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How to Look Up a Stegosaurus's Skirt

A Stegosaurus graveyard presents the first concrete evidence of sexual dimorphism in non-avian dinosaurs.
April 22, 2015, 6:50pm
​All spiked up and nowhere to go. Credit: Heinrich Harder

Jeff Goldblum's role as Ian Malcolm in Jurassic Park is legendary for its lavish quotability, with lines ranging from the philosophical "life finds a way" to the scatological "that is one big pile of shit." Among the most memorable of these one-offs is a question Malcolm poses to BD Wong's character, as baby raptors casually hatch in the background. "How do you know they're all female?" he asks. "Does somebody go out into the park and pull up the dinosaurs' skirts?"

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Well, as it turns out, determining dinosaur gender isn't just fodder for Crichton-Spielberg plot points—it is a huge sticking point for real paleontologists as well. And if it was tricky for Jurassic Park's scientists to keep track of the sex of their living dinosaurs, you can imagine how much more difficult it is to reconstruct the sexual differences between animals that died 150 million years ago.

Nevertheless, paleontologist Evan Saitta has found compelling evidence that the famous spikey herbivore Stegosaurus was sexually dimorphic, meaning that male and female stegosaurs were anatomically distinct from each other.

Saitta has spent six summers excavating the remains of several members of the species Stegosaurus mjosi in a newly discovered fossil-rich quarry in Montana, and examining their individual features. He found that there was a variation in the shape and size of the animals' characteristic spinal plates, most likely due to sex differences. His research is published today in PLoS ONE.

"The evidence provided here is the first support for sexual dimorphism in a non-avian dinosaur that rules out all other possible explanations for the observed morphological variation," Saitta said in the study.

The two plate types Saitta studied. Image: PLOS ONE/Evan Saitta

It has been easier to speculate about sex differences in avian dinosaurs, because we can observe their extant descendants—birds. Modern bird species frequently exhibit extreme sexual dimorphism, which suggests that their Mesozoic forerunