New look at the sprawl in gator's gait.

New look at the sprawl in gator's gait

With their sprawling belly-walk and other primitive features, today's crocodiles, alligators and other crocodilians look as if they've crawled directly out of the most ancient reptilian past. But living crocodilians are also known to walk, and at times even gallop, in an erect posture, with legs held close to their bodies in a manner characteristic of the more advanced dinosaurs and mammals.

Because today's crocodilians are thought in some ways to be living relics of prehistoric times, and because they display both kinds of walking postures, paleontologists have long believed that the earliest crocodilians represented a link between primitive, lizard-like reptiles that sprawled and the more erect reptiles that evolved later.

But according to the most recent issue of PALEOBIOLOGY (Vol. 13, No. 4), this theory is out of step with the fossil evidence. Paleontologist J. Michael Parrish, at the University of Colorado Museum in Boulder, concludes that the earliest crocodilians and their nearest relatives (collectively known as crocodylomorphs) stood and walked erect. The sprawling stance and gait that living crocodilians use while sliding into water, he says, are more recent adaptations, made when the animals moved from a primarily terrestrial home to an aquatic one.

Parrish's conclusion is "quite reasonable," comments paleontologist Kevin Padian at the University of California at Berkeley. "He's straightened out a little bit about the sequence of evolution of these gaits, how they have changed and even reversed themselves in the history of a group."

In the past, says Parrish, most studies have concentrated on systematics, or the evolutionary relationships among animals. "I've been trying to integrate systematics with other approaches," he says. In particular, he has focused on limb mechanics, examining how the hind leg, pelvic and ankle bones fit and move together in 190-million- to 210-million-year-old crocodylomorph fossils and in modern specimens. He has also incorporated paleoecological studies, which suggest that the early crocodylomorphs spent most of their time on land.

Paleontologists still believe that reptiles progressed from a sprawling gait to an erect one. But if the already-erect early crocodylomorph was not the link between these two postures, says Parrish, the question becomes: What animal was?