Indeed it’s very difficult to tell a male and a female Stegosaurus apart.
It is over a century since the first Stegosaurus fossils were described, whether it’s a male or a female. Scientists think they may eventually help work out another intriguing question: how on Earth did these mighty beasts have sex? Today, however, is a big challenge.
Researchers seem to agree that there’s been no convincing evidence so far for sex-based differences — like the mane on a male lion, a phenomenon scientists call “sexual dimorphism” — in any dinosaur, despite those differences being common among modern animals.
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, this is not the case for the family known as Stegosauridae, who were named after its most famous species, 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 partner is getting impaled.
One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph,” said Saitta.
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 added as 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 the female mate choice was likely the driving evolutionary mechanism rather than male-male competition.”
On the other hand, 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.
via Evolution Literacy