Leading Edge / Summer 2003
Evolutionary News

Different physical traits discovered between sexes of the same prehistoric species

When it comes to press coverage of the prehistoric era, dinosaurs have received most of the attention. But for Robert Reisz, a biology professor at the University of Toronto at Mississauga, the genus Diictodon – which existed 180 million years before T. Rex – is far more fascinating. A distant relative of mammals, Diictodon was a plant-eating burrowing animal (think prairie dog) with a beaked skull and scales. “People ask, ‘What’s so interesting about these animals? They are a dime a dozen, and they don’t have the spectacular history of dinosaurs,’” says Reisz. “Yet, you find really interesting evolutionary stories associated with them.”

Here’s a case in point: while studying fossils unearthed in South Africa, Reisz and his team – master’s student Corwin Sullivan, now doing his PhD at Harvard, and Roger M.H. Smith of the South African Museum in Cape Town – were able to prove that only the male Diictodon had tusks. What they revealed is the oldest clear example of sexual dimorphism; that is, different physical traits between the sexes of the same species. The study was featured in the January issue of the Proceedings of the Royal Society of London B.

The formidable tusks indicate ritualistic behaviour; they may have been used for defence, to attract females, or for both purposes. “The very fact that there is armament suggests mammalian-like behaviour patterns,” says Reisz. “That’s what’s interesting: here is an animal, millions of years before dinosaurs, exhibiting very complex social behaviour, and yet it is in the mammalian lineage rather than the reptilian lineage.”

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