Tyrannosaurus: Hyena of the Cretaceous
Of all the organisms scientists have found in the fossil record, Tyrannosaurus rex is the most prominent ambassador for paleontology. No dinosaur hall is complete without at least some fragment of the tyrant dinosaur, and almost anything about the dinosaur is sure to get press coverage. We simply can't get enough of old T. rex. It was no surprise, then, that a census of Tyrannosaurus specimens from Montana's Hell Creek Formation published by Jack Horner, Mark Goodwin and Nathan Myhrvold in PLoS One gained wide media coverage, but there was a sub-story that many news outlets missed. Rather than overturning the image of Tyrannosaurus as a predator, as some reports claimed, the conclusions of the new study actually brought Horner's stance on the iconic dinosaur close to what other experts thought.
The story behind the new PLoS One study began eighteen years ago. The film Jurassic Park had just triggered a wave of dinomania unlike any seen before, and paleontologists were quick to take advantage of the interest that the film had generated. Among them were Gary Rosenberg and Donald Wolberg, who organized 1994's Dino Fest event at Indiana University-Purdue University at Indianapolis, and one of the invited speakers was Jack Horner. One of the chief consultants on Jurassic Park, Horner had helped bring the film's terrifying Tyrannosaurus to life, but in his talk he presented a different picture of the dinosaur.
Horner's lecture was titled "Steak Knives, Beady Eyes, and Tiny Little Arms (A Portrait of T. rex as a Scavenger)," and a transcript of it was printed in the collected proceedings of the conference. With Jurassic Park fresh in the audience's mind, Horner explained that the real animal probably wasn't as speedy or ferocious as the film made it seem. "In fact," Horner said, "I think the only thing that Tyrannosaurus rex would have done in that movie is eat that lawyer."
In Horner's view, Tyrannosaurus was built for scavenging. Despite possessing a huge head full of serrated teeth the size of rail spikes, the tyrant dinosaur had puny, rigid arms, and Horner argued that strong arms would have been essential for an active predator to catch and subdue prey. Furthermore, Horner pointed to the apparently small eyes of Tyrannosaurus and the large olfactory lobe of the dinosaur's brain. Horner asserted his uncertainty about these features—"I don't know if it's worth anything," he said—but hinted that they might be consistent with the idea of Tyrannosaurus as a scavenger that was better at sniffing out carcasses than following live prey. Since the hadrosaurs and horned dinosaurs of the time lived in giant herds, Horner suggested that tyrannosaurs followed them to pick at the carcasses of those that died as the herds trod around the landscape. Horner concluded:
Picture Tyrannosaurus rex. He has no arms, can't run fast, appears to have a large olfactory lobe and he's big. Interestingly enough if you think about it, one of the best things to be if you are a scavenger is big so you can chase away anything else around the carcass.
Horner's book "The Complete T. rex", published that year with science writer Don Lessem, presented the "obligate scavenger" hypothesis to a wider audience. Similar ideas had been proposed before, but Horner's public suggestion that Tyrannosaurus was a lazy scavenger stirred immediate controversy. This was not so much an academic debate as a tug-of-war over who would mold the image of Tyrannosaurus.
Among the early responses to Horner's ideas was a 1997 lecture delivered by Theagarten Lingham-Soliar to the British Association for the Advancement of science (later printed in Geology Today) titled "Guess who's coming to dinner: A portrait of Tyrannosaurus as a predator." Tyrannosaurus would have scavenged when the opportunity arose, Lingham-Soliar said, but the reinforced skull and impact-resistant teeth of the dinosaur were clearly well-suited to handling struggling prey. Even juveniles had these features, and given their small size it was probable that they were actively hunting smaller fare instead of relying on scraps from carcasses already obliterated by adults.
Responses like Lingham-Soliar's did little to quell the debate. The scavenger hypothesis was popularized in books, news reports and documentaries. Horner's influence even turned Tyrannosaurus into a scavenger during an early scene of Jurassic Park III. Horner hinted that part of his motivation for proposing the obligate scavenger idea was to get scientists and dinosaur fans to think critically about commonly accepted ideas. Despite the amount of attention the idea received, other paleontologists were not convinced.
