Cannibalism: A Perfectly Natural History(21)
Coelophysis bauri was one of the earliest dinosaurs—a carnivorous and remarkably birdlike biped that lived approximately 200 million years ago across what is now the southwestern United States. A fast runner, it stood about a meter tall at the hips and had an overall length of about three meters from snout to tail. Equipped with a mouthful of recurved and bladelike teeth, Coelophysis was thought to feed on smaller animals like lizards.
In 1947, a team from the American Museum of Natural History (AMNH) working at Ghost Ranch in New Mexico unearthed a huge bone bed composed of hundreds of Coelophysis skeletons. After examining the fossils, famed AMNH paleontologist Edwin Colbert claimed that the abdominal cavities of some of the specimens contained the bones of smaller individuals of the same species. Thus was born the “cannibal-Coelophysis hypothesis” and the subsequent portrayal of Coelophysis and other dinosaurs as cannibals. Reminiscent of the misconceptions concerning male-munching black widow spiders, the depiction of dinosaurs as cannibals remained unchallenged for decades.
In 2005, another group of researchers from the AMNH set out to determine whether or not claims of dinosaur cannibalism could be supported. Led by paleontologists Sterling Nesbitt and Mark Norell, they performed detailed morphological and histological analyses of the bones (something Colbert did not do). Soon enough, the scientists uncovered a slight problem—not only were the bones in question not from juvenile specimens of Coelophysis, they weren’t even dinosaur bones. Instead, the fragments recovered from the abdominal cavities of the two relevant Coelophysis specimens belonged to crocodylomorphs, a group that includes crocodiles and their extinct relatives—but not dinosaurs.
Investigating further, Nesbitt and Norell determined that another example of reputed dinosaur cannibalism was also problematic. In a much-publicized case, paleontologist Aase Jacobsen reported in 1998 that bite marks on the bones of tyrannosaurs from the Dinosaur Park Formation in Alberta, Canada, were consistent with bite marks from conspecifics. The AMNH researchers, however, called this cannibalism claim into question by pointing out that there were actually two tyrannosaur genera, Gorgosaurus and Daspletosaurus, living at the site. Since even by its loosest definition, cannibalism requires that the participants be conspecifics, the potential that the predation took place between separate species led Nesbitt and Norell to raise a warning flag. They concluded that while cannibalism “may be expected in non-avian dinosaurs,” it was “not as prevalent as was once supposed.”
In the limited realm of dinosaur cannibalism, an additional piece of evidence came from paleontologist Nicholas Longrich and several high profile co-authors in 2010. The researchers discovered bite marks on four museum specimens of Tyrannosaurus rex. They reasoned that since T. rex was the only large predator alive at that time, the score marks and gouges on the skeletons must have been made by conspecifics, and that the most likely scenario was that the marks had been made during scavenging of carcasses. Longrich and his coworkers concluded that their results provided solid evidence that “cannibalism seems to have been a surprisingly common behavior in Tyrannosaurus, and this behavior may have been relatively common in carnivorous dinosaurs.”
I interviewed Mark Norell on a beautiful mid-September afternoon at the American Museum of Natural History. His lab is a dinosaur lover’s dream—a remarkable, fossil-filled space that opens onto one of the museum’s famous turrets. The view from Norell’s high-ceilinged office was nothing short of spectacular, and as we talked it was difficult not to look over his shoulder at the wide swath of Central Park below us.
“I think there’s very little evidence at all for dinosaur cannibalism,” Norell told me. “Although a lot of it really depends on what you’d call cannibalism. If a tyrannosaur dies and another tyrannosaur comes along and eats it, is that cannibalism? Or is that just scavenging a dead carcass? I have a picture around here someplace of a camel eating a dead camel that was lying there. Is that cannibalism?”
I told him that I didn’t consider hunting and killing to be prerequisites for cannibalism and that, from what I’d learned, scavenging your own species was cannibalism. I used the example of besieged cities, where the victims of starvation or exposure were consumed, sometimes by their own relatives.
As scientists began to study cannibalism in nature, some of the behaviors they observed expanded on the definition of cannibalism coined by Elgar and Crespi in the early 1990s. The requirement that the cannibal kill the conspecific before eating it was dropped (enabling certain forms of scavenging to be considered cannibalism), as was the necessity that the cannibal consume the entire victim. As a result, some forms of behavior now straddle a line between cannibalism and something else—in this case, scavenging.
But even allowing for a broad definition of cannibalism, in the case of dinosaurs, Norell had some major problems with the evidence that Longrich had presented. For example, he said that the gouges on Longrich’s T. rex bones could have been inflicted by conspecifics fighting each other, not necessarily eating each other.
According to Norell, the only compelling evidence for dinosaur cannibalism appeared to have occurred in the late Cretaceous theropod Majungasaurus crenatissimus, an example uncovered by geologist Raymond Rogers from multiple bone beds from a Madagascan rock formation thought to be between 70.6 and 65.5 million years of age.
In a 2014 phone conversation, I asked Rogers how he had come to the conclusion that Majungasaurus was a cannibal. He explained that he’d been looking at dinosaur bones for decades and that specimens from a particular region in Madagascar (dated to approximately 70 million years ago) had a remarkably high number of teeth marks.