In Crocodile Evolution, the Bite Came Before the Body

John Roach
for National Geographic News
August 25, 2004
Today the crush of a crocodile's mighty jaws signals lights-out for many a fish or other water-loving animal. But according to a new study, the croc's characteristic jaws evolved on dry land—and long before its swim-tuned body.

The finding stems from the discovery of a well-preserved fossil of an ancestor of crocodilians in northwestern China. A crocodilian is any member of an order of reptiles that includes crocodiles, alligators, caimans, gavials, and related extinct forms.

The discovery will be reported in tomorrow's issue of the journal Nature.

The creature, named Junggarsuchus sloani, was a three-foot-long [one-meter-long] sphenosuchian—one of a class of small, slender, land-dwelling crocodilians that lived from about 230 million to 150 million years ago.

Junggarsuchus's skull shares many characteristics with skulls of modern crocodilians. But its body is much more similar to those of sphenosuchians, according to James Clark, an associate professor of biology at the George Washington University in Washington, D.C.

In fact, the forelimbs of Junggarsuchus are more adapted to walking on land than those of its sphenosuchian contemporaries, according to the study led by Clark. Co-authors include Xu Xing and Yuan Wang, from the Chinese Academy of Sciences in Beijing, and Catherine Forster, from Stony Brook University in New York State.

Based on their analysis of the new fossil, the team concludes in Nature that the skull of modern crocodiles evolved while the legs and body were evolving toward greater walking ability, rather than toward greater swimming ability.

Specializations for land walking are mainly found in the forelimbs, including a ball-and-socket joint in the shoulder, like that of mammals. "These specializations indicated the forelimbs were held underneath the body, not out to the side as in living crocodilians," Clark said.

The finding does not surprise Hans-Dieter Sues, the associate director for research and collections at the Smithsonian Institution's National Museum of Natural History in Washington, D.C.

"The fact that today's semiaquatic crocodilians—crocodiles, alligators, and their relatives—are descended from land-dwelling ancestors was already established in the 1920s by German and South African researchers," he said.

"This new find," he added, "is noteworthy for its good preservation and particular combination of features."

Sues is a member of the National Geographic Society's Committee for Research and Exploration.

Crocodilians and Sphenosuchians

Modern crocodilians have sprawling limbs and broad bodies, giving them a distinctive side-to-side swagger when walking on land.

The sphenosuchians, in contrast, had an erect stance, like dinosaurs and mammals.

In water, crocodilians tuck their limbs into their bodies and swish their powerful tails back and forth to swim. Nostrils located on the top of their snouts allow them to breathe while keeping their bodies submerged.

Opportunistic feeders, crocodilians lie in wait for prey to cross their paths and then lunge with lightninglike speed to capture it between their powerful jaws. Once they impale it with their sharp teeth, crocodilians swallow prey whole.

According to Sues, nobody knows what factors led modern crocodilians to adapt a semiaquatic lifestyle, but, he said, "The ability to crush prey with their jaws was already well established in far more remote ancestors of crocodilians [than Junggarsuchus]."

Fossil Find

Found in Xinjiang, China, the Junggarsuchus fossil comprises the front half of the skeleton and is the most complete known skeleton of a nonmarine crocodilian ancestor.

The fossil dates to the Middle Jurassic period, about 175 million years ago. "This part of Asia, which is the land area most removed from any seacoast in the world, was beginning its long history of seasonal aridity then," Clark said.

Conifer and palmlike cycad trees and ferns dominated the landscape. There were no flowering plants.

Several rivers flowed from the mountains, and floods spreading out from stream channels trapped many of the fossils that researchers find today, Clark said.

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