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All dinosaurs once pranced, strolled or lumbered about on two legs. But some took to occasionally resting or running on all fours for greater stability and over time evolved into quadrupeds. During the transition, the forelimbs were shorter than the hind limbs, raising the question of how the intermediate animals leveled out the tilted stance from those stubby appendages: Did they walk on their “fingertips” or their palms? New research suggests the latter—some early dinosaurs and their close relatives may have stepped straight down on the front of their palms.
Dinosaurs are closely related to alligators' ancestors and consequently share many structural features with gators. So biologist Joel Hutson and geologist Kelda Hutson compared the forelimb mechanics of alligators with fossils from Postosuchus—a relative of early dinosaurs and an ancestor of alligators and crocodiles—to learn more about joint mobility. The Hutsons measured movement of each joint in alligator specimens in multiple states: intact, without scales, without muscles and tendons, without ligaments and, finally, without cartilage. The team found that the ability of bone-on-bone specimens to hyperextend matched that of the fossils. They also verified that with cartilage in place, the alligator digits easily hyperextended backward, suggesting that Postosuchus would have been capable of hyperextension as well. Thus, perhaps dinosaurs making the transition from bipedalism walked in such a way, too—walking on their palms with hyperextended fingers. The results were published online in March in the Journal of Zoology.
Range-of-motion comparisons among dinosaur fossils and fresh, intact tissues have rarely been performed, says Mason Meers, a biologist at the University of Tampa who researches the evolution of crocodile locomotion. “The work's 100 years overdue,” he adds. And although the study is small, the results shed more light on exactly how strange early dinosaurs would have looked as they stalked about, Joel Hutson says. For instance, while in the process of developing four legs dedicated to locomotion, dinosaurs might have used their wrists and palms as if they were stilts.