After analyzing the fossil in relation to modern seal skull structure with the assistance of Carleton PhD candidate Danielle Fraser, Cullen found the evidence suggested that sexual dimorphism existed in seals somewhere between 20 million and 27 million years ago, near the base of all pinniped evolution and much longer ago than was previously thought. Cullen became one of the first researchers to pin down a timeline for the phenomenon. The discovery has implications for the past and future of the species.
"Early pinnipeds likely formed harems, the way a sea lion would," says Cullen. "Up to this point, it was not widely expected that early pinnipeds would behave this way."
Once Cullen and his team addressed the question of when sexual dimorphism in pinnipeds evolved, he was able to turn to the question of why it happened.
"Our interpretation is that these changes were happening at a time when the Earth was experiencing major climate and ocean circulation changes. Harem colonies were likely located at ocean upwelling sites that concentrate nutrients in otherwise nutrient-poor water. We think that this environmental factor, this concentration of large numbers of pinnipeds into one area, pressured them into developing the harem mating system and sexual dimorphism."
Cullen's finding sheds light on the history of pinnipeds, but when taken in the modern context of climate change, it has major implications for the future of the species.
"Climate change today appears to be having an effect on the Arctic and Antarctic more than on the temperate and equatorial latitudes. Most Arctic and Antarctic pinnipeds aren't really sexually dim
|Contact: Dan Smythe|
Canadian Museum of Nature