The ultimate take-down of Horner's hypothesis was published by tyrannosaur expert Thomas Holtz in the 2008 book "Tyrannosaurus rex: The Tyrant King." Right at the outset, Holtz pointed out that flesh-eating animals do not break down into neat categories of "scavenger" and "predator." Spotted hyenas—traditionally believed to be almost pure scavengers—have been found to be active hunters, and even lions, iconic hunters, obtain a significant part of their food through scavenging. Large carnivorous animals both hunt and scavenge food. Tyrannosaurus would not have been different.
Holtz's paper was the first comprehensive and scientific critique of Horner's idea. The ideas had been batted around in talks, documentaries and popular books, but Holtz actually put in the scientific legwork to see if the traits Horner associated with scavenging truly indicated that Tyrannosaurus relied almost entirely on carrion.
Holtz's analysis dismantled what Horner had proposed. The eyes of Tyrannosaurus were not atypically small; the proportions of its legs would have allowed it to run faster than other large theropods (and, more importantly, potential prey species); it had deeply rooted teeth that would have been able to cope with the stresses generated by struggling prey; and its small forelimbs would not have prohibited it from hunting and killing other dinosaurs. Oddly enough, some of the best evidence of tyrannosaur hunting come from two animals that escaped attacks by the dinosaur: an Edmontosaurus with a partially healed bite along its tail and a Triceratops skull showing a similar type of damage. Since Tyrannosaurus was the only gigantic predator known from the habitats in which the injured herbivores were found, it is probable that the dinosaurs were survivors of Tyrannosaurus attacks.
Tyrannosaurus almost certainly scavenged—something that has been supported by the recent discovery of cannibalism and an instance of scavenging by the related Tarbosaurus—but there was nothing about the dinosaur that barred it from being a formidable hunter. "here is no evidence to suggest that tyrannosaurs were radically different in diet from living large-bodied carnivores, which obtain food both predation and scavenging," Holtz said.
As reconstructed by Holtz, Tyrannosaurus may have been the spotted hyena of its day. The hyenas do not have large claws or muscular arms like lions. Instead, they primarily catch, kill and consume prey with their robust jaws, which is what the tyrant dinosaur would have done as well. Especially after Holtz's paper, the idea that Tyrannosaurus hunted and scavenged should not have surprised anyone. So why did so many media sources act with astonishment at the statements by Horner and his team in reference to their new PLoS One paper?
The recent publication of a paper that explicitly attacked Horner's hypothesis set the stage. A few weeks ago, Chris Carbone, Samuel Turvey and Jon Bielby published a study suggesting that smaller meat-eating dinosaurs would have destroyed most of the available carcasses before Tyrannosaurus had a chance to get to them, making it unlikely that the giant dinosaur relied on carrion for food. There were a few problems with the lists of dinosaurs the authors drew up to create their estimates, but the study still made the important point that Tyrannosaurus probably would have competed with numerous other dinosaurs for carrion. Scavenging would not have been as easy a gig as Horner initially proposed.
The study by Carbone and co-authors cast doubt on the ability of Tyrannosaurus to find—much less consume—dinosaur carcasses. But a little more than a week later, Horner, Goodwin and Myhrvold concluded that the tyrant must have scavenged.
Horner and colleagues based their hypothesis on a census of dinosaurs found in the vicinity of Fort Peck Reservoir in northeastern Montana during the decade-long Hell Creek Project. The goal of this effort is to "create a comprehensive biotic foundation from which paleobiological and geological hypotheses could be tested," including an understanding of dinosaur abundance at the end of the Cretaceous. The new paper presented some preliminary results from the census, and Tyrannosaurus turned out to be more common than expected.
Outcrops sampled by the Hell Creek Project were divided into three sections: lower, middle and upper slices. The top and bottom sections were the focus of the PLoS One report, and within each portion many remains of Triceratops, Edmontosaurus, and Tyrannosaurus were found. Triceratops was the most common in each section, but, surprisingly, Tyrannosaurus was just as common, if not slightly more common, than the hadrosaur Edmontosaurus. In the upper Hell Creek section, for example, the census included twenty two Triceratops, five Tyrannosaurus, and five Edmontosaurus.
(The dinosaurs Thescelosaurus, Ornithomimus, Pachycephalosaurus, and Ankylosaurus were also included in the breakdown, but were relatively rare. Small predatory dinosaurs, such as Troodon, were reported as being rare and are not included in the breakdown.)
The relative number of Tyrannosaurus skeletons seems high for a predatory species. Why would a large predator be about as common as one of its prey species? Perhaps there was some kind of bias in preservation or collection. If Edmontosaurus was one of the main sources of food for Tyrannosaurus, for example, the skeletons of these dinosaurs were probably destroyed on a regular basis and therefore did not enter the fossil record. The census records what was preserved and discovered, but is not a perfect snapshot of local ecology. Even so, Tyrannosaurus does seem to be abundant in each section of the Hell Creek Formation that was sampled, and the authors of the new paper suggest that this was because the dinosaur was an opportunistic feeder.
Contrary to the conclusions of Carbone and colleagues, the PLoS One study proposes that Tyrannosaurus scavenged regularly. How else could the area have supported so many tyrant dinosaurs? "Tyrannosaurus may have acquired a larger percentage of meat from carrion sources than did smaller theropods," Horner and co-authors suggested, "therefore filling the role of a more generalized, carnivorous opportunist such as a hyena."
The conclusion of the new paper corresponds with what Holtz suggested several years ago, but frustratingly, Horner and colleagues do not specify what kind of hyena they imagine Tyrannosaurus as. This is not just a bit of nit-picking. Despite their reputations of scavengers, the large spotted hyenas actually obtain the majority of their prey by hunting. The degree to which spotted hyenas hunt varies from place to place, but carrion may make up as little as five percent of the diet of some populations, such as Kenya's "Talek clan." The smaller brown and striped hyenas, by contrast, are primarily scavengers that also take live prey when they can. Horner, Goodwin, and Myhrvold do not specify which species they are talking about—they refer to hyenas in a general sense—and so their exact idea of Tyrannosaurus feeding habits is left unclear.
Significantly, though, the authors of the PLoS One paper note that the feeding habits of individual Tyrannosaurus may have changed as they grew. Young Tyrannosaurus may have been more predatory, whereas the more powerful jaws of adult individuals allowed them to more effectively scavenge, meaning that Tyrannosaurus actually occupied a range of predatory niches throughout its life. Perhaps this is why smaller predatory dinosaurs are relatively rare in the Fort Peck Reservoir deposits: young Tyrannosaurus may have filled the "small predator" role.
That Tyrannosaurus was an opportunistic carnivore that both hunted and scavenged isn't news. Paleontologists have been saying this in response to Horner's "obligate scavenging" hypothesis for years, and Holtz specifically drew comparisons to predators like spotted hyenas. What is noteworthy is that Horner appears to have softened his original hypothesis to the point where I was surprised that Holtz's paper was not cited as a more direct source of support for Tyrannosaurus as an opportunistic feeder. The abundance of Tyrannosaurus in the Fort Peck Reservoir area is a significant surprise, but the paper's conclusions about the lifestyle of Tyrannosaurus is not as shocking as news reports made them out to be.
Cooper, S., Holekamp, K., & Smale, L. (1999). A seasonal feast: long-term analysis of feeding behaviour in the spotted hyaena (Crocuta crocuta) African Journal of Ecology, 37 (2), 149-160 DOI: 10.1046/j.1365-2028.1999.00161.x
Hayward, M. (2006). Prey preferences of the spotted hyaena (Crocuta crocuta) and degree of dietary overlap with the lion (Panthera leo) Journal of Zoology, 270 (4), 606-614 DOI: 10.1111/j.1469-7998.2006.00183.x
Holtz, T.R. 2008. "A Critical Reappraisal of the Obligate Scavenging Hypothesis for Tyrannosaurus rex and Other Tyrant Dinosaurs." in Larson, P. and Carpenter, K. (eds) Tyrannosaurus rex: The Tyrant King. Bloomington: Indiana University Press.
Horner, J.R. 1994. "Steak Knives, Beady Eyes, and Tiny Little Arms (A Portrait of T. rex as a Scavenger." in Rosenberg, G.D. and Wolberg, D.L. (eds) Dino Fest. The Paleontological Society Special Publication No. 7.
Horner, J., Goodwin, M., & Myhrvold, N. (2011). Dinosaur Census Reveals Abundant Tyrannosaurus and Rare Ontogenetic Stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016574
LINGHAM-SOLIAR, T. (1998). Guess who's coming to dinner: A portrait of Tyrannosaurus as a predator Geology Today, 14 (1), 16-20 DOI: 10.1046/j.1365-2451.1998.014001016.x