July 15, 2010
Back in the 1990s, paleontologist Jack Horner proposed that Tyrannosaurus rex—popularly cast as the most fearsome predator of all time—was really a giant-sized scavenger. With its small arms, a large part of its brain devoted to analyzing smells, and a mouth full of rail-spike-sized teeth, the tyrant dinosaur seemed to be better-suited to processing the carcasses of dead animals than chasing down live prey. Journalists and filmmakers ate it up. For years afterward, magazine articles and documentaries covered the debate around the feeding habits of Tyrannosaurus, and a new paper just published by David Hone and Mahito Watabe in Acta Palaeontologica Polonica would seem to feed into this paleo-controversy.
In 1995 paleontologists recovered the nearly complete skeleton of the large, herbivorous hadrosaur Saurolophus from approximately 70-million-year-old Cretaceous rock in the western part of the Gobi Desert. Yet, despite being relatively well-preserved, the dinosaur had obviously been damaged before it was completely buried, as evidenced by bite marks on its left humerus (upper arm bone). Something had been picking at the carcass, and now, based upon the size and shape of the bitemarks, Hone and Watabe propose Tarbosaurus—a cousin of Tyrannosaurus which lived in prehistoric Asia—as the probable culprit.
Interestingly, when Hone and Watabe looked at the skeleton of the herbivorous dinosaur they found no signs that it had been attacked and killed by a Tarbosaurus. There were no bitemarks where one might expect a large predator to attack in an attempt to bring down prey. Instead, it seems that the Saurolophus had already died and was mostly buried, leaving only a little bit of its body exposed above the surface. This would have been a free meal, and this specimen seems to represent the first identified case of scavenging by a large tyrannosaur.
Despite its size and power of its jaws, however, it appears that the Tarbosaurus that fed on the dinosaur did not simply crunch through the arm bones. (And, in a study Hone published with colleague Oliver Rauhut last year, the scientists did not find direct evidence that large, predatory dinosaurs were in the habit of crunching up whole bones as a regular part of their diet.) Instead the Saurolophus humerus shows several different kinds of bits marks, including punctures and scrapes, suggestive of the scavenging Tarbosaurus stripping the muscle off the bone instead of just chomping it off and swallowing the shattered pieces. As large as it was, these bite marks suggest that Tarbosaurus—as well as its kin among the tyrannosaurus—could be delicate eaters.
So what does this mean for the long-running debate over whether large tyrannosaurs were predators or scavengers? This is the first case in which paleontologists have been able to unequivocally identify scavenging by a large tyrannosaur, but the fact that such traces should exist will come as no surprise to seasoned paleontologists. While the “T. rex—predator or scavenger?” angle is often played up in television shows and articles, many (if not most) paleontologists agree that Tyrannosaurus was neither only a hunter nor an obligate scavenger. Among professional paleontologists, at least, the predator vs. scavenger debate is pretty much dead, with an excellent review by tyrannosaur specialist Thomas Holtz in the book Tyrannosaurus rex, the Tyrant King being the last nail in its coffin. Healed bite marks on the skeletons of herbivorous dinosaurs provide evidence that large tyrannosaurs hunted live prey, while specimens such as the Saurolophus skeleton show that they would not be above consuming carrion when the opportunity presented itself. (And, as Horner hinted in an interview I conducted with him two years ago, his comments about Tyrannosaurus were at least partially motivated by wanting to get scientists to test what had long been assumed about the dinosaur.) Personally, I find the fact that Tarbosaurus could be so delicate with its jaws (relatively speaking) to be much more interesting. While it certainly could have crushed the Saurolophus arm bone, instead it used its teeth to strip meat off the bone, providing evidence that these dinosaurs could be quite careful with their meals when they wanted to be.
Hone, D., & Watabe, M. (2010). New information on scavenging and selective feeding behaviour of tyrannosaurs Acta Palaeontologica Polonica DOI: 10.4202/app.2009.0133
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