Stegosaurs Walked Like Sauropods

Paleontologists certainly have their hands full finding and describing new dinosaurs, but there is still much to learn about the already familiar species. The configuration of different parts of dinosaur anatomy, for one thing, is an area of research in which scientists are constantly re-examining how ancient bones would have been arranged in life. Now a study by Fayetteville State University paleontologist Phil Senter suggests that we have to tweak our understanding of stegosaur forelimbs.

Senter's new study focuses on the hands of stegosaurs and sauropod dinosaurs, specifically a set of bones between the wrist and the fingers called the metacarpals. Traditionally, these two types of dinosaurs were thought to have had very different arrangements of these bones. If you were to look at the metacarpals of a stegosaur from the top down you would see them arranged in a slight arc, while in many sauropods these bones had a more circular arrangement which, as Senter states, formed "a vertical tube" which helped support the massive weight of these animals. (Even in relatively early or archaic sauropod dinosaurs, the metacarpal bones were arranged in enough of a circle to create "a half tube" shape.) Hence, for over a century, stegosaurs have often been portrayed as having slanted hands with slightly spread fingers while sauropods supported themselves upon fleshy pillars.

The problem with the traditional view of stegosaurs is that some specimens, such as a skeleton of Stegosaurus armatus found nearly a century ago, had metacarpals articulated in a different disposition. Rather than matching the diagrams printed by paleontologists such as O.C. Marsh and C.W. Gilmore, they had a semicircular arrangement similar to that of the sauropod Camarasaurus. The same is true of another old Stegosaurus specimen examined by Senter, in which the metacarpals best articulated in a semicircle rather than in the more traditional, spread-out arrangement. Rare stegosaur trackway evidence, too, is more consistent with a semicircular articulation of the hand bones than the older model.

What all of this means for stegosaurs is that their metacarpals were more important during walking than their fingers. Instead of their fingers mainly being in contact with the ground and pushing off, as in other dinosaurs, the fingers of stegosaurs were reduced in size and shifted out of the way so that the part of the foot supported by the metacarpals was primarily in touch with the ground during each step. Under this hypothesis both stegosaurs and sauropods were adapted to have similar forelimb anatomy, and this revision will certainly play into future discussions about how these dinosaurs moved.

References:

Senter, P. (2010). Evidence for a Sauropod-Like Metacarpal Configuration in Stegosaurian Dinosaurs Acta Palaeontologica Polonica, 55 (3), 427-432 DOI: 10.4202/app.2009.1105

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.