May 25, 2012
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.
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
May 24, 2012
Some dinosaur lineages are more famous than others. I can say “tyrannosaur” and most anyone immediately knows what I’m talking about: a big-headed, small-armed predator similar to the notorious Tyrannosaurus rex. The same goes for “stegosaur,” and of course it helps that Stegosaurus itself is the famous emblem of this bizarre group. But public understanding hasn’t kept up with new discoveries. In the past two decades, paleontologists have identified various dinosaur lineages vastly different from the classic types that gained their fame during the Bone Wars era of the late 19th century. One of those relatively obscure groups is the abelisaurids: large theropod dinosaurs such as Carnotaurus with high, short skulls and ridiculously stubby arms that make T. rex look like Trogdor the Burninator. And paleontologists Diego Pol and Oliver Rauhut have just described an animal close to the beginning of this group of supreme predators—a dinosaur from the dawn of the abelisaurid reign.
Pol and Rauhut named the dinosaur Eoabelisaurus mefi. Discovered in roughly 170-million-year-old Jurassic rock near Chubut, Argentina, the mostly complete dinosaur skeleton is about 40 million year older than the next oldest abelisaurid skeleton. Eoabelisaurus, placed in context with other theropod dinosaurs of the same era, represents a time when predatory dinosaurs were undergoing a major radiation. Early members of many terrifying Cretaceous predators such as the tyrannosaurs and abelisaurids had already appeared by the Middle to Late Jurassic.
Not all of these Jurassic predators looked quite like their later Cretaceous counterparts. Jurassic tyrannosaurs such as Juratyrant and Stokesosaurus were relatively small predators, unlike their bulky, titanic relatives from the Late Cretaceous. Eoabelisaurus was a little closer to what was to come.
Despite being many tens of millions of years older than relatives such as Carnotaurus and Majungasaurus, the newly described dinosaur displays some tell-tale features that characterize the group. While a significant portion of the dinosaur’s skull is missing, the head of Eoabelisaurus had the short, deep profile seen among other abelisaurids. And this dinosaur already had distinct forelimbs. Much like its later relatives, Eoabelisaurus had a strange combination of heavy shoulder blades but wimpy forelimbs, with a long upper arm compared to the lower part of the arm. The dinosaur’s condition was not as extreme as in Carnotaurus—a dinosaur whose lower forelimbs were so strange that we have no idea what, if anything, Carnotaurus was doing with its arms—but they were still comparatively small and tipped with little fingers good for wiggling but probably useless in capturing prey.
And with a 40-million-year gap between Eoabelisaurus and its closest kin, there are plenty of other abelisaurids to find. The question is where they are. Is their record so poor that very few were preserved? Or are they waiting in relatively unexplored places? Now that the history of these blunt-skulled predators has been pushed back, paleontologists can target places to look for the carnivores.
Pol, D., Rauhut, O. (2012). A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs. Proceedings of the Royal Society B, 1-6 : 10.1098/rspb.2012.0660