Fossils found covered with ancient bite marks suggest at least one type of large dinosaur was in such dire straits, it began dining on members of its own species.
A remarkable 29 percent of 2,368 fossil bones unearthed since 1981 from the late Jurassic Mygatt-Moore Quarry have bite marks on them; this is six times more than is typically found at similar sites elsewhere, researchers report today in the journal PLOS One. The bones include evidence that Allosaurus, the most common carnivore at the site by far, was munching on its kin.
Cannibalism among dinosaurs is not necessarily surprising. Many large predators, such as crocodiles and alligators, eat members of their own species under certain circumstances. “Every major predator today will eat its own species, whether you’re a Komodo dragon or a bear or a lion,” says Mark Loewen, a paleontologist at the Natural History Museum of Utah in Salt Lake City. “If there’s a dead animal, meat eaters will eat it.”
What’s unusual is to find evidence of cannibalism in the fossil record, says lead author Stephanie Drumheller, a paleontologist at the University of Tennessee in Knoxville. “We only have good evidence for cannibalism in a couple of other theropod species.” Knowing when and where such gruesome events were happening can reveal key details about the state of prehistoric environments.
“Maybe something weird was going on this ecosystem, and these animals were having to work for every nutrient they could find, and were really extensively scavenging any remains left out on the landscape,” Drumheller suggests.
Her team believes the dinosaurs there may have died around an ephemeral waterhole that suffered through long periods of drought. Their carcasses were then buried by sediment, but it was a slow process.
“We keep joking that if you were able to go back in time and visit, it would probably just smell horrible because everything is telling us that these carcasses and remains would lay out on the landscape for long stretches of time,” she adds.
Bite marks tell the tale
The Mygatt-Moore fossil site is part of the Morrison Formation, an extensive layer of rocks that dates to about 150 million years ago. This layer stretches across the western United States and has been among the nation’s most productive sources of dinosaur fossils.
At most other major dinosaur sites in the Morrison, the bones bear far fewer bite marks. At the Cleveland-Lloyd Dinosaur Quarryin Utah, for example, “way less than 5 percent of the 20,000 bones actually have bite marks,” says Loewen, who described a new species of Allosaurus earlier this year and is not one of the study’s authors.
Mygatt-Moore, in contrast, “is a place that’s being fed upon,” he says. “What’s cool about this study is that they found lots and lots of theropod bite marks on bones. That means bodies were sitting out on the surface available for scavenging.”
While most of the 684 fossil bones with bite marks belonged to herbivorous long-necked dinosaurs called sauropods, 83 were bones that once belonged to theropod dinosaurs—members of a group that included all the carnivorous species.
The vast majority of carnivorous dinosaur bones at the quarry are thought to be those of the 30-foot-long killer Allosaurus, while a handful of others may be those of a more primitive predator called Ceratosaurus. Several other large theropods—Torvosaurus and Saurophaganax—have been found at other Morrison Formation fossil sites of around the same age.
“We have this hugely Allosaurus-dominated assemblage, with these theropod bite marks on everything,” says Drumheller. “It lets us know that at least some of the bite marks are coming from Allosaurus, and we are finding them on Allosaurus.” She believes all of the species of carnivorous dinosaurs here may have been consuming one another on occasion.
Munching on toes
While there were hints of Allosaurus cannibalism in work done several decades ago, the new study presents “the strongest evidence so far” that these dinosaurs were eating one another, says Thomas R. Holtz, a paleontologist who studies carnivorous dinosaurs at the University of Maryland in College Park.
Out of the various theropods in these late Jurassic ecosystems, Allosaurus “has the most robust teeth and is thus better suited for tooth-on-bone contact,” he argues. That, along with the shape and size of the scrapes and the specific striations created by the serrated teeth of Allosaurus, means “the weight of the evidence points to these being Allosaurus bites, and thus cannibalism.”
Drumheller and her co-author Julia McHugh of the Museums of Western Colorado, which manages the Mygatt-Moore Quarry, argue that in the majority of cases these allosaurs were probably eating their already-dead brethren, rather than killing and then devouring members of their own species. Many bite marks are on some of the scrappiest and least nutritious parts of the carcasses, such as toe bones, making a good case for scavengers picking over long-dead remains.
“Whoever is eating those parts was pretty late in the process of breaking down those remains, because you’d never go for a toe if you had the belly cavity still available,” she explains.
Holtz, who was not a study author, adds that the findings are also interesting because “unambiguous evidence of scavenging in dinosaurs is rare, as it is hard to directly document.”
Drumheller says that, while paleontologists typically hope to find fossil skeletons that are as pristine and complete as possible, it’s bones like those found at Mygatt-Moore that are “a little bit more dinged up” that really thrill her.
“Most people see this beat-up bone with gouges taken out of it and holes punched in it, and they think that’s awful, but I get giddy,” she says.
“If what you’re interested in is the environment and how these animals interacted—so who was eating whom, and what happened after they died—then the ugly stuff is even more useful.”