Sauropods - long-necked dinosaurs like Brontosaurus and Astrodon - were remarkable creatures. They were the largest land animals the world has ever known. Their crane-like necks were incredible feats of bioengineering. As a group, they were incredibly long-lasting, living all over the world for 140 million years. But one feature of sauropods often goes underappreciated: they were armored.
The backs and flanks of many sauropod species were studded with osteoderms. These small, teardrop-shaped bones were embedded in the animals’ skin. Traditionally, paleontologists have assumed defense was the main function of sauropod osteoderms. However, researchers at the National University of Distance Education in Madrid have proposed another possibility: osteoderms were calcium reserves for egg-laying females.
When modern birds are preparing to lay eggs, they grow a temporary layer of densely mineralized bone on the interior surfaces of their hollow limb bones. The birds use this layer, called medullary bone, as an extra supply of calcium while eggshells are forming in their bodies. This prevents debilitating bone resorption – making eggshells requires a lot of calcium! Several dinosaur species, including Tyrannosaurus and Allosaurus, are also known to have produced medullary bone. According to Daniel Vidal and his colleagues in Madrid, sauropod osteoderms may have served a similar function in pregnant females.
Sauropod osteoderms are generally rare, perhaps because they are not connected to the rest of the skeleton and scatter shortly after the animal dies. However, an unusually large number have been collected at Lo Hueco, a late Cretaceous fossil site in east-central Spain. Vidal and colleagues examined 17 osteoderms from Lo Hueco inside and out, using CT scans to see the interior surfaces. They found that while most osteoderms had a solid bone core, 30% of the fossils had hollow centers. In both cases, the researchers observed a dense network of neurovascular canals radiating out from the osteoderm cores.
This suggests that, like medullary bone in birds, the osteoderms were reseviors of calcium that the animal could draw on in times of need. Notably, only a small percentage of the osteoderms had drained cores. Vidal and colleagues argue that the calcium reserves were therefore only being used by portion of the population – females preparing to lay eggs. If the calcium reserves were being drawn on during a time of environmental stress, like a drought, all the sauropods would have similarly depleted osteoderm cores.
Vidal, D., Ortega, F., Gascó, F., Serrano-Martinez, A., and Sanz, J.L. 2017. The internal anatomy of titanosaur osteoderms from the Upper Cretaceous of Spain is compatible with a role in oogenesis. Scientific Reports 7:42035.
Schweitzer, M. H., Wittmeyer, J. L. and Horner, J. R. 2005. Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex. Science 308:1456-1460.