Student identifies difference between the dinosaur sexes

Evan Saitta

PHOTO BY DENISE APPLEWHITE

THE DISCOVERY OF A SINGLE ANATOMICAL DIFFERENCE between males and females of a species of Stegosaurus provides some of the most conclusive evidence that some dinosaurs looked different based on sex, according to research published in PLoS One and conducted by Evan Saitta while he was an undergraduate at Princeton.

The study found that the back plates of the species Stegosaurus mjosi came in two varieties that indicated the animal’s sex — short and wide, and tall and narrow. Females had one type of plate and males the other. The lack of a particular female-specific bone tissue found in birds and some dinosaurs, however, made it difficult to determine which sex had which plate type.

Saitta, who graduated from Princeton in 2014 and conducted the research for his senior thesis project, drew from existing animals, particularly horned animals, to suggest that the distinct shape of male and female S. mjosi plates indicated two different functions. He supposes that the tall, narrow plates belonged to females, who would have needed the pointier plates to defend themselves against predators. The wide plates, which were 45 percent larger in surface area, likely served as “billboard” displays males used to attract females, similar to the plumes of the male peacock.

Beyond the implications for Stegosaurus, the research establishes that sexual dimorphism — in which males and females of a species have distinct physical forms — could exist in non-avian dinosaurs, a group that includes iconic reptiles such as Tyrannosaurus and Brontosaurus, Saitta said. Existing work on sexual dimorphism in non-avian dinosaurs had been inconclusive. Saitta is now a graduate student at the University of Bristol in the United Kingdom.

Andrew Farke, the Augustyn Family Curator of Paleontology at the Raymond M. Alf Museum of Paleontology in Claremont, California, said that the work provides a potential foothold for other researchers wanting to explore sexual dimorphism in Stegosaurus and possibly other non-avian dinosaurs.

“This is very species specific, so there’s a lot of work that needs to be done to extend this to other animals,” said Farke, who is familiar with the study but had no role in it. “It’s not the end of the road, but I think it will stimulate people to look at this issue in Stegosaurus.”

–By Morgan Kelly

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