A comprehensive analysis of thousands of fossilized bones excavated from a Wyoming bonebed has provided rare physical evidence that *Tyrannosaurus rex* fed on the carcasses of the duck-billed dinosaur *Edmontosaurus annectens*. The findings, published today in the journal *PLOS ONE*, suggest that these apex predators scavenged the remains of the herbivores during the Late Cretaceous period, approximately 72 to 66 million years ago.
The study, led by paleontologist Bethania Siviero of Loma Linda University and her colleagues, examined 3,013 bones collected between 1997 and 2017 from the Lance Formation near Hanson Ranch Station. The research team aimed to distinguish genuine bite marks from other bone modifications, such as disease, erosion, or natural anatomical features.
Rigorous Criteria for Identifying Tooth Traces
The researchers emphasized that identifying the origin of perforating lesions on fossil bone is often difficult, as many features previously labeled as tooth traces may actually be the result of post-mortem processes or biological factors like joint disease.
To address this, the team developed a refined set of criteria for future paleontological research. Their methodology involves assessing a mark’s shape, depth, edges, surface texture, and location, while also checking for evidence of healing in the surrounding bone.
A notable example of the importance of these standards was a jawbone fragment initially thought to bear crocodile bite marks. Upon closer inspection, including CT scans, the team determined the holes were actually natural passageways for blood vessels and nerves. Furthermore, the bone was re-identified as belonging to a triceratopsin rather than an *Edmontosaurus*.
Evidence of Scavenging Behavior
Out of the 3,013 bones analyzed, the researchers confirmed genuine tooth traces on 12 specimens, including ribs, vertebrae, a radius, and an ulna. Of those, four bones preserved distinct patterns—including fine grooves and curved scores—that allowed the team to attribute the bites to *Tyrannosaurus rex*.
The team compared the spacing and serration patterns of the marks against the tooth structure of other known carnivores from the same ecosystem, such as *Nanotyrannus*, *Pectinodon*, and *Acheroraptor*. The wider spacing of *T. rex* teeth matched the fossil evidence, while other predators were excluded due to their denser serration patterns.
Crucially, the study found that most of the bitten bones showed no signs of healing. This lack of biological response indicates that the injuries were sustained at or near the time of death, supporting the conclusion that the *T. rex* acted as a scavenger. While the researchers noted that they could not rule out active predation in every instance, the overall evidence points toward scavenging, potentially involving the consumption of internal organs.
Refining the Prehistoric Ecosystem
The bonebed, which is overwhelmingly composed of *Edmontosaurus annectens* remains, offers a window into the interactions of the Late Cretaceous. The study notes that fewer than one-half of one percent of the bones exhibited unambiguous tooth marks, highlighting that such evidence is rare.
The authors of the study, which includes Bethania C. T. Siviero, Elizabeth Rega, Matthew A. McLain, Leonard R. Brand, David Nelsen, and Art V. Chadwick, stated that distinguishing between different types of bone modifications is essential for understanding both the condition of an animal before death and the environmental processes that affected its remains afterward.
By establishing a clear, standardized guide for identifying bite marks, the researchers hope to reduce the “misleading interpretations of animal behavior” that have historically complicated the study of fossilized remains. This effort to categorize punctures, holes, and furrows serves as a vital tool for future efforts to piece together the complexities of extinct ecosystems.
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