Scientists May Be Able to Sex Dinosaurs — Tail Fractures Offer a Surprising Clue

New research suggests healed tail fractures could identify female hadrosaurs

Researchers at Queen's University Belfast, working with international collaborators, report a recurring pattern of healed fractures in the tail vertebrae of hadrosaurs that may be linked to mating behaviour. The findings, published in iScience, are based on statistical analysis and computer simulations of roughly 500 pathological caudal vertebrae from hadrosaur specimens collected across North America, Europe and Russia.

Filippo Bertozzo—who began studying hadrosaur pathologies during his PhD at Queen's and now works at the Institute of Natural History in Brussels—first noticed a cluster of healed fractures on the neural spines near the base of the tail of an ornate crest-bearing hadrosaur, Olorotitan arharensis, during a 2019 visit to Blagoveschensk, Russia.

"I was puzzled by the observation. I have seen this pattern in other similar species, but usually only in isolated vertebrae. Here, the fractures were concentrated in the vertebrae at the upper base of the tail, and did not extend to the tip." — Filippo Bertozzo

Earlier work by Canadian paleontologist Darren H. Tanke (who has collected hadrosaur remains from Alberta's Dinosaur Provincial Park since 1979) noted similar injured caudal vertebrae and proposed an explanation in 1989 based on North American finds. When Bertozzo contacted Tanke in 2019, the project expanded to include many more specimens and regions.

The team found a consistent vertical-to-oblique fracture pattern localized to the dorsal (upper) part of the tail's neural spines. Computer models and comparative statistics indicate these breaks are consistent with downward force applied to the tip of the spinous process — a force direction compatible with a mounting male placing weight on a female during mating.

Importantly, many of the affected bones show clear signs of healing, and some bear evidence of repeated trauma, suggesting the injuries were typically non‑fatal and could recur over an individual’s life.

Professor Gareth Arnott of Queen's, who advised on animal behaviour, compared the fossil evidence to reproductive aggression and mounting-related injury seen in some modern animals (for example, sea lions, turtles and some birds). He warns that reproductive competition can take forms that look costly but are common across taxa.

Caveats and next steps: The mating hypothesis is plausible and supported by the consistent location and orientation of fractures, but it is not yet definitive. Alternative explanations—such as intraspecific fighting, accidental trauma, predation attempts or taphonomic processes—cannot be completely ruled out. To strengthen the claim, researchers will need corroborating lines of evidence (for example, consistent association with other sex-linked traits, population-level patterns, or biomechanical studies) and broader sampling across taxa and life stages.

If the interpretation holds up, individuals bearing these healed upper-tail injuries could be reliably inferred to be female. That would be a major advance: paleontologists could then study sexual dimorphism, growth patterns, and behaviour with sex-specific resolution for a group of dinosaurs that left abundant fossils.

Study authors and credit: The paper is led by Filippo Bertozzo and co-authored by Eileen Murphy, Gareth Arnott, Alastair Ruffell and Darren H. Tanke. The work combines fossil data, statistical analysis and computer simulations and was published in iScience.