Dinosaur skeletons are marvelous things. The reconstructed bones of Allosaurus, Stegosaurus, Styracosaurus, Barosaurus and the like are beautiful monuments of natural architecture. But what really makes the skeletons so fantastic is that we know they once cradled viscera and were wrapped in flesh. It’s impossible to look at a dinosaur’s skeleton and not wonder about how the animals looked and acted in life.

How social dinosaurs were is one of the most persistent mysteries of their natural history. Rare trackways record the steps of dinosaurs that walked together, and bonebeds containing the bones of multiple individuals of a particular species have sometimes been taken as evidence that the dinosaurs must have been traveling together when they died. But the evidence is never straightforward. Sometimes multiple dinosaurs walked over the same patch of ground at different times, creating trackway slabs that record the independent activities of several dinosaurs rather than a coordinated herd. And just because dinosaurs were preserved together doesn’t necessarily mean that they composed a social group—natural disasters such as drought and flood, as well as transportation of carcasses by water, can create assemblages of animals that didn’t actually flock together in life. Great care is required in piecing together dinosaur lives.

With this in mind, I was curious to read a paper by Leonardo Salgado and colleagues in the latest Journal of Vertebrate Paleontology about possible evidence for social sauropods from Cretaceous Patagonia. While searching for a previously discovered dinosaur quarry in Argentina, Salgado and collaborators stumbled across a small bonebed containing the jumbled remains of three sauropods. The deposit was formed over 100 million years ago.

The largest dinosaur at the site—presumably an adult—was primarily represented by strings of articulated vertebrae arranged in the classic dinosaur death pose, while two smaller sauropod skeletons were scattered in other parts of the quarry. The dinosaurs are still undergoing study and don’t have a formal identity yet, but they appear to be rebbachisaurids, a group of sauropods that were distant cousins of the more familiar Diplodocus.

The juvenile dinosaurs alone were a significant find—no one had identified juvenile rebacchisaurids before. But the association of those skeletons is the focus of the new paper. Evidence from trackways and bonebeds has hinted that different sauropods had distinct social structures. Some, such as Alamosaurus, seemed to group together in small herds as juveniles and either become solitary as they grew or form age-segregated adult herds. Other sauropods seemed to live in mixed-age herds, where juveniles remained with older individuals. In the case of the bonebed in Argentina, it would seem that juveniles and adults traveled together.

But how do we know these dinosaurs really lived together? The skeletons are incomplete and mostly disarticulated—perhaps they were all washed up to the same spot and buried. Salgado and co-authors present a different interpretation. The bonebed doesn’t seem to be a trap or mire, and the paleontologists noted that the skeletons show “few signs of transport.” It would seem that the sauropods died all at once. The reason why is a mystery. While they frustratingly do not provide details about this scenario, the researchers speculate that “the death of the adult triggered the death of the two juvenile individuals.”

The fact that the three dinosaurs were preserved in place, without evidence of transport, seems to be fair evidence that this species of sauropod was social. But even that hypothesis brings up a series of other questions. Did individuals stay with the herd from the time they were born? Was there any form of parental care after the babies left the nest? Did these dinosaurs really form large herds, or did the young simply stick with one of their parents? We still have a lot to learn about the lifestyles of the big and extinct.

References:

Myers, T., & Fiorillo, A. (2009). Evidence for gregarious behavior and age segregation in sauropod dinosaurs Palaeogeography, Palaeoclimatology, Palaeoecology, 274 (1-2), 96-104 DOI: 10.1016/j.palaeo.2009.01.002

Salgado, L., Canudo, J., Garrido, A., & Carballido, J. (2012). Evidence of gregariousness in rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the Early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina Journal of Vertebrate Paleontology, 32 (3), 603-613 DOI: 10.1080/02724634.2012.661004