The mark of scavenging, written in unhealed bone.
Sixty-six million years after the last Cretaceous sunset, the teeth of Tyrannosaurus rex have left their testimony in Wyoming stone. Paleontologists examining thousands of fossilized bones from a Lance Formation bonebed have found rare, direct evidence that the great predator fed not as a killer but as a scavenger, consuming duck-billed Edmontosaurus carcasses that had already passed beyond life. The discovery reminds us that even the mightiest creatures in Earth's history were bound by the same ancient economies of survival — and that the difference between hunter and scavenger is a distinction that reshapes how we understand an entire lost world.
- Direct physical evidence of what T. rex actually ate is vanishingly rare, making every confirmed bite mark a significant crack in the fossil record's silence.
- Out of more than 3,000 examined bones, only twelve carried genuine tooth traces — a reminder of how easily researchers can be misled by natural bone damage masquerading as predation.
- Four bones preserved mark patterns precise enough to be matched to T. rex dental anatomy, anchoring the identification to something more than inference.
- The absence of healed bone around the bite sites is the critical clue: these animals were not attacked and survived, nor were they killed in the act — they were already dead when the predator arrived.
- Beyond the behavioral finding, the study sharpens the scientific tools themselves, offering clearer standards for distinguishing real bite marks from false ones in future fossil research.
In northeastern Wyoming, a bonebed buried in Cretaceous rock has yielded something paleontologists rarely find: physical proof of what an extinct predator actually ate. Over two decades, Bethania Siviero of Loma Linda University and her colleagues collected 3,013 fossilized bones from the Lance Formation near Hanson Ranch Station, eventually turning their attention to marks that might have originated from teeth.
The work demanded caution. Natural features and post-depositional damage frequently mimic bite marks on fossil bone, and such misidentifications have produced false conclusions about ancient animal behavior before. Applying CT scanning and rigorous comparative analysis, the team winnowed their candidates down to twelve bones — ribs, vertebrae, and limb elements — bearing genuine tooth traces. Four of these preserved patterns precise enough to be matched against known ichnospecies, and by comparing the spacing and geometry of the marks to predators known from the same ecosystem, the researchers identified Tyrannosaurus rex as the most probable source.
What the marks revealed about behavior proved equally significant. Nearly all of the bitten bones showed no signs of healing — no remodeling, no evidence of survival. The animals had been bitten at or after the moment of death, meaning T. rex was not the cause of their end. It was feeding on creatures that had already died from other causes, scavenging rather than hunting. That distinction carries real weight for understanding how these animals actually functioned within their ecosystem.
The study, published in PLoS ONE, also offered the broader field a more rigorous framework for identifying tooth marks on fossil bone — a methodological contribution that may prove as durable as the behavioral finding itself.
In northeastern Wyoming, buried in rock layers from the Cretaceous period, lies a bonebed that tells a story of death and hunger written in tooth marks. Paleontologists examining 3,013 fossilized bones from a site near Hanson Ranch Station have found rare physical evidence that Tyrannosaurus rex scavenged the carcasses of Edmontosaurus annectens, a large duck-billed plant-eater that roamed the same landscape roughly 66 million years ago.
The discovery matters because direct evidence of what extinct predators actually ate is scarce. Researchers led by Bethania Siviero of Loma Linda University spent two decades collecting bones from the Lance Formation, and when they began examining them closely, they found something unexpected: marks that looked like they might have come from teeth. But identifying such traces is treacherous work. Many depressions and perforations on fossil bone get mistaken for bite marks when they are actually natural features or damage from other causes. These misidentifications have led paleontologists astray before, producing false conclusions about how ancient animals behaved.
Out of the 3,013 bones, only thirteen showed marks that resembled tooth traces at all. When Siviero's team applied CT scanning and rigorous analysis, they eliminated one as a natural anatomical feature. That left twelve bones with genuine bite marks—ribs, vertebrae, a radius and an ulna. Four of these bones preserved distinctive patterns that matched known tooth trace patterns, called ichnospecies. By comparing the spacing and geometry of these marks to the dental anatomy of predators known to have lived in the same ecosystem, the researchers concluded that Tyrannosaurus rex was the most likely culprit.
But the bite marks told a larger story about behavior. Most of the bitten bones showed no signs of healing—no bone remodeling, no evidence that the animal had survived the injury. This pattern pointed to a specific conclusion: the dinosaurs were bitten around the time they died or after death had already occurred. They were not killed by the T. rex that fed on them. Instead, the predator was scavenging, consuming animals that had already perished from other causes. This distinction between active predation and scavenging is fundamental to understanding how these creatures lived and died.
The work also served a second purpose. Siviero and her colleagues used the Wyoming bonebed to refine the scientific standards by which paleontologists identify tooth marks on fossil bones. Correctly distinguishing genuine bite marks from other bone damage is crucial because misidentification can lead to false interpretations of predator-prey relationships and ecosystem dynamics. The team published their findings in PLoS ONE, offering both specific evidence of T. rex scavenging behavior and a clearer framework for future researchers trying to read the marks that ancient teeth left behind.
Notable Quotes
Correctly identifying bone depressions and perforations is important because not all of these features are tooth marks.— Bethania Siviero and colleagues
The Hearth Conversation Another angle on the story
Why does it matter whether T. rex killed these dinosaurs or just ate them after they died?
Because it changes the entire picture of how the ecosystem worked. A scavenger plays a different ecological role than an active predator. It tells us something about T. rex's hunting strategy, its competition with other carnivores, and what food sources were actually available.
How confident are you that these marks came from T. rex and not some other predator?
The tooth spacing and the patterns they left match T. rex dental anatomy better than anything else that lived there. But we're not claiming absolute certainty—we're saying it's the most likely explanation given what we know about the animals present.
You said most bones showed no healing. What does that actually look like under a microscope?
No remodeling of the bone tissue around the bite. When an animal survives a bite, the bone responds by laying down new tissue to repair the damage. These bones just have the mark—clean, unhealed. It's the signature of post-mortem feeding.
Is this the first time we've known T. rex ate hadrosaurs?
Not the first time we've suspected it, but direct physical evidence like this is genuinely rare. Most of what we know comes from indirect sources—coprolites, or educated guesses about diet. This is different. These are the actual marks.
What was the hardest part of the analysis?
Distinguishing genuine tooth marks from everything else that can create holes and depressions in bone. Natural anatomical features, damage from fossilization, marks from other processes—they can all look similar if you're not careful. That's why the team had to develop clearer criteria.