Smithsonian.com

Gorgosaurus as envisioned by Lambe. Clockwise from the upper left - standing, sitting, in repose, and feeding. From Lambe, 1917.

There just doesn’t seem to be any way around it—almost any time a new study about the feeding habits of Tyrannosaurus comes out, there is at least one news story that frames the research with the question of whether the great Cretaceous carnivore was exclusively a predator or a scavenger. There’s no reason for journalists to keep going back to the well for the same opener. The overhyped argument made a splash during the mid- to late 1990s thanks to Jack Horner and Don Lessem’s book The Complete T. rex and a number of cable documentaries, but the debate has been over for years. As articulated by tyrannosaur specialists such as Thomas Holtz, Tyrannosaurus was an active predator but was not above scavenging if there was an easy meal to be had. In this way, Tyrannosaurus may have been something akin to a modern day spotted hyena—an adept hunter, but one also capable of crushing through bone and making the most of any Triceratops carcasses that might be around.

Part of the reason why the idea of Tyrannosaurus as an obligate scavenger took off was because it was presented as a novel and heterodox idea championed by a famous paleontologist. In television documentaries, especially, the argument was framed as a rebuttal to the classic idea of Tyrannosaurus as a powerful and nigh-unstoppable predator. But, as Horner himself pointed out in The Complete T. rex, “T. rex as a scavenger isn’t a new idea.”  About a century ago, when the tyrannosaurs were strange and new, the Canadian paleontologist Lawrence Lambe hypothesized that the huge carnivores relied upon rotting carcasses to survive.

Lambe named and initially described Gorgosaurus in 1914. The skeleton of the giant, carnivorous dinosaur was mostly complete, and Lambe focused on the basic description of the dinosaur in his first paper on the specimen. How Gorgosaurus made a living, however, Lambe saved for a more comprehensive 1917 paper. The picture that emerged was of an imposing flesh-eater, but one that was incredibly lazy.

While pointing out that the reconstruction of a prehistoric animal’s appearance and lifestyle relied on some amount of conjecture, Lambe felt that certain anatomical portions of the Gorgosaurus skeleton could be taken as tell-tale signs of the animal’s habits. For one thing, the large boot at the lower edge of the pubic bone indicated to Lambe that this part of the hip bore most of the dinosaur’s weight while it was taking a break from bumbling around the landscape. “This position of rest, and particularly the recumbent one of repose at full length,” Lambe wrote, “were probably those most frequently assumed by a reptile having the form, and the supposed sluggish disposition of Gorgosaurus.” This creature was the antithesis of the dynamic, active theropod dinosaurs in the paintings of Charles R. Knight. As Lambe put it:

Was this reptile agile, alert, and quick of movement? Was it capable of capturing its prey by a sudden rush from some place of concealment, or by overtaking it after a pursuit possibly of some length? Was its victim eaten when killed? Did it engage in spirited encounters with its own kind as depicted in Knight’s well known restoration of Dryptosaurus in accordance with Cope’s views subsequently modified? The writer believes that Gorgosaurus was sluggish and not a quick mover, and that it fed, not on the fresh flesh of animals necessarily of its own killing but rather on carcasses found or stumbled across during its hunger impelled wanderings.

Gorgosaurus must not have been a picky eater, either. Lambe suggested that anything from dead hadrosaurs to decaying turtles would have “enticed” the dinosaur’s appetite. A giant dinosaur that feeds entirely on carrion can’t afford to pass up any meals.

Of course, part of Lambe’s reconstruction relied on the idea that dinosaurs were simply big reptiles. The notion that they were active, more bird-like animals—which was entertained by naturalists such as Richard Owen, E.D. Cope, and O.C. Marsh during the late 19th century—was falling out of fashion. But Lambe cited another line of evidence. The teeth of Gorgosaurus showed almost no signs of wear.

Lambe’s reasoning went like this. Even though we now know that dinosaurs replaced their teeth throughout their lives, Lambe thought Gorgosaurus had only one set of adult teeth (as in us mammals). Since the teeth of Gorgosaurus showed almost no signs of damage or abrasion, then, Lambe proposed two hypotheses: either his large Gorgosaurus specimen was a juvenile animal that had not eaten enough food to have conspicuous marks on its teeth, or the dinosaur ate only soft flesh. Lambe tossed the first idea. The Gorgosaurus skeleton was far too large to belong to a juvenile. Instead, Lambe proposed that the dinosaur let decay soften up its meals before digging in:

It is believed, therefore, that Gorgosaurus confined itself to feeding upon carcasses of animals that had not been freshly killed, that it was not as an intrepid hunter but as a scavenger that it played its useful part in nature, and no doubt its services were fully required when we consider the immense numbers of trachodonts, ceratopsians, stegosaurs, and other dinosaurs and reptiles that lived and died at this particular time of the Cretaceous period.

This same line of reasoning could be applied to other tyrannosaurs as they were understood at the beginning of the 20th century. All were big, had short arms and were often reconstructed as being on the more rotund and slow-moving side of scale. Tyrannosaurus may have had enough star power to regularly overcome this type of imagery, but the point is that the idea that tyrannosaurs were obligate scavengers was not a brand new idea that came out of nowhere in the 1990s. Paleontologists had considered the possibility and, eventually, opinions changed. Horner’s hypothesis stirred the pot again using different lines of evidence. But the anatomy of tyrannosaurs and healed bite wounds on herbivorous dinosaurs are consistent with the idea that tyrannosaurs hunted, and toothmarks on other bones indicate that tyrannosaurs could make the most of carcasses and probably scavenged. Choosing one extreme or the other doesn’t make sense. Tyrannosaurus was a complex animal in a complex ecosystem; the Cretaceous was not a world of black-and-white.

References:

Horner, J.; Lessem, D. 1993. The Complete T. rex. New York: Simon & Schuster. pp. 203-217

Lambe, L. 1914. A new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon. The Ottawa Naturalist XXVIII, 13-20

Lambe, L. 1917. The Cretaceous theropodus dinosaur Gorgosaurus, Memoirs of the Geological Survey of Canada. 100, pp. 1–84.