Stegosaurus, a dinosaur familiar from textbooks and known for the large, bony discs on its neck, back and tail in two staggered rows, may have displayed differences in those plates depending on its sex, a new study suggests.
The finding may be the first convincing evidence of discernible sexual difference in any dinosaur species, researchers argue in the journal PLOS One.
Paleontologists Michael Benton and Evan Saitta first thought differences they were seeing in fossil stegosaurus skeletons meant they were looking at two different species, but soon became convinced what they were seeing were male and female examples of the same species, Stegosaurus mjosi.
The differences - some of the 150-million-year-old skeletons had wide plates while others had thinner, taller plates - may have distinguished males from females, says study author Saitta, who began the study as part of his senior thesis at Princeton University and is now a graduate student of paleobiology at the University of Bristol in Great Britain.
Such anatomical differences are known as sexual dimorphism.
Saitta, who has studied stegosaurus fossils since 2009, became intrigued by the dinosaurs' plates and the differences he saw, even in skeletons from the same regions and time periods that were almost certainly from a single species.
Determining the sex of dinosaurs is extremely difficult except when, for example, a fossil can be identified as female because it contains eggs.
However, without obvious signs like that scientists can't know a dinosaur's sex.
Previous claims of sexual dimorphism in dinosaurs have been questioned because there are other possible explanations for physical differences, including species differences or the difference between a young and old individual of the same species.
Extensive study of the S. mjosi fossils have ruled out almost any other explanation, though, strongly suggesting plate size and shape can identify a male or female stegosaurus, Saitta says.
"It's the most convincing evidence we have so far of sexual dimorphism in a dinosaur," he said.
And which is which, when it comes to a boy or girl stegosaurus?
"As males typically invest more in their ornamentation, the larger, wide plates likely came from males," Saitta says. "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."
Other differences seen in some dinosaur fossils - such as extra large head crests or longer nose horns - may have also been examples of sexual dimorphism, the researchers suggest.