Scientists have found the earliest evidence yet of bodily differences between males and females in a group of land animals that lived 260 million years ago.
The animal is Diictodon, a herbivorous barrel-shaped creature that lived at least 30 million years before the dinosaurs. It measured one metre in length, had a large, beaked skull and a short tail. And the male of the species had one very distinguishing feature: a pair of tusks.
Over the last two decades, biology professor Robert Reisz, of U of T’s Mississauga campus, and his colleagues in South Africa have unearthed nearly a hundred skeletons of this species. They believe the male Diictodon had two large tusks extending down from the upper jaw, and used them as weapons for ritualistic or physical combat.
Since the tusks are not found in females fossils and do not show any signs of wear, the researchers have ruled out other uses for the tusks, such as feeding and digging.
“Our findings give very clear evidence of complex social behaviour,” said Reisz. “Evidence suggests [these animals] formed groups. The very fact that there is sexual dimorphism with armament—meaning one of the sexes has a weapon-like structure in the form of a large tusk—indicates [they were] doing things similar to their distant mammalian herbivorous relatives.”
The presence of these tusks exclusively in the Diictodon males is an indication of sexual dimorphism—bodily traits that differ between the sexes of the same species.
Sexual dimorphism is not unusual, explained Reisz, but is typically associated with body size. The tusks of the male Diictodon are therefore very striking features from an evolutionary standpoint.
“This is the first such instance that we see in the fossil record, [and it] tells us that the kinds of social behaviour we see in modern living mammals probably was around a long time ago.”
Photograph by Simon Turnbull
