What Male and Female Stegosaurus Looked Like

Paleontologists are just now confirming what has long been suspected: male and female dinosaurs sometimes — and perhaps always — looked different from each other.

At least that appears to be true for Stegosaurus, a large plant-eating dinosaur living 150 million years ago, that sported rows of bony plates on top of its long back, and formidable-looking spikes at the end of its tail.

Authors of the new research, published in the journal PLOS One, argue it’s first convincing evidence for sexual differences in a species of dinosaur.

Initially, scientists Michael Benton and Evan Saitta thought that certain Stegosaurus-like skeletons represented different species. Then they started to realize that they were actually looking at male and female members of the same species.

“Evan made this discovery while he was completing his undergraduate thesis at Princeton University,” Benton, who is director of the Masters in Paleobiology Program at the University of Bristol, said in a press release.

Anatomical differences between males and females of the same species are collectively known as “sexual dimorphism.” Not all species exhibit this, but it’s obviously very common among living animals. Think of the manes of male lions, for example, or the antlers of male deer.

In the case of Stegosaurus, it appears that the primary difference had to do with the look and arrangement of their bony plates. Some Stegosaurus had wide plates while some had tall, with the wide plates being up to 45 percent larger than the tall plates.

Benton and Saitta studied Stegosaurus fossils unearthed in Montana. Puzzled by the plate variations, they CT-scanned samples from the plates, to see if the observed differences might reflect distinct growth stages. This was ruled out, however, because the bone tissues had ceased growing in both types.

The researchers also noted that the skeletal differences were not associated with a separation of ecological niches, which would have been expected if the two types were different species.

With these and other possibilities ruled out, Saitta concluded that the best explanation for the two varieties of plates is that one type belonged to males and the other to females.

“As males typically invest more in their ornamentation, the larger, wide plates likely came from males,” he explained. “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.”

Read more at Discovery News