Tarbosaurus Gangs: What Do We Know?

Tarbosaurus, the great tyrannosaur of Cretaceous Mongolia, hunted in packs. That is the exceptional claim made by University of Alberta paleontologist Philip Currie in a press release, and news outlets all over the world have picked up the story. Just imagine rapacious tyrannosaur families tearing over the prehistoric countryside; it is a terrifying notion that the press release heralds as a “groundbreaking” discovery that will forever change paleontology.

But does the actual evidence live up to all the hype? Unfortunately, the answer is no. The proposal of pack-hunting dinosaurs is old news in paleontological circles, and the hard evidence to support the claims about Tarbosaurus has not yet been released.

Packaged under the theme “Dino Gangs,” the media release, book, and cable-network documentary arranged by Atlantic Productions hinge on a Tarbosaurus bonebed found in Mongolia’s Gobi Desert. The site was one of 90 Tarbosaurus localities surveyed by Currie and the Korea-Mongolia International Dinosaur Project, but it is unique in that it preserves the remains of six individual animals of different life stages. How the animals died and became buried is unknown. Even so, the press claims that these dinosaurs were a single family group that hunted together.

There was no scientific paper attached to the release, and I received no reply from Atlantic Productions when I inquired whether a technical description of the site will soon be published. The media release–reporting conclusions without providing evidence–was presented on its own.

This is not the first time tyrannosaurs have been reconstructed as living in packs. In 1997 Currie relocated a rich dinosaur bonebed in Alberta, Canada that had been discovered by the fossil hunter Barnum Brown in 1905. The site was dominated by remains of the tyrannosaur Albertosaurus—at least a dozen individuals of this species were found in this one place. Why one site should contain so many tyrannosaurs was difficult to explain, but in a 1998 paper published in Gaia, Currie proposed that the Albertosaurus were living in a social group and that the site was evidence of gregarious behavior among the dinosaurs. More than that, Currie proposed that there was a “division of labor” within the Albertosaurus packs. Compared with the adults, juvenile Albertosaurus would have been much faster runners thanks to their different leg proportions, and so Currie suggested: “The faster, more agile juveniles may have been responsible for driving potential prey towards the larger, more powerful adult tyrannosaurids.” Currie has suggested the same thing for Tarbosaurus in the “Dino Gangs” press release.

But the idea that young and old tyrannosaurs worked together to tackle prey rests upon the inference that the bonebeds contain social groups. This is not necessarily so. There are many ways to make a bonebed, and the fine geological details of such fossil-rich sites contain essential information about how the bodies of the different individuals became preserved together. Proximity does not always indicate sociality, as Currie himself noted in a paper published with David Eberth last year about the Albertosaurus quarry.

Although the idea that the Albertosaurus quarry indicates complex social interactions among pack-hunting dinosaurs is a sexy hypothesis, Currie and Eberth noted that the animals could have been brought into close association by some kind of environmental catastrophe. “he evidence for a significant storm and associated flooding event at the site and in the surrounding area is well documented,” the scientists wrote, and they suggested that solitary Albertosaurus might have been driven together into a small area by the floodwaters. Pack behavior among the animals could not be taken as a given. The Albertosaurus were together when they died, but exactly how they died and why they were so close to each other remains unclear.

In the 2005 book Carnivorous Dinosaurs, Currie and several co-authors reported on a bonebed found in Montana that contained several hadrosaurs and remains of three tyrannosaurs identified as Daspletosaurus. Though the scientists suggested that the tyrannosaurs might have been interacting socially before they died, how the animals died and became buried was unknown. The same was true of a site in Argentina described by Currie and colleague Rodolfo Coria. The bonebed contained seven individuals of a large predatory dinosaur unrelated to tyrannosaurs named Mapusaurus. Although the site could have represented a social group, Currie and Coria concluded that “It is conceivable that this bonebed represents a long term or coincidental accumulation of carcasses.”

There is no slam-dunk evidence that tyrannosaurs or other large predatory dinosaurs hunted in packs. Even in the case of Deinonychus—a small, sickle-clawed “raptor” traditionally thought to be a cooperative hunter—evidence of multiple individuals in association with prey species has recently been questioned. In the end, trackways that record the footsteps of multiple raptors moving together has provided better evidence that these dinosaurs were sometimes social. No such evidence exists for tyrannosaurs yet. (Only one footprint attributed to a tyrannosaur has been found so far.)

Various processes can bring bones together into a single fossil deposit. A bonebed might represent a social group killed and buried by a flood, scattered bodies or bones that were washed together by water currents, or a natural trap where multiple individual animals died over a long period of time, among other possibilities. How the animals died, how long it took for the fossil deposit to accumulate, and other questions must be answered before hypotheses about behavior can be drawn out. As for the Tarbosaurus bonebed, no technical details of the site have yet been released. There is no science to talk about at this point. The site might record the death of a dinosaur pack, but that is just one of many possibilities that have yet to be ruled out.

The hubbub over the “Dino Gangs” press release is intensely frustrating. No scientific information is available, and the supposedly jaw-dropping findings are almost exactly the same as those proposed on the basis of a different site in 1998. The press release is full of bombastic language about how it is now time to rewrite the dinosaur books and how this discovery will forever change our understanding of dinosaur behavior. None of the information provided so far will do any such thing. The new find is one more discovery that will add to our understanding of dinosaurs, but is not wildly different from what has been discovered or proposed before. If there is something truly exceptional about the Tarbosaurus bonebed, it has yet to be revealed.

A discovery isn’t important simply because a press release says it is. Scientific findings should not be judged by how glitzy a documentary is or how well a book sells. By the sound of it, Currie and his colleagues have found a spectacular fossil site that is brimming with information about prehistoric life. None of the details have been published yet, and, consequently, they have not been submitted to the process of scientific debate, so no one can definitively say how the Tarbosaurus bonebed will affect our understanding of these dinosaurs. The discovery of the fossil site is just one part of the story. The rest, including how the Tarbosaurus lived and died, will take time to draw out.

References:

Coria, R., and Currie, P. (2006). A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina Geodiversitas, 28 (1), 71-118

Currie, P. (1998). POSSIBLE EVIDENCE OF GREGARIOUS BEHAVIOR IN TYRANNOSAURIDS Gaia, 271-277

Currie, P., & Eberth, D. (2010). On gregarious behavior in Albertosaurus Canadian Journal of Earth Sciences, 47 (9), 1277-1289 DOI: 10.1139/E10-072

Currie, P.; Trexler, D.; Koppelhus, E.; Wicks, K.; Murphy, N. (2005) An unusual multi-individual, tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA), in Carpenter, K. (ed.), The Carnivorous Dinosaurs. Indiana University Press, Bloomington; Indianapolis: 313-324.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.