Bicentenaria and the Rise of the Coelurosaurs

When paleontologists at the Argentine Museum of Natural Science in Buenos Aires threw the curtain back on the new dinosaur Bicentenaria argentina last month, they showed off a beautiful mount of tussling dinosaurs. But I couldn’t help but wonder about the reconstruction. Just how much of the dinosaur had been found, and was there any direct evidence that these dinosaurs fought each other?

Frustratingly, I couldn’t obtain immediate answers. The press event preceded the actual paper describing Bicentenaria. But last night I finally got my claws on the description of this archaic, peculiar dinosaur and its possible behavior.

Although Bicentenaria is new to science, the dinosaur’s remains were first discovered years ago. In 1998, during a drop in the water level at Argentina’s Ezequiel Ramos Mexía Reservoir, Rauel Spedale discovered and collected the disarticulated, scattered remains of several Bicentenaria from a small quarry. There was no single complete skeleton, but the quarry contained multiple skull and postcranial bones from several animals. The largest of these dinosaurs would have been about 10 feet long.

According to the analysis of the accumulated bones by paleontologist Fernando Novas and colleagues, Bicentenaria was an archaic form of coelurosaur. This is the major group of theropod dinosaurs that includes tyrannosaurs, the fluffy compsognathids, the sickle-clawed deinonychosaurs, the utterly strange therizinosaurs and birds, among other disparate lineages. Bicentenaria didn’t belong to any of these subgroups but was near the base of the coelurosaur family tree.

Yet, despite its old school anatomy, Bicentenaria was definitely not the ancestral coelurosaur. Not even close. Coelurosaurs were already a diverse group by the Late Jurassic, meaning that they started to proliferate before 150 million years ago. Yet Bicentenaria lived around 95 million years ago during the Late Cretaceous. It was over 55 million years too late to be a true ancestor of the other coelurosaur groups.

Bicentenaria can still help paleontologists visualize the anatomy early coelurosaurs, though. Based on the evolutionary analysis in the new paper, Bicentenaria preserved features seen in much, much older dinosaurs that were at the base of the coelurosaur family tree. While not an ancestor of coelurosaurs, the skeleton of Bicentenaria can help scientists figure out what the actual progenitors of the group were like.

The study also speculated about the dinosaur’s social life. Since the small quarry yielded multiple individuals, Novas and collaborators concluded that these dinosaurs must have been socializing when they died. More than that, the paleontologists tie in other theropod bonebeds to suggest that a gregarious lifestyle was the ancestral condition of theropod dinosaurs, “if not Dinosauria as a whole.”

I’m not so sure. The fact that multiple dinosaurs of the same species died in the same place, by itself, isn’t evidence that the animals lived together. It is only evidence that the dinosaurs were buried together. Even though there have been many claims of “dino gangs” and “dueling dinosaurs” based upon associated skeletons, we need to know the details of how those animals died and became buried before we can accurately reconstruct their behavior. Just because we find dinosaurs buried together doesn’t necessarily mean they were socializing before they perished. Some bonebeds really do seem to contain dinosaurs that were in a social group when they perished, while others represent assemblages of individuals that died at different times and were later washed together. The geologic and taphonomic context is critical.

In this case, unfortunately, Spedale did not take any notes on the arrangement of the bones or the context in which they were found. That data is lost. But one quarry block indicates that the bones of the dinosaurs were transported by water and stirred together. The dinosaurs died elsewhere and only parts of them ultimately became preserved in the same spot. This complicates the social Bicentenaria hypothesis. Did all the dinosaurs in the quarry die together, or did their bodies accumulate in a particular place–perhaps due to a drought or other event–over time before being washed together? We don’t know. Bicentenaria very well could have been a social dinosaur, but the evidence isn’t strong enough to say for sure, much less hypothesize that a gregarious lifestyle was the ancestral condition for all theropods. There’s a lot that we can learn about dinosaur lives from their bones, but the intricacies of their social lives remains obscured by the quirks of the fossil record.

Reference:

Novas, F., Ezcurra, M., Agnolin, F., Pol, D., Ortíz, R. 2012. New Patagonian Cretaceous theropod sheds light about the early radiation of Coelurosauria. Rev. Mus. Argentino Cienc. Nat., n.s. 14(1): 57-81 (PDF)

Presentación de Bicentenaria argentina, una nueva especie de dinosaurio carnívoro

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Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.