Desperate Dinosaurs Turned to Cannibalism to Survive, Study Suggests

Cannibalism: It’s frequently what’s for dinner in the animal kingdom. Eating other people may be a taboo in human cultures, but it is a common feeding strategy for all kinds of creatures such as spiders, sharks, prairie dogs, lions, and chimpanzees, among many others.

Scientists assume that extinct animals, like dinosaurs, also regularly dined on the flesh of conspecifics, but it’s very difficult to identify clear signs of these intraspecies meals in the fossil record.

Now, a team of paleontologists has managed to do just that by meticulously examining more than 2,300 bones for signs of bite marks from meat-eating dinosaurs. The results yielded “powerful evidence of cannibalism in Allosaurus,” one of the Jurassic’s most abundant apex predators, which “represents the first time cannibalism has been reported in this taxon and its encompassing clade,” according to a study published on Wednesday in the journal PLOS ONE.

Paleontologists led by Stephanie Drumheller, a paleontologist at the University of Tennessee, Knoxville, obtained this rare glimpse of dinosaur cannibalism by searching for bites on thousands of bones sourced from the Mygatt-Moore Quarry in Colorado.

“Previous studies of dinosaur-dominated sites reported a very low frequency of bite-marked bones,” as in under five percent, Drumheller said in an email. "This led to interpretations that maybe dinosaurs didn’t target bone as a food resource, and that any bite marks they left were essentially accidental.”

“Our assemblage has nearly 30 percent of the bones preserving bite marks,” she continued. “This is more in line with mammalian predators, who actively seek out marrow as a resource, or crocodylians, who smash and twist off sections of prey, swallowing bones and fragments whole.”

There are a few possible reasons why the Mygatt-Moore Quarry contains such an abnormally high number of fossils etched with bite marks, which totalled 684 specimens in this study. One explanation is the recent switch to a technique of “bulk collection” at the site, meaning the extraction of practically every specimen from the quarry. This contrasts with normal procedures that prioritize the collection of fossils with perceived scientific or aesthetic significance.

“In a perfect world where money, space, and person-hours are no object, we would love to collect every single fossil at a site, from the most perfectly preserved, articulated skeletons down to the most battered, broken piece of bone,” Drumheller explained.

“Logistically, this isn’t often possible,” she said, adding the gentle hint: “Support your local museums.” (For instance, the Museums of Western Colorado is the institution housing all these bulk fossils.)

As Drumheller’s team has demonstrated, major insights can be extracted from fossils that aren’t destined for a special public display case at a museum. Perhaps other sites also contain plentiful bite marks, but the fossils that could have told that story were left behind in favor of prettier or more intact specimens.

However, it’s also possible that this ancient habitat was genuinely different in some way that caused flesh-eating dinosaurs to scavenge dead animals at a higher rate than has been seen elsewhere. If live prey was relatively scarce in this area, for instance, theropod dinosaurs such as Allosaurus and Ceratosaurus may have resorted to scavenging more frequently to survive.

“If something odd is going on at Mygatt-Moore, such as a stressed environment leading theropods to eat every available resource, that also tells us something different about the environment,” Drumheller said. “We knew previously that the Mygatt-Moore ecosystem experienced wet and dry seasons, and other Morrison Formation sites preserve evidence of major droughts and fires, so how did that affect the animals living there?”

“Maybe when times were rough, they ate absolutely everything they could, whether that meant scavenging remains that were no longer fresh or even resorting to cannibalism,” she speculated.

While evidence of dinosaur cannibalism has been reported before—especially from the charismatic bone-cruncher Tyrannosaurus—the new study paints a more intricate portrait of a full ecosystem in which this type of feeding strategy was common.

The team was able to reconstruct these details through many techniques, including looking at the “scavenging sequence,” which is a pattern that helps distinguish between tooth-marks left in active attacks of live prey and bites inflicted while eating a carcass. The most nourishing “high-economy” parts of an animal's body are normally consumed first, often immediately after a kill, while the tougher “low-economy” bits are chewed over later by scavengers.

About 55 percent of the bite marks found on both the plant-eating prey species and the meat-eating predators fell into the “low-economy” category, suggesting a high rate of scavenging. The abundance of presumed Allosaurus bites in the sample, often found on other Allosaurus bones, is what led the team to the conclusion that this iconic predator fed on the dead bodies of its own kind.

It will take more research to figure out whether the Mygatt-Moore Quarry yielded such unusual results because of the team’s collecting technique, a resource-stressed Jurassic environment, or perhaps some combination of these factors.

“We are hoping to expand this type of collection and research at other dinosaur-dominated sites,” said Drumheller. “Right now, we don’t know if what we are seeing is ‘normal’ for a Mesozoic assemblage, and the differences we are seeing are related to collector bias, or if there was truly something odd going on in the Mygatt-Moore ecosystem, which was driving the theropods there to more completely consume available carcasses.”

“This type of study is more common with more recent deposits, like in zooarchaeology,” she concluded, “but it’s very rare in paleontological research.”