Inside Sauropod Armor

Everyone knows the sauropod body plan: thin at one end, much thicker in the middle, and then thin again at the far end. Yet simply calling these dinosaurs “long necks” or focusing on their often enormous size doesn’t do justice to the diversity of forms within this group. Different sauropods had vacuum-shaped heads, whiplash tails, long bony spines jutting out of their necks, tail clubs and, among other things, armor. Regarding this latter feature, some sauropods within the titanosaur subgroup had bones embedded within their skin—called osteoderms—that would seem to have strengthened their hides against attack. According to a new Nature Communications report by paleontologist Kristina Curry Rogers and colleagues, however, an inside look at two such osteoderms yielded new evidence that these bones might have had a different function.

The pair of osteoderms that are the focus of the new study were found in association with two different specimens of Rapetosaurus, a titanosaur estimated to have reached an adult length of about 50 feet. These dinosaurs lived sometime between 70 million and 65 million years ago on what is now the island of Madagascar. One piece of armor was found next to the tail vertebrae of a juvenile individual. As seen in osteoderms of other animals, the bone had a dense outer layer surrounding spongy bone inside.

When the paleontologists used CT-scanning technology to look inside a larger, approximately 22-inch-long osteoderm found near the hips of an adult Rapetosaurus, however, they found something unusual. The inside of the osteoderm was mostly hollow. What’s more, the thickness of the outer layer of bone varied around the internal cavity, and the microscopic bone structure inside the osteoderm showed signs that bone was actually being resorbed by the body.

Maybe the osteoderms in the adult animals were not actually armor at all. A mostly hollow, relatively thin-walled bone is not exactly the sort of structure that is going to protect a sauropod from attack, especially since Curry Rogers and co-authors suggest that sauropods like Rapetosaurus were probably not fully covered in osteoderms, anyway. Instead, the paleontologists take the bone resorption within the larger osteoderm as a clue that these bones might have been mineral reservoirs for when times got tough or when egg-laying dinosaurs required extra calcium to give their a hard shell. While small Rapetosaurus might have had relatively solid osteoderms, adult individuals may have drawn upon the calcium and phosphorous in these bones to meet the demands of growing, reproducing, or living in an arid environment poor in such minerals. These dinosaur decorations may have had little to do with attack or defense.

References:

Curry Rogers, K., D’Emic, M., Rogers, R., Vickaryous, M., & Cagan, A. (2011). Sauropod dinosaur osteoderms from the Late Cretaceous of Madagascar Nature Communications, 2 DOI: 10.1038/ncomms1578

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