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Excavation Yields Tantalizing Hints of Earliest Marine Reptiles

Peking University, Dec. 3, 2010: “It's exciting; We are all hoping for more!” says Olivier Rieppel, a specialist on Mesozoic marine reptiles at The Field Museum in Chicago, Illinois.

 

What thrilled Rieppel was the latest finding of a research on the earliest marine reptiles, led by Prof. Jiang Dayong of PKU School of Earth and Space Sciences.

 

A special report was published on the Nov. 26 issue of Science, written by Richard Stone.

 

Eons ago, convulsions warped the land here on the edge of town, thrusting slabs of mudstone and limestone high into the sky. Buried in the rock are traces of a mass extinction that wiped out about 90% of oceanic life forms and brought the Paleozoic era to a shuddering close 251 million years ago — the end of the Permian period and beginning of the Triassic. When life rebounded in the early Triassic, a new kind of top predator arose: ichthyosaurs. Marine reptiles would dominate the oceans until another mass extinction ended the Cretaceous 65 million years ago.

 

The origins of marine reptiles are an enigma. Their ancestors came from the land, but scientists can only imagine what sort of animals ventured into the sea and evolved into the three successful lineages of marine reptiles: sleek tuna-shaped ichthyosaurs; sauropterygians, including the long-necked plesiosaurs; and mosasaurs, which gulped down prey with double-hinged jaws like those of snakes. “The invasion must have happened very fast, within 4 million or 5 million years after the end of the Permian,” says paleontologist Jiang Dayong of Peking University. But the fossil record of this ocean colonization is largely a blank slate. “We have very little material from the lower Triassic,” says Rieppel.

 

Jiang Dayong and Ryosuke Motanihope (right) to unearth spectacular marine reptiles before farmers do. (Richard Stone/Science)

 

Answers may be entombed at Majiashan quarry, in a 150-meter-thick outcropping dappled with shades of gray that spans 6 million years of geologic history. “We expect to find new species here,” including forms marking the land-sea transition, says Ryosuke Motani, a paleontologist at the University of California, Davis, who, with Jiang in September, launched the first systematic excavation at Majiashan. The dig's first fruits — including an ancestor of the plesiosaurs—are already generating a buzz.

 

Majiashan in east China's Anhui province is a fixture on the paleontological map. In 1972, railway workers here found an unusual fossil that two scientists at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing — Young Chung-chien and Dong Zhiming—identified as a new species of ichthyosaur, Chaohusaurus geishanensis. Since then, a couple dozen more Chaohusaurus skeletons from the early Triassic have streamed out of this site and nearby Guishan; most were unearthed by quarry workers and farmers, and most have ended up in private collections. Besides looking more like land reptiles than other ichthyosaurs, Chaohusaurus was about 70 centimeters long — much smaller than ichthyosaurs from the late Triassic and the Jurassic, which could top 15 meters in length.

 

Motani and Jiang have their eyes on a bigger prize: undiscovered pint-sized marine reptiles, presumed to be tens of centimeters long, with vestigial limbs, or other terrestrial features such as lungs and ears better adapted to life on shore. “We hope to find the oldest marine reptiles. Then we'll look for signs of their ancestors,” says Jiang. Worldwide, few outcroppings from the Permian-Triassic boundary, the beginning of the Mesozoic era, are as promising as Chaohu. “This is where the fossils have to be,” says Nick Fraser, a paleontologist at National Museums Scotland in Edinburgh. “But you still have to get lucky to find them.”

 

The first fossil fragments of a marine reptile—those of an ichthyosaur—were unearthed at the turn of the 18th century. The species drew scientific interest only a century later, in 1811, when a 12-year-old girl collected the skull and vertebrae of Temnodontosaurus platyodon on the Dorset coast in England. Mary Anning became one of the great fossil hunters of all time. Among her many other discoveries is the first fossil of a plesiosaur, Plesiosaurus dolichodeirus, which she found in 1821. The creature was so unlike living animals that it helped convince skeptics that species could go extinct—a new and not widely accepted concept at the time.

Since then, several dozen species of extinct marine reptiles have come to light, mostly members of the three dominant lineages. Ichthyosaurs, soon after arising in the early Triassic, morphed into a variety of body plans. It took about 20 million years, however, for plesiosaurs—four-flippered behemoths resembling overgrown sea lions—to have evolved from early sauropterygians. The ancestors of mosasaurs, which arose in the late Cretaceous, are entirely obscure, as is the kinship of a less-known fourth lineage of marine reptiles, the lizard-like thalattosaurs of the middle to late Triassic, which appear in the fossil record from 247 million to about 200 million years ago.

 

Researchers are beginning to get a handle on the physiology of these beasts. Recent findings suggest that ichthyosaurs and plesiosaurs—which cruised for food—were warm-blooded, allowing them to maintain constant muscle activity (Science, 11 June, p. 1379; p. 1361 depicts evolutionary relationships between Mesozoic marine reptiles). Mosasaurs were ambush predators more like crocodiles, their body temperature matching that of the water.

 

Plenty of questions about early Mesozoic marine species remain unanswered, and Anhui holds particular appeal as a possible source of information. During the Triassic, two large oceans intersected in this region: the Panthalassa, which covered half the globe, and the Tethys, which gave rise to the Indian Ocean and the Mediterranean Sea. Bivalves from both oceans have been found in Early Triassic sediments in Chaohu, Motani says.

 

On an exploratory trip to Majiashan in September 2008, Jiang tracked down farmers and quarry workers who had collected fossils from the site and was shown some intriguing specimens. “The farmers called them all Chaohusaurus, but it's clear that some are other kinds of reptiles,” Jiang says. The skeletal fragments were too big to be Chaohusaurus; in Jiang's estimation, they represent at least two new species.

 

Majiashan is famed for Chaohusaurus, an early diminutive ichthyosaur. (Courtesy of Jiang Dayong and Ryosuke Motani)

 

The researchers are racing to recover more of these treasures, but time is not on their side. Over the years, quarry workers at Majiashan have gouged deeply into the Permian-Triassic boundary formation. “We only have this much limestone,” says Motani, gesturing to an outcropping with spray-painted red splotches indicating layers that have been dated. “When it's gone, the early Triassic is gone.” But his team has a big advantage: The geological layers here are stacked like books on a shelf, which makes it possible to survey millions of years of rock without climbing high. And specimens can easily be carted out after excavation.

 

To protect the site, the Chaohu government has vowed to close the quarry and the nearby cement factory it supplies, says Jiang. And to cut down on illegal fossil sales, his team, working with Jiang Li'ai, director of Anhui Geological Museum in Hefei and president of the Paleontological Society of Anhui, plans to educate residents and local authorities about the fossils' scientific value.

 

Back in Beijing, Prof. Jiang Dayong has begun analyzing the specimens pried from the rock in September. They include the remains of a new kind of early ichthyosaur and skeletal fragments of a sauropterygian that was less than a meter long — a tiny ancestor of the mighty plesiosaurs.

 

 

 

Translated by: Chen Long

Edited by: Jacques

Source: PKU News (Chinese)