Female dinosaurs can be identified by traumatic bone injuries in tailbones, according to research conducted at Queen's University in Belfast.
Determining whether a dinosaur is male or female has long been a difficult task for palaeontologists.
However, using statistics and computer simulation, the Queen’s researchers, working as part of an international team of palaentologists, have found that a common pattern of bone fractures at the dinosaurs’ tail were likely caused during mating, where the male dinosaur crushed the backbones of the female.
The research pertains to duck-billed dinosaurs, also known as hadrosaurs, which were a common group of herbivorous dinosaurs during the late cretaceous period, from 100 to 66 million years ago.
The study is based on around 500 tailbones from different hadrosaur species from North America, Europe and Russia.
The backbone injuries were found to be strikingly similar between species, showing a "vertical-to-oblique injury", which researchers say were likely caused by "vertical pressure having been placed on the tip of the vertebral spinous process".
The injuries were not fatal, however, with many bone specimens showing signs of healing.
Researchers found that in some cases, the bones even displayed evidence of a second injury, indicating repeated behaviour.
Professor Eileen Murphy, who is one of the paper’s co-authors, said it is "a peculiar situation".
"Often when you find a caudal vertebra from the upper end of a hadrosaur tail, there is a strong chance of it bearing a healed injury. This is a clear pattern, and we set out to try to explain its cause."
Another co-author, Professor Gareth Arnott, said that while "aggressively pursuing a female during reproduction might sound evolutionary disadvantageous for the continuation of the species", this pattern is seen in many modern species such as " sea lions, turtles, and some species of birds".
"Reproductive competition is one of the most complicated topics in animal biology, especially for extinct species," he said.
Findings a 'gamechanger'
Dr Filippo Bertozzo, who published the findings, said he was "puzzled by the observation".
"I have seen this pattern in other similar species, but only in isolated vertebrae. Here, the fractures were visibly concentrated in the vertebrae at the upper end of the tail, without extending down to its tip.
A hypothesis on injured dinosaur vertebrae was made as far back as 1989 by Canadian palaeontologist Darren H Tanke.
However, his observation was based on a limited number of specimens and species, all from North America.
In 2019, Dr Bertozzo contacted Mr Tanke and invited him to collaborate on the project.
That same year, during a research visit to Blagoveschensk in Russia, Dr Bertozzo was studying a large hadrosaurid with an ornate fancy head crest, called Olorotitan arharensis.
It was then that he noticed healed fractures on the spines of many of the tailbones of the dinosaurs.
Dr Bertozzo, who began studying duckbill dinosaur pathologies for his PhD at Queen's in 2019, said he believes the findings will be a "gamechanger" in palaeontology.
"If the mating hypothesis is correct, we can infer that an individual with the injuries is female.
"This will be a game changer since it will enable other questions to be answered about differences between male and female dinosaurs."
Researchers plan to develop the study further with a wider dataset.
