Stegosaurs plates may have differed between males and females, according to a study published April 22, 2015 in the open-access journal PLOS ONE by Evan Saitta from University of Bristol, UK.

Anatomical differences between males and females of the same species, also known as sexual dimorphism, is common in living animals, yet is surprisingly difficult to determine in extinct species, especially dinosaurs.

Stegosaurus, a large, herbivorous dinosaur, lived roughly 150 million years ago and had two staggered rows of bony plates along its back. Some individuals had tall plates and others had wide plates, which could be up to 45% larger than the tall plates.

The author of this study examined a group Stegosaurus mjosi with varied plates, excavated from a Stegosaurus 'graveyard' in central Montana, in addition to previously excavated specimens of this species.

Evan Saitta found that the tall-plated Stegosaurus and the wide-plated Stegosaurus were not two distinct species, nor were they individuals of different age: they were actually males and females.

CT scanning and microscope analysis of the plates showed that the differences were not a result of growth, as the bone tissues had ceased growing in both varieties. The presence of sexual dimorphism in an extinct species may provide scientists with a much clearer picture Stegosaurus behavior than would otherwise be possible.

Saitta speculated about the differences: "As males typically invest more in their ornamentation, the larger, wide plates likely came from males. 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. These stegosaurs seem to provide the first really convincing evidence for sexual dimorphism in a dinosaur species (excluding birds, which are technically dinosaurs themselves)."

Saitta ET (2015) Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA. PLoS ONE 10(4): e0123503. doi:10.1371/journal.pone.0123503