It is well over a century since the first Stegosaurus fossils were described, and scientists think they have finally solved one of the great mysteries of the famous dinosaur family: how to tell the sexes apart. The discovery may eventually help work out another intriguing question: how on Earth did these mighty beasts have sex?
Genitalia seldom fossilize well, but for some species of dinosaurs it is possible to tell the males from the females in other ways. In certain species, specimens show signs of interrupted growth, suspected to coincide with reproductive cycles. Many sauropods have fused vertebrae that would have helped them keep their tails raised. In species such as Apatosaurus, this feature only occurs in half the specimens, suggesting it was used by females during mating.
However, for the family known as Stegosauridae, named after its most famous species, this is not the case. Paleontologists have been left to wonder whether one sex was larger than the other or if there is any other way to tell them apart. The lethal plates on their backs have also raised the question of how mating could occur without one of the partners getting impaled.
However, Evan Saitta, a master's student at the University of Bristol, noticed that specimens of Stegosaurus mjosi came in two different types, or morphs. In PLOS One, he reports: “One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph.” Evidence from other dig sites demonstrates that the plates were not differently shaped at various positions on the body, but consistent throughout an individual's spine.
These were not, Saitta concludes, opposite ends of a spectrum, but represent distinct types with no representatives falling in between. Saitta is also confident that the different morphs are from the same species, since a recently unearthed quarry in Montana shows at least five specimens that appear to have been buried together and were probably living in a social group. Nor do the differences appear to be a result of age. “Both morphs occur in fully-grown individuals,” Saitta writes. He established this through CT scans showing that the animals had stopped growing.
Such findings strongly suggest the two morphs represent male and female S. mjosi.
This still leaves the challenge of working out which morph is which. The differences appear to have more to do with appearance than practicality. Showy features, such as the peacock's tail, are overwhelmingly more likely to be seen in males of a species than females, and Saitta argues, “Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species.” On this basis, he thinks the larger plates belonged to the male Stegosaurs and “Female mate choice was likely the driving evolutionary mechanism rather than male-male competition.”
It is unusual enough for a master's student to be the sole author of a paper in such a prestigious journal, but according to Bristol's Professor Michael Benton, “Evan made this discovery while he was completing his undergraduate thesis at Princeton University,” noticing something that had eluded the greatest dinosaur hunters of the last century. But we still don't know how they had sex without the mates getting skewered.