Carnivorous Dinosaurs Like Allosaurus Were Cannibals

Not far from the Colorado-Utah border, the Mygatt-Moore Quarry bonebed is brimming with hundreds of fossils from Jurassic favorites like Apatosaurus and Allosaurus. Similar dinos have been found at spots around the American West, but the carnivores found here must have been hungry. Dozens of bones bear the toothmarks of massive meat-eaters—including the bones of carnivores themselves, suggesting at least a little dinosaur cannibalism.

“The site probably smelled terrible”

Bitten bones and broken teeth are rare finds at dinosaurs digs. That’s because rapid burial is usually required for fossil preservation, and if that happens, then scavengers don’t get much chance to pick at the free meat. However, in a survey of more than 2,000 bones from Mygatt-Moore published today in the journal PLOS ONE, University of Tennessee-Knoxville paleontologist Stephanie Drumheller-Horton and colleagues found that 28 percent of the fossils were punctured, bitten and scratched by carnivores, a far greater percentage than other sites in the Morrison Formation, a huge span of rock in the American West that contains the Mygatt-Moore.

“While tooth traces in bone are not uncommon in the Morrison, the sheer number of chewed bones at Mygatt-Moore is surprising,” says University of Wisconsin Oshkosh paleontologist Joseph Peterson, who was not involved in the study. The dinosaurs here acted differently, and the environment might offer a clue as to why.

During the part of the Jurassic encapsulated by the Morrison Formation (146 to 156 million years ago) the area now preserved as the Mygatt-Moore quarry would have changed dramatically with the annual wet and dry seasons. In rainy months, the spot was probably a watering hole. The fact that fish, amphibians, crocodiles and other aquatic species are rare in the deposit suggests the water evaporated in the dry season. This setting may have given carnivores more of a chance to pick at carcasses before the returning rainstorms of the wet season washed enough sediment over the bones to bury them and preserve them as fossils. Drumheller-Horton says other details on the fossil bones, such as signs of trampling, indicate they were exposed for a stretch before burial.

“We think that carcasses would have persisted on the landscape for a pretty long time,” she says. “We’ve been joking that the site probably smelled terrible.”

The evidence also suggests the carnivores that left teeth marks at the site weren’t in a feeding frenzy but took each bite with intent. “Predators will usually target the high-economy anatomical regions first, like the viscera and the meatiest long bones,” Drumheller-Horton says, “and then work down to the lowest-economy bones, like toes.” If paleontologists find bite marks on the parts without much good meat, then the carnivore was probably late to the party and the more desirable parts were already gone. At Mygatt-Moore, Drumheller-Horton says, the team found a smattering of bite marks all over, so some carnivores may have taken down the prey and gotten first pick while others were left to gnaw on the carcasses later.

When Allosaurus eats Allosaurus

Many of the bones with bite marks come from Apatosaurus, a long-necked herbivore. But the team also found tooth marks on the bones of the carnivorous Allosaurus.

“The pattern of bite marks indicates that non-theropods have them in nutritious areas of the skeleton, whereas theropods elements tend to be better in lower-nutrition areas,” says bone injury specialist Ewan Wolff, who was not involved in the study. In other words: Whatever was biting the Allosaurus would seem to have gotten to them late, or at least focused on areas that didn’t have much flesh on them.

The question is: Who bit them?

The three-horned carnivore Ceratosaurus could have made the marks, but Ceratosaurus is rare at Mygatt-Moore. The only other carnivore that could have made them is Allosaurus itself, which is much more abundant at the site and would mean Allosaurus were eating some of their own.

Why would Allosaurus eat other Allosaurus? Evidence of dinosaur cannibalism is rare in the fossil record. To date, only two other predatory dinosaurs—Tyrannosaurus and Majungasaurus—have been shown to feed on the carcasses of their own species. Drumheller-Horton notes that cannibalism isn’t all that rare among modern carnivores, though. “Almost no predator will turn down a free meal, so the line between predators and scavengers is fuzzy at best,” she notes.

The bite marks at the Mygatt-Moore quarry might represent dinosaurs under stress, such as droughts and fires. “The unusually high frequencies of bites we found might be evidence of carnivores trying to scrounge up every available resource to survive rougher parts of the year,” Drumheller-Horton says. Similar trends have been found at places like the La Brea asphalt seeps in Los Angeles, where stressful ecological times match up with a greater number of carnivores gnawing on bones.

Ancient bites, new insights

Scientists once thought dinosaurs like Allosaurus rarely ate bones because their bite marks are uncommon at other quarries. The Mygatt-Moore findings rewrite that story. “It is easy to look at a predatory dinosaur like Allosaurus and make a lot of assumptions about how they lived,” Peterson says. Paleontologists have been studying Late Jurassic bonebeds and the dinosaurs within since the 1800s, Peterson notes, yet studies like this new one highlight just how little we know about these familiar environments.

These findings also might revise how researchers approach other dinosaur sites. In the past, Drumheller-Horton notes, expeditions often focused on well-preserved bones suitable for display or anatomical study. Bones that were damaged or not as aesthetically-pleasing were often left behind or even destroyed. The new insights from Mygatt-Moore partly came from a bulk collecting effort, including damaged and beaten bones in the sample.

Besides, the toothmarks may reveal the presence of carnivores that have yet to be seen in the bonebed. Striations on one particular bone, created by the serrations of a carnivore’s tooth, are too big to have been created by an average Allosaurus. The biter was either an exceptionally large Allosaurus, a much rarer Jurassic predator called Torvosaurus, or a dinosaur not yet seen. One predator’s bite has left paleontologists with a new mystery to solve.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.