutionary intermediate between the shorter-bodied land creatures from which they evolved - the first diapsids such as Petrolacosaurus - and the more advanced ichthyosaurs that swam like tuna.
Motani's method of removing the distortion from fossils long buried in moving rock could have application broadly 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, UC Berkeley's Miller Institute for Basic Research in Science and
the Fukada Geological Institute.
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