Are your toy dinosaurs boys or girls? One Science News editor decided her plush stegosaurus was male and named him Franklin. It was as good a guess as any since there’s really no way to tell a dinosaur’s gender just by looking at it. Even paleontologists can’t always determine gender. Female dinosaurs that were pregnant when they died had a specific type of bone tissue (and sometimes eggs inside), but dinos without that tissue could have been either males or (nonpregnant) females.
A new study, however, proposes that at least one species of dino, Stegosaurus mjosi, did have distinct gender differences in morphology. In this species, the big, bony plates that distinguish a stegosaur may have differed in shape between males and females.
This is the conclusion of an undergraduate project by Evan Saitta, now a grad student at the University of Bristol in England. Saitta examined nearly a dozen S. mjosi fossil specimens, including five new ones that had been found together in a quarry near Grass Range, Mont. He published his findings April 22 in PLOS ONE.
S. mjosihad two forms, he found. Some individuals had plates that were tall, skinny and pointy. Others had plates that were shorter, rounder and wider. Other than the plates, the bones of the animals were so similar that they must have come from the same species. And all the individuals were adults, so the plate shapes didn’t differ according to maturity. The most likely explanation for the different morphologies, Saitta reasoned, was that they came from different sexes.
But which was the male and which was the female? There was none of that special bone tissue or any eggs to definitively mark any of the dinos as females. So Saitta made some deductions based on anatomical differences in modern bovids — large herbivores including antelopes and bison that could be a stegosaur stand-in. In bovids, males tend to have large ornamentation. If that was also true in S. mjosi, then the male fossils are the ones with the larger, rounder plates.
If these two morphs truly belonged to the same species and represent different sexes, then this would be one of the first cases of sexual dimorphism found in a non-avian dinosaur. It’s something that’s pretty common in animals today (and in avian dinos, like cardinals), but it’s been hard to find in dino fossils.
What this means in your house, though, is that you probably can tell the gender of your toy stegosaurus. If it’s got wide, round plates across its back, your dino is probably a boy. If those plates are tall and pointy, you’ve got a girl.
Editor's note: This post was updated on April 22 to note that this would be one of the first, not the first, cases of sexual dimorphism in a non-avian dinosaur.