According to paleontologist Evan Saitta of the University of Bristol, UK, stegosaur plates may have differed between males and females.
Silhouettes of male and female Stegosaurus mjosi. Image credit: Evan Saitta.
Stegosaurus is a genus of armored, herbivorous dinosaurs. They lived in what is now the western United States during the Late Jurassic, approximately 150 million years ago.
They were 6 – 9 meters (20 – 30 feet) long, had two staggered rows of bony plates along their back and two pairs of spikes at the end of their tail. The skull and brain were very small for such a large dinosaur.
Some individuals had wide plates, some had tall, with the wide plates being up to 45 % larger overall than the tall plates.
According to the study by Saitta, M.Sc. student at the University of Bristol’s School of Earth Sciences, tall- and wide-plated stegosaurs were not two distinct species, nor were they individuals of different age – they were actually males and females.
Saitta examined five specimens of Stegosaurus mjosi with varied plates, excavated from a new quarry in Montana (JRDI 5ES Quarry), in addition to previously excavated individuals of this species.
CT scanning and microscope analysis of the stegosaur plates showed that the differences were not a result of growth, as the bone tissues had ceased growing in both varieties.
“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,” said Saitta, who is the author of the paper published in the journal PLoS ONE.
“The tall plates might have functioned as prickly predator deterrents in females.”
Stegosaurs may not have been the only dinosaur to exhibit sexual dimorphism. Other species showed huge crests or nose horns, which were potentially sexual features.
Not only does the new study show that dinosaurs exhibited sexual dimorphism, it suggests that the ornamentation of at least some species was used for sexual display.
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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
