Courtesy Mark RyanPaleontologist Kristina Curry Rogers, a former curator here at the Science Museum of Minnesota, has dedicated much of her career studying sauropod dinosaurs, their behaviors, and the histology of their bones. Sauropods are a diverse group of large, long-necked dinosaurs that include, among others, the popular Apatosaurus (Brontosaurus), Diplodocus, Barosaurus, and Brachiosaurus. The four-legged, herbivorous sauropods made their first appearance in the Late Triassic Period about 220 million years ago, then rose to prominence during the Late Jurassic, growing to 80 tons or more. The Jurassic giants gave way to the titanosaurs in the Cretaceous, a group that thrived until 65 million years ago when a major extinction event wiped out all the mega-fauna dinosaurs.
Courtesy Fritz Geller-Grimm via Wikipedia Creative CommonsNow an assistant professor at Macalester College in St. Paul, Curry Rogers' latest research centers on titanosaur osteoderms. Osteoderms are bony deposits embedded in the skin and used as defensive protection or for other functions such as temperature regulation. The bumpy armor can be seen today in alligators, crocodiles and many other reptilian forms (see photo). So far, titanosaurs are the only sauropod dinosaurs known to have had osteoderms.The smaller ornithopod dinosaurs such as ankylosaurs and stegosaurs used osteoderms to create bony exteriors to strengthen their hide against attack. On the predator side of the equation, the carnivore Ceratosaurus is the only theropod dinosaur found with osteoderms.
Curry Rogers has been studying the osteoderms of two titanosaur specimens (Rapetosaurus krausi), one from an adult and the other from an immature individual. Both specimens were found in the 1990s in the sandstones of the Maevarano Formation exposed on the northwest corner of the island of Madagascar. During the Late Cretaceous the climate in the region was seasonal with periods of rain and drought alternating with periods of semi-arid conditions.
Courtesy Mark Ryan That a 50-foot adult Rapetosaurus would even need osteoderms at first mystified the paleontologist. A full-grown Rapetosaurus was just too enormous to require body armor.
Using a CT scanner and core-sampling to analyze the two samples, Curry Rogers found that the juvenile osteoderm had a thick, dense outer lining on the outside and was filled with a spongy bone on the inside. The larger adult osteoderm - 22 inches in length and the largest ever found - appeared much the same from the outside but surprisingly the shell wasn’t as thick and the inside was almost completely hollow. Microscopic examination of the inside lining of the outer structure showed signs of it being resorbed into the body
The new data made the paleontologist wonder. Why would the osteoderms appear one way in an earlier life stage and change into another form in adulthood? Did their functions change as the titanosaurs passed through different stages of life?
Eventually the paleontologist came up with a hypothesis: the osteoderms were probably used as armor when the Rapetosaurus was small and still growing, and easy prey for predators. But as it neared adult size the osteoderms were no longer needed for protection and instead could have served as reservoirs for minerals, such as calcium and phosphorus. These would have been useful as the dinosaurs' size increased or when the adult titanosaurs needed extra minerals during times of environmental stress (like the frequent droughts occurring on Madagascar during the Late Cretaceous). Drawing minerals from its arm or leg bones would have weakened them making it difficult for the multi-ton animals to stand or walk. The mineral stores could also be used during reproduction when calcium was necessary for egg shell production.