A 75-million-year-old fossil reveals a shocking tyrannosaur secret

This finding comes from a study by the Department of Geoscience at Aarhus University, recently published in the journal Evolving Earth.

Fossil Bite Marks Reveal Cannibal Feeding

The research was led by Josephine Nielsen, a Master's student in geoscience. She used advanced 3D scanning techniques to examine a fossilized metatarsal (foot bone) that belonged to a large tyrannosaur more than 75 million years ago. Her analysis uncovered 16 distinct bite marks on the bone.

"I have analyzed the depth, angle, and placement of the marks in a virtual 3D environment and can document that these bite marks did not occur by chance. They are precise impressions from the teeth of a smaller tyrannosaur that fed on a much larger relative," says Josephine Nielsen.

The results suggest that tyrannosaurs did not let resources go to waste. Even tough bones with little meat were consumed late in the decay process, after most of the soft tissue had already been eaten.

"The bone shows no signs of healing after the smaller dinosaur bites into it. Since the marks are located on the foot, where there is very little meat, it suggests that the dinosaur was 'cleaning up' and eating the last remains of an old carcass," she explains.

Digital Models Help Decode Ancient Behavior

Rather than working directly with the original fossil, Nielsen studied a digital model and a 3D-printed version produced at Aarhus University.

"It would, of course, have been a special experience to work with the real bone, but it is far too risky to send it through the mail to Denmark," she explains.

The metatarsal measures 10 centimeters in length and came from a tyrannosaur that likely reached 10-12 meters and weighed several tons in life. The fossil was discovered by an amateur collector in the Judith River Formation in Montana, a region known for preserving a rich record of a 75-million-year-old ecosystem. It has since been donated to the Badlands Dinosaur Museum in Dickinson, North Dakota.

Using digital tools provided key advantages. High-resolution scans allowed Nielsen to closely inspect tiny details that would be difficult to see otherwise.

"What makes this study special is not just the knowledge of how the food chain functioned among dinosaurs millions of years ago, but the technique used to read the details. By creating a digital version, I've been able to zoom in on very small details. To ensure the analysis was objective, I used the systematic CM (Category-Modifier) classification system. This method categorizes each individual mark based on fixed criteria, allowing us to distinguish everything from glancing tooth strikes to deep crushing bites. It has been like solving an ancient murder mystery, with metatarsal evidence."

Technology Brings New Insight Into Dinosaur Life

Nielsen's work was supported by Canadian paleontologist Taia Wyenberg-Henzler and museum curator Denver Fowler, who served as external supervisors during the undergraduate project that led to the published study.

"I got in touch with Denver Fowler and Taia Wyenberg-Henzler while volunteering at an excavation camp in Montana in the summer of 2024. This set me on the path of my project, and it has been incredibly valuable to build international relationships already during my studies," says Josephine Nielsen.

The study highlights how modern techniques can reveal detailed information about dinosaur behavior from even small traces preserved in fossils.

"Now we can extract detailed information about their behavior from quite small traces. By using the CM system, we have established a common scientific language to describe bite marks. This means we are no longer just guessing that 'it looks like a bite,' but can precisely document when and why the small tyrannosaur sank its teeth into the large one," says Josephine Nielsen.