Sexing the Stegosaurus

The tall-plated Stegosaurus and wide-plated Stegosaurus are not two distinct species but actually males and females! The findings were published in PLoS One last week.

Herbivorous Stegosaurus roamed North America approximately 150 million years ago, long before Tyrannosaurus rex. These dinosaurs are distinctive due to the two staggered rows of armour plates along their backs, and their spiky tails.

Fossils of Stegosaurus to date always have had one of two distinct plate types - either wide or very narrow - which until now has been unexplained. Several theories have been suggested, including species differences, dinosaur age or gender. Whilst studying at Princeton, Evan Saitta carefully examined the iconic back-plates of Stegosaurus individuals, including five found buried together in Montana, to help solve the mystery.

The first clue was that the remains of five individuals were discovered in close quarters. This indicates that they would have lived together, as a herd. Given that inter-species co-existence is rare in modern animals, it was the first piece of evidence suggesting they might belong to the same species.

The next clue came from microscopic analyses showing that the plates had stopped growing, ruling out age as a possible factor in plate type. Finally, CT scans revealed that, other than the plates, there were no other skeletal differences, which is again surprising if it was to be believed they were different species.

Gender variance was thus concluded as the only possible explanation.

The next stage was to determine the plate type for each gender and for this Saitta looked to the animal kingdom. Males are typically bigger than females and invest more in ornamentation to attract mates, so it makes sense for the wide plates to be from males.

"These broad plates have more overlap so it creates a big display surface along the silhouette of the animal" he says.

However, if the plates are for ornamentation, why do females also have them? The answer to this comes again from a living animal: the rhino. Both male and female rhinos have horns. Males use them to compete but also protect themselves against predation. Females don't have to compete for mates but they do use them as a weapon when needed. It is probable therefore that the tall, pointed plates were to protect the female stegosaurus whereas the wide plates of males were used to attract mates.

The physical differences between males and females are easily spotted in modern animals, for example in peacock's plumage and lion's manes. However, it is much more difficult to prove in dinosaurs. Often researchers have a big pile of bones and need to determine whether it is more than one animal, different species, different genders or just one complex individual! Sample size and fragmented remains are other issues to deal with too. Although there has been a lot of research into gender variation for all different types of dinosaurs, in most cases alternative explanations weren't fully explored. 

"These Stegosaurs have actually provided the best evidence for sexual variation in any dinosaur", Saitta concluded. Proving sexual variation exists is a step towards getting a complete understanding of dinosaur behaviour.