When it comes to the stegosaurus, it might be very easy to tell the ladies from the gents. A new study of the dinosaurs' ridges of fierce plates shows that males and females sported very different armor.
The study of these dinos' sexual dimorphism, published in the journal PLOS One, reveals fresh insight into their evolution and could lead scientists to look for such sex-based body types in other dinosaur species.
Until now, "conclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive," study author Evan Thomas Saitta of the University of Bristol in England wrote in the paper.
Stegosaurus, which lived roughly 150 million years ago during the Late Jurassic epoch, is one of the more famous of the large-bodied plant-eating dinosaurs. It had a small head, stumpy legs and featured a dramatic double-layered set of staggered plates running like a Mohawk from neck to tail, which it probably used as a defense against large predators. At the very end of the tail were spikes, which the animal could whip around to do some serious damage if it was being attacked.
But there seems to be a strange level of variety in the sizes and shapes of these plates. A collection of Stegosaurus mjosi fossils in a quarry in Montana, called JRDI 5ES -- where it seems that at least five stegosauruses were buried at the same time -- shows that some of the plates were tall and sort of thin, while others were large, wide and rounded. In fact, the wide ones were 45% larger than the tall and thin ones.
The problem is, when you only have fossils to represent long-gone animals, two individuals in the same species might look very different depending on factors like age, sex or random genetic variation. It was unclear whether these plates were from two different species of stegosaurus, whether they reflected some individual variety or changes as young animals matured.
To get at this question, Saitta studied 40 stegosaurus plates found at the Montana site, sizing them by hand using calipers and a tape measure as well as scaled photographs. After analyzing their sizes and shapes, Saitta identified a clear pattern of sexual dimorphism -- that the two sexes in the same species had very different shapes.
Sexual dimorphism is very common across the animal kingdom. Male peacocks sport iridescent blue-green feather fans while the female peahens have plain brown plumage, for instance, and male lions have manes while females do not.
Saitta thinks that the male stegosauruses had the large, rounded plates, all the better for showing off to females, while the females had the smaller, thinner, more angular plates. It's likely the females' preference is what drove the evolution of the males' showier armor, he wrote.
"Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition," Saitta wrote.
It's similar to the antlers on stags or the horns on bulls today, except, Saitta said, that the plates were used for ornamentation rather than for competing for access to females.
"The larger wide morph plates were probably under sexual selection like male bovid horns and functioned to create a broad, continuous display surface along the animal's back, like a billboard," he wrote.
The findings could change the way paleontologists look at other dinosaur species' fossils as well, Saitta added. For one, there might be clear differences between the sexes in fossils of the well-known triceratops. Features that were once seen as signs of "juvenile" animals might actually be from fully grown animals, he pointed out.
"These results may suggest that previously described ornament variations in dinosaurs are actually cases of sexual dimorphism," he wrote. "For example, the ontogeny of Triceratops horns seems to match sexual variation seen in modern bovid horns, where males have horns with downward pointing tips that are better for sparring and resistance of lateral stresses and females have thinner and straighter horns with tips that point up and away from the skull that are more efficient stabbing weapons."