Paleontologists have long wrestled with how to distinguish between male and female dinosaurs based on their fossils.
But new research may bring scientists closer to determining the sex of one group of dinosaurs.
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 on many continents.
Some hadrosaur fossils show evidence of healing of traumatic bone injuries, all in the same place: on the vertebrae behind the base of the tail.
There are no soft tissues found in the fossil record that preserve evidence of dinosaur reproductive organs, and the differences seen in the fossils are often thought to be due to species or age rather than sex. Additionally, evidence of fossilized eggs in dinosaurs is difficult to find.
Authors new research published Tuesday in the journal iScience suggest the injuries occurred during mating—and could be used to indicate which fossils belong to female hadrosaurs.
“This will be a game-changer because it will answer other questions about the differences between male and female dinosaurs,” said study lead author Dr. Filippo Bertozzo, a researcher at the Operations Directorate for Earth and Life History at the Royal Belgian Institute of Natural Sciences.
Identifying patterns in fossils
Canadian paleontologist Darren H. Tanke first noticed damage to hadrosaur vertebrae on bones he discovered at the Alberta Dinosaur Provincial Park in 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 worked on his doctorate on diseases of duck-billed dinosaurs while at Queen's University Belfast. Bertozzo invited Tanke to join him in conducting new research.
“To support behavioral claims, you need to have a large data set that can cover biases associated with fossilization, preservation and collection,” Bertozzo said. “Also, biomechanical modeling of dinosaurs was not as advanced as it is today, and Darren could not test his hypothesis.”
Together, the researchers and their colleagues analyzed about 500 tail vertebrae from different hadrosaur species held in museums in North America, Europe and Russia.
The injuries and deformities they found in the middle part of the tail in different individuals were strikingly similar.
“At the base of the tail, between the sacrum and the midpoint of the tail, the neural spines (the elongated bars at the top of the vertebra, what we call the 'spinous process' in human anatomy) are broken at the tip, sometimes along the main part of the spine,” Bertozzo said.
The tips were either tilted, swollen, tilted or even missing – meaning the injury had caused the tip to break off and reabsorb into the body, he added.
Bertozzo said injuries were visible on several vertebrae of each specimen, suggesting the damage extended along the main line of the tail.
The team ran several simulations to see if the injuries could have been caused by other activities in the dinosaurs' daily lives, such as accidentally stepping on each other's tails or muscle strain while moving, fighting, hunting, feeding or walking. However, neither scenario would have resulted in the permanent damage seen in the fossils, according to the study.
Extensive damage was found on the tail of a hadrosaur named Edmontosaurus. It is on display at the Badlands Dinosaur Museum in North Dakota. – Filippo Bertozzo/iScience
Instead, the authors speculate that the males may have mounted the side-lying females and pressed on the females' tails during mating, accidentally breaking the nerve spines.
“The mating hypothesis currently provides the best explanation for our observations and data,” Bertozzo said. “The best evidence is the fact that we find this condition in many different species in different places and at different times, which suggests that this was not something specific to the species, but rather an event that would be conserved across all of them, happening to all of them, i.e. mating.”
The injuries also did not prove fatal, as signs of healing and even signs of a second injury were visible.
“Aggressively pursuing a female during reproduction may seem evolutionarily disadvantageous for the continuation of the species, but we already see similar phenomena in many modern species, such as sea lions, turtles and some bird species,” study co-author Gareth Arnott, a professor in the school of biological sciences at Queen's University Belfast, said in a statement. “Reproductive competition is one of the most challenging topics in animal biology, especially for extinct species.”
Dr Albert Prieto-Marques, a researcher at the Dinosaur Ecosystems Research Group at the Miquel Cruzafont Catalan Institute of Palaeontology at the Autonomous University of Barcelona, said the consistent pattern of damage in the form of healed fractures of the middle tail was likely a genuine signal rather than a coincidence. He was not involved in the study and applauds the researchers' creativity, but believes more evidence is needed.
“Right now we can't identify male and female dinosaurs just by looking at tail injuries,” he said. “Of course, we need to find the same pattern in individuals that we know are female, for example those with preserved eggs or medullary bone.”
Medullary bone is a temporary tissue that forms within the cavity of the long bones of birds and dinosaurs to provide calcium to eggshells and has previously been used in research to identify a pregnant tyrannosaurus rex in 2016.
For example, in chickens, medullary bone is formed one to two weeks before the first egg is laid and is reabsorbed approximately three weeks after the last egg is laid, according to research.
If the same pattern had been observed in dinosaurs, any of them containing this evidence would have died within about five weeks during the breeding season, the authors write.
The difference between a woman and a man
If the hypothesis can be proven, it could open up a whole new way to understand entire hadrosaur fossils that have such damage, Bertozzo said.
“This could lead to a cascade of consequences, such as testing whether cranial crests differ between sexes, or revealing anatomical features that were previously attributed to new species,” he added.
But more data is needed. Although the team has collected a large data set, they are looking to study fossils from China and South America to further make comparisons. They also want to use more powerful computer modeling to account for tail movements and muscle volumes under different injury scenarios, Bertozzo said.
Bertozzo is also curious to see if injuries are present in other types of dinosaurs, such as long-necked sauropods. He was surprised to find no similar lesions in iguanodons, which were the ancestors of hadrosaurs and are also very common in the fossil record.
The illustration shows Edmontosaurus annectens, a hadrosaur from the Late Cretaceous of North America. – Roman Garcia Mora/Stocktrek Images/Getty Images
“The story is just beginning, and I hope our story is just one of the first steps toward better understanding this aspect of dinosaur life,” Bertozzo said.
Determining whether a dinosaur is male from female based on bones requires a high burden of proof, which has historically been difficult, according to Steve Brusatte, a professor of paleontology and evolution at the University of Edinburgh in Scotland who was not involved in the study.
“The authors have made their case and I think they make a compelling case, but as with many aspects of dinosaur behavior, we simply weren't around tens of millions of years ago to observe them as true animals, so there will always be some degree of doubt, some degree of uncertainty in these interpretations,” he said. “We often just can’t tell boys from girls.”
Dr Yoshitsugu Kobayashi, professor of vertebrate paleontology at the Hokkaido University Museum in Japan, believes the study is one of the most creative he has seen in recent dinosaur paleobiology and believes the modeling in the study rules out other possible causes. While Kobayashi has discovered new species of hadrosaurshe was not involved in the new study.
However, although Kobayashi finds this hypothesis intriguing, he considers it a bold starting point rather than conclusive evidence.
“This study opens a fascinating window into the private lives of dinosaurs,” he wrote in an email. “Even if not all the findings are correct, it shows that bones can preserve traces of behavior that we never thought we'd see. It's exciting to think that even the scars of these ancient creatures could reveal moments of their most intimate lives, literally the dinosaurs' 'love lives' written in their bones.”
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