Cannibalism: A Perfectly Natural History(22)
“Quickly enough, you start thinking about who could have made these bite marks, and [in this instance] there are very few candidates.” According to Rogers, there were only a few large carnivores living at the site, which they named MAD05-42. One was a crocodile, “which would have made no comparable traces on the bones” and the other a small theropod dinosaur, “which had tiny little teeth.”
“And then you’ve got Majungasaurus, which had large teeth. When you look at the scoring patterns on [Majungasaurus] bones you can match them to the spacing of Majungasaurus teeth and their actual denticle patterns.”
“So there’s no potential that you might be missing another large predator—something you just haven’t dug up yet?” I asked.
“Right, I don’t think we’re missing anything. And if we are missing something it would have to be big, and arguably it would have to be rare. But the bite marks are anything but rare. So . . . whatever it is, it would have to be really big, really cryptic, really rare, and it would have to bite everything, which doesn’t make any sense. Basically, there are tons of bite marks that match the teeth of Majungasaurus and nothing else matches those traces.”
I pressed on. “So how do you know that these tooth marks on Majungasaurus bones weren’t made during combat between conspecifics?”
Rogers said he had good evidence to prove that hadn’t been the case. Elements of the vertebral column showed evidence of having been scraped and possibly gnawed. This type of “late stage scavenging” took place after the limb muscles and the guts had been consumed, and when the scavenger had to work hard to obtain any further nutrition from the carcass.
“There’s no evidence, whatsoever, for killing or jostling with conspecifics. There’s evidence for feeding. And the evidence we have for feeding is consistent with some pretty focused effort. These bones were scraped.”
It was beginning to sound like Majungasaurus had been following two of Gary Polis’s cannibalism-related generalizations: first, that incidents of the behavior increased during stressful environmental conditions; and second, that the absence of alternative food sources often led to cannibalism. So I asked Rogers if he believed that stressful environmental conditions could have driven Majungasaurus to scavenge conspecifics.
“It kind of looks that way,” he replied. “The overall reconstruction that we put together is pretty well supported. It seems like these ancient ecological systems were devastated again and again and a lot of things died. I think that if you’re a large meat-eating dinosaur, you’ll capitalize on whatever there is to feed upon, and I have no doubts that a creature like Majungasaurus would take the resources that were available.”
Finally, I asked the paleontologist how prevalent he thought cannibalism had been among the dinosaurs.
“I doubt that it was uncommon,” Rogers replied. “What’s uncommon is evidence. Because I think that more often than not, animals can probably get their meals without heavily working bone, and when you’re dealing with fossils . . . that’s all you got. If it’s not written on the bone, you’re not going to get the story. So . . . I don’t think cannibalism was unique to any particular theropod group. When you drive around Montana today and see ground squirrels eating ground squirrels and you read papers about dogs eating dogs, and lizards eating lizards . . . it’s pervasive. I have no doubts that cannibalism was widely practiced by dinosaurs. The fact that there have only been two cases of dinosaur cannibalism . . . that’s just an artifact of paleontology and the [scarcity of the] fossil record.”
So was dinosaur cannibalism a rare event? There appear to be two contrasting issues here. First, knowing what we do about the prevalence of cannibalism in the animal kingdom, it makes sense that dinosaurs might have exhibited the behavior for the same reasons other animals do—namely overcrowding, predation, competition, and hunger. Alternately, cannibalism is relatively rare in birds, the only surviving link to the Mesozoic dinosaurs. The problem with comparing the two groups, though, is that birds are gape-limited predators while carnivorous dinosaurs certainly were not.
Then there’s the lack of widespread physical evidence that dinosaur cannibalism occurred, and because of this, some paleontologists, like Mark Norell and his colleagues, are unwilling to make any sort of speculative leap. Instead their explanations for the strange bite marks on ancient T. rex bones fall back onto behavior they do have evidence for—like fighting. According to Norell, “There’s nothing like a smoking gun that anyone has ever presented to me and said, ‘This is it!’ ”
“Can you give me an example of a paleontological ‘this is it’ moment?” I asked.
Norell brought up the now-generally-accepted claim that modern birds were actually theropod dinosaurs and would, therefore, show similar anatomical and behavior traits.
“We found evidence that dinosaurs sat on their nests,” he said. “We were able to show that. Dinosaur feathers—we were able to show that. In our lab we tend to be incredibly careful about what we say about this stuff . . . but no one has ever been able to come up with a total case for dinosaur cannibalism, like a member of the same species that’s inside the body cavity. Like Coelophysis was supposed to be.”
In a telling side story, Raymond Rogers provided another example of just how attractive the word cannibalism is to the media. “I took this story of dinosaur cannibalism to the Society of Vertebrate Paleontology meetings, and I called it “Conspecific Scavenging,” which is what I think it is. I remember that a guy from Science News looked at it, but nobody else really took much notice at all. I went home and thought about it, and I was like, ‘You know, why don’t I just call it cannibalism?’ So I did . . . and after that the story got in Nature and it was on the front page of Google News for about a week.”