Sorting the Stegosaurus Boys From the Stegosaurus Girls

Peacocks have their tails. Male lions have their manes. And now, new research suggests, male stegosaurs may have distinguished themselves—and signaled their prowess to the ladies—with rounder back plates.

Because of the nature of fossils, it’s often hard to tell whether anatomical differences in dinosaurs are truly due to sex differences. But in a study published in the journal PLOS ONE, University of Bristol researcher Evan Saitta seems to have found convincing evidence that the shape of back plates may have been an important sex marker for these Jurassic titans.

Oval and Wide Versus Spiky and Almond-Shaped

In his work, Saitta examined a selection of Stegosaurus mjosi fossils unearthed in Montana. He found that back plates could be sorted into two distinct types. One group was ovoid and wider than they are tall, while the other was spikier and kind of almond-shaped.

To rule out age or individual variation among the dinosaurs as alternate explanations for the different shapes, Saitta had the plates scanned with a CT machine, and also analyzed tiny sections cut from them. The histological features of the plates show that plates of both types had stopped growing, so the different shapes weren’t caused by age. And the fact that the plates came in two distinct groups, with no intermediate shapes exhibited, means that the differences probably can’t be put down to individual variation, Saitta argues.

So Which Is Female and Which Is Male?

So, if the differences are a form of sexual dimorphism, who bore the wide plates and who sported the tall? Determining gender in dinosaurs is pretty tricky, as there’s no soft tissue to go off of and skeletons can be deceiving. Unlike female mammals, female egg-laying animals usually don’t have particularly distinct pelvis shapes. Paleontologists have been trying various other methods of determining dinosaur gender. One telltale sign is the existence of medullary bone, a particular kind of bone tissue also found in pregnant female birds. It’s a calcium-rich tissue that aids the birds in making eggshells.

Saitta didn’t find any evidence of medullary bone in the leg bones he tested. That isn’t too surprising, as the tissue is only present during breeding, so a dinosaur has to die at just the right time for researchers to see it. That makes it hard to definitively assign sex to either type of plate. But Saitta has a theory.

“As males typically invest more in their ornamentation, the larger, wide plates likely came from males,” Saitta said in a statement. “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.”

Taller plates on a Stegosaurus are the shape most commonly depicted in dinosaur reproductions. So if Saitta’s hypothesis holds up, the image of a Stegosaurus most of us are familiar with, whether through movies or plastic toys, is probably a female.

Image: iStock.com / ugurhan