Stegosaurus' Bony Plates May Determine Sex

Stegosaurus were known for their spiked tails and armor-like exterior, and new research shows that their iconic bony plates may have determined the sex of these dinosaurs, according to new research.

During the Late Jurassic period roughly 150 million years ago, there were all sorts of Stegosaurus roaming the western United States. Some individuals had wide plates, some had tall, with the wide plates being up to 45 percent larger overall than the tall plates. University of Bristol researchers say these tall-plated and wide-plated Stegosaurus were not two distinct species, nor were they individuals of different age: they were actually just males and females.

Sexual dimorphism - that is, distinct anatomical differences between males and females of the same species - is very common in living animals. For example, the manes of lions or the antlers of deer indicate male members of these species. And yet, it is surprisingly difficult to determine sexual dimorphism in extinct species.

While previous research has suggested this phenomenon existed among dinosaurs, some scientists claim these studies did not rule out other possible explanations for why differences in anatomy might be present between fossil specimens. It has been said that two individuals that differ in anatomy might in fact be two separate species, a young and an old individual, or a male and a female individual.

So after unearthing what is the first ever Stegosaurus 'graveyard' in central Montana, a team of researchers set out to get to the bottom of this mystery.

The group of excavated dinosaurs, Stegosaurus mjosi, was strikingly similar except for their bony plates, which are staggered in two rows along their back. Other skeletal differences indicating separation of ecological niches would have been seen if the two were different species.

The study also found that the two varieties were not a result of growth. CT scanning, as well as thin sections sampled from the plates for microscope analysis, showed that the bone tissues had ceased growing in both varieties. Neither type of plate was in the process of growing into the other.

By ruling out all other possibilities, the team came to the conclusion that the distinct plates are the result of a difference in gender, male and female.

"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," lead study author Evan Saitta explained in a statement.

Saitta and his colleagues also suggest that Stegosaurus wasn't the only dinosaur that exhibited sexual dimorphism. Other species boasted extra-large crests or nose horns, which were potentially sexual features used to attract mates - like red deer or peacocks today.

These findings not only provide scientists with a much clearer picture of Stegosaurus behavior than would otherwise be possible, but also show that sexual dimorphism in fact did exist among dinosaurs long ago.

The results were published in the journal PLOS ONE.

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