How to tell if a Stegosaurus is male or female

Evan Saitta with a Stegosaurus plate. Image:
Billings Clinic, Montana

Scientists believe they have finally determined the differences in looks between female and male Stegosaurs.

Stegosaurus, a big herbivorous dinosaur, measuring nine metres long and four metres tall, with two rows of bony plates along its back and two pairs of spikes at the end of its tail, lived about 150 million years ago.

Researchers now believe tall-plated Stegosaurus and wide-plate Stegosaurus are not distinct species but are actually females and males.

Anatomical differences between males and females, known as sexual dimorphism, is common in all animals but is difficult to determine from fossils, especially for dinosaurs.

The discovery was made by undergraduate student Evan Saitta while he was completing a thesis at Princeton University.

“As males typically invest more in their ornamentation, the larger, wide plates likely came from males,” he says. “The tall plates might have functioned as prickly predator deterrents in females.”

The study also found that the two varieties were not a result of growth.

The difference between male and females.

CT scanning at Billings Clinic in Montana 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.

With other possibilities ruled out, the best explanation for the two varieties of plates is that one type belonged to males and the other to females.

The study by Evan Saitta, now at the University of Bristol, is published in the journal PLOS ONE.

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