About a three hour drive to the east of where I’m sitting as I write this, low down on the famous quarry wall of Dinosaur National Monument, there’s an unusual bone. 

It’s not the shape of the bone, or even that species, that makes it odd. It’s the postmortem fate of the Jurassic antique that makes it stand out. You can even see the clues from the gallery floor. The femur, just one of many in the immense osteological logjam and cataloged as DINO 5119, is marked by multiple grooves running almost perpendicular to the bone’s orientation, as well as smaller scratches running in a different direction - bite marks. In over 1,500 vertebrate remains found at the quarry, this slightly-masticated bone is the only one of its kind.

But who bit the bone? Given the size of the grooves running across the thigh bone, there are at least three contenders. The bones of Allosaurus have been found in the same quarry, and this large predator was the most common carnivore of its time. The odds are with Allosaurus. But paleontologists have also turned up the bones of the tri-horned Ceratosaurus and the monstrous Torvosaurus from the same deposit. Allosaurus, Ceratosaurus, and Torvosaurus were the big three of their time, the apex predators of the fern-covered floodplains now preserved as the Morrison Formation across the west, and any of them could have made those grooves. 

There’s also the question of how the culprit did so. A bite mark is prehistoric behavior. It records how a creature proceeded with stuffing its craw with food. In this case, how a carnivore stripped flesh. Previous studies have cataloged similar traces. Paleontologists have determined that tyrannosaurs not only crunched through bone, for example, but could also use closely-spaced teeth to deftly strip flesh from skeletons. We know something about their behavior from the traces they left behind. That means the bite marks on DINO 5119 are a way to look back to life about 149 million years ago, a few moments of Jurassic time.

DINO 5119. Credit: Brian Switek

To that end, paleontologists Dave Hone and Dan Chure took a detailed look at the sauropod femur to suss out who was doing the eating, and how they were doing it. As it turns out, determining a theropod’s identity through the dino damage they left behind is no easy task.

Even though the teeth of Allosaurus, Ceratosaurus, and Torvosaurusall differ from each other in details like cross section and serrations, the offending theropod in this case didn’t leave any teeth in the sauropod bone itself. There are only the bite marks, and a variety of factors - from angle of the bite to differences in height between the tooth crowns - confound pinning the bite marks on a specific dinosaur. Allosaurus is most likely given its abundance, but, Hone and Chure write, “it is not possible to make a confident attribution to one of the candidate theropod genera here.”

Yet while who was doing the biting is unclear, how they were biting is more in focus. It follows a pattern seen in other places and times. 

The feeding dinosaur wasn’t biting randomly. The bite marks, Hone and Chure write, are centered around where major muscle groups of the thigh attached. When fresh, this could have been a meaty prize, but the researchers note that the presence of bite marks at all - a rarity in deposits like these - might indicate that the carnivore was trying to detach the last available flesh from a bone that had mostly been denuded already. The big carnivores of the Morrison Formation might have been more like cats, in this way, preferentially feeding on soft parts only scraping bone when necessary. The bite marks on DINO 5119 might be signals of desperation.

The spacing and depth of the marks suggest that the feeding carnivore bit the femur at least twice, dragging its teeth through the outer layers of bone as it attempted to strip flesh off. This is unexpected for Allosaurus and its neighbors. Puncture-pull feeding - which is pretty much exactly what it sounds like - is a hallmark of the huge tyrannosaurs that would dominate western North America about 70 million years after the era of Allosaurus. The behavior wasn’t as common during Jurassic time, particularly because the big carnivores of the day hadn’t evolved the heavy-duty skulls that allowed the likes of Tyrannosaurus to pulverize bone so easily, but they still employed a similar method of stripping flesh close to the bone. 

What runs beneath these discussions of tooth spacing and bite scrapes is that carnivorous dinosaurs were careful feeders. They didn’t indiscriminately mouth carcasses like some stop-motion aberration of 20th century dino cinema. The big predators of the Late Jurassic had jaws that were both powerful and precise, removing flesh in a dedicated and even delicate manner. This leaves us with a hazy image of Morrison Formation time that is nonetheless underscored by science. 

Imagine an Allosaurus stalking along a dried out floodplain, snuffling at desiccated bones and body parts scattered here and there - the remains of other dinosaurs that died in the intense seasonal drought. One isolated femur, dismembered from its original body, still has some rotten flesh hanging from the end. It’s hardly a mouthful, but, bracing the bone with its hind foot, the carnivore dips its head and drags its front teeth across the femur, scraping up bone and tattered flesh before repositioning to do so again. The Allosaurus will have to find more if its going to survive the height of the dry season, and so, the morsels only stoking the carnivore’s hunger, the dinosaur snorts and continues along in hopes of a fetid breeze that will lead it to something meatier.