The male Stegosaurus had billboard-like back plates that it used to attract potential mates, while females had prickly spikes that kept predators at bay, scientists have discovered.
The finding is the most convincing evidence to date that male and female dinosaurs differed in appearance, and suggests that they may have had mating selection rituals comparable to those of modern species such as birds.
“The discovery of sexual dimorphism suggests that the males were displaying. It gives us insights into dinosaur behaviour,” said Evan Saitta, a masters student at the University of Bristol who carried out the research.
Stegosaurus was a large herbivore that lived roughly 150 million years ago and is famous for its two staggered rows of bony plates along its back.
Saitta carried out an anatomical survey of the shapes of back plates from at least five skeletons discovered in a Stegosaurus “graveyard” in central Montana, as well as plates from at least six individuals discovered elsewhere.
The analysis revealed that some had tall, sharp plates and others had wide rounded plates, which were up to 45% larger in surface area.
The scientists ruled out the possibility that some individuals - male or female - were simply spikier than others.
“The 40 plates we looked at belonged to two distinct categories of shape without any real intermediates,” said . “It’s a stark distinction of wide versus tall. If it was normal variation you’d expect more of a bell-shaped curve.”
Analysis of other bones in the pit showed that the skeletons were otherwise anatomically very similar, which the authors said excluded the possibility that they were looking at separate species.
Having convinced themselves that they were looking at a male-female difference, the question remained of which was which. “We don’t have a smoking gun piece of evidence, so you just have to go by intuition,” said Saitta.
They attributed the larger wide plates to males, in keeping with the “handicap hypothesis”, which sees males invest in cumbersome body parts to prove their evolutionary fitness. “Males typically invest more energy into growing their ornamentation,” said Saitta. “You can imagine the wider display area along the back being like a billboard. The tall plates might have functioned as prickly predator deterrents in females.”
The findings shed no light on the sex of the Stegosaurus specimen, Sophie, currently on display at the Natural History Museum in London, as it belongs to a different Stegosaurus subspecies to those in the study.
The findings are reported in the journal PLOS One.