It is fairly easy to tell male animals from female animals in most cases. It is more difficult to tell the difference in extinct species, especially when fossilized bones are all you have to work with.
New research, however, suggests that the bones themselves may provide answers when it comes to the Stegosaurus and possibly other dinosaurs as well.
Sexual dimorphism is a term used to describe physical, anatomical differences between male and female animals of the same species. It is common in living animals, deer antlers for example, but scientists have generally dismissed it as a technique for determining the gender of dinosaurs.
Physical differences in extinct animals, the argument goes, could be a sigh that the dinosaurs were different species or different ages. Without a way to exclude other possibilities, sexual dimorphism can’t be confirmed.
After spending six summers excavating a “Stegosaurus graveyard” in central Montana, Evan Saitta thinks that he can rule out other possibilities and show that male and female Stegosaurs can be identified by the plates on their backs.
Stegosaurus was a dinosaur that lived in the Late Jurassic period, about 150 million years ago, in the western United States. The stegosaur had two pairs of spikes at the end of its tail and two rows of plates along its back.
Some of the animals had wide plates and others had tall plates. According to research published online at PLOS ONE, the tall plated and wide plated stegosaurs were not different species, just different genders.
Saitta is convinced that he has sufficiently ruled out other possible explanations for the physical differences between the species.
CT scans and microscopic analysis of various plates show that the bone was done growing. Neither type of plate was in the process of growing into the other type. This rules out the possibility that one variety or the other was younger.
Additionally, the number of specimens excavated at the Montana site indicates large numbers of both types living in the same area at the same type. This makes it highly unlikely that they were two different species. Even two species sharing the same territory would be expected to display other skeletal differences indicating that they filled different ecological niches.
With these possibilities ruled out, the one that remains is that the animals were of different genders. Unfortunately, it is not clear which gender was which. That part of the equation is largely based on speculation.
“As males typically invest more in their ornamentation, the larger, wide plates likely came from males. These broad plates would have provided a great display surface to attract mates. The tall plates might have functioned as prickly predator deterrents in females,” said Saitta in a statement.
While which dinosaur was which may still be open to debate, the discovery that stegosaurs exhibit sexual dimorphism is important. It raises the possibility that other species of dinosaurs did too.
“Evan made this discovery while he was completing his undergraduate thesis at Princeton University. It’s very impressive when an undergraduate makes such a major scientific discovery,” said Professor Michael Benton, Director of the Masters in Palaeobiology at the University of Bristol.
