Sexual Dimorphism Of Stegosaurus

Stegosaurus is amongst the most iconic of the dinosaurs

Stegosaurus is one of the most iconic dinosaurs. Several metres long with a spiked tail and a series of large pentagonal plates along its back, it was well equipped to survive the predator filled world of the Jurassic. The function of the plates, however, is disputed.

They may have acted as solar panels, allowing the sun to heat the blood within a dense network of vessels. In turn, their spiked tails were a perfect means of defence against their carnivorous cousins, the theropods. All in all it is a well known and studied species, yet the familiar can still yield surprises. A recent study has shown that the shape of a Stegosaurus's plates relates to gender.

Sexual dimorphism is the technical term for members of the same species, but different genders, possessing different morphologies. Female angler-fish are much larger than males, male birds-of-paradise have resplendent plumages whilst females have not. Identifying sexual dimorphism in fossils is more difficult due to its potential confusion with the morphological differences between juvenile and adult, as well as male/female. Having spent six summers in central Montana, as part of an excavation crew, Evan Saitta from the University of Bristol was able to identify conclusively sexual dimorphism in the species Stegosaurus mjosi.

The plates in Stegosaurus mjosi give an indication as to
the gender of the individual from which they come.

The ecological composition of the Montana site showed that individuals of Stegosaurus mjosi differed only in the shape of their plates. CT scans and thin sections taken from plates in different specimens showed that they had stopped growing. This could not be put down to different stages of morphological development. The most viable explanation was gender, with the supposed male plates 45% larger than those from females.

'Males typically invest more in their ornamentation,' said Saitta. '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.'

The identification of sexual dimorphism in Stegosaurus is an important step forward as it gives us an indication as to how we may identify the trait in other dinosaur species. In turn, an understanding of gender balance in dinosaur populations will impact on how we model Mesozoic ecosystems and the ecological interactions which comprised them.