Mating injuries may lead scientists to identify dinosaurs’ sex
Paleontologists have long wrestled with how to differentiate between male and female dinosaurs based on their fossils.
But new research may bring scientists closer to identifying the sex of one group of dinosaur.
Hadrosaurs, also known as duck-billed dinosaurs, were common during the Late Cretaceous Period (100.5 to 66 million years ago), and the bones of these herbivores have been found across multiple continents.
Some hadrosaur fossils show evidence of healing from traumatic bone injuries, all in the same location: on their vertebrae past the base of the tail.
No soft tissues preserving evidence of dinosaur reproductive organs have been found in the fossil record, and differences observed in fossils are often thought to be due to species or age, rather than sex. Additionally, evidence of dinosaurs containing fossilized eggs has been hard to come by.
The authors of a new study published Tuesday in the journal iScience believe the injuries occurred during mating — and they could be used to indicate which fossils belong to female hadrosaurs.
“This will be a game changer since it will enable other questions to be answered about differences between male and female dinosaurs,” said lead study author Dr. Filippo Bertozzo, a researcher at the Royal Belgian Institute of Natural Sciences’ Operational Directorate Earth and History of Life.
Identifying a pattern in fossils
Canadian paleontologist Darren H. Tanke first noticed the injuries on hadrosaur vertebrae on bones he discovered in Alberta’s Dinosaur Provincial Park throughout the 1980s.
Tanke’s original hypothesis was that they were caused by male hadrosaurs mounting females during the mating process, but he based his claims largely on fossils found in Canada.
Bertozzo noticed the same injury while studying a fossil belonging to the hadrosaur species Olorotitan arharensis, during a research trip to Russia in 2019. He was working on his PhD about disease in duckbilled dinosaurs while at Queen’s University Belfast. Bertozzo invited Tanke to join him in conducting new research.
“In order to support behavioral claims, you need to have an extensive dataset able to possibly cover bias from fossilization, preservation, and collection,” Bertozzo said. “Also, the dinosaur biomechanical simulations weren’t as developed as those of today, and Darren couldn’t test his hypothesis.”
Together, the researchers and their collaborators analyzed nearly 500 tail vertebrae across different hadrosaur species kept at museums in North America, Europe and Russia.
The injuries and deformities they discovered on the middle part of the tail across different specimens were strikingly similar.
“At the base of the tail, between the sacrum and the mid-point of the tail, the neural spines (the elongated bars atop the vertebra, what we call the “spinous process” in human anatomy) are broken on their tip, sometimes along the main body of the spine,” Bertozzo said.
The tips were either tilted, swollen, inclined or even missing — meaning the injury caused the tip to break off and become reabsorbed by the body, he added.
The injuries were visible on several vertebrae for each specimen, suggesting the damage was spread along the main line of the tail, Bertozzo said.
The team carried out multiple simulations to see if the injuries could have been sustained through other activities during the dinosaurs’ daily lives, like accidentally stepping on each other’s tails, muscular strain during motion, fighting, being hunted, feeding or walking. But none of the scenarios would have consistently produced the injuries seen on the fossils, according to the study.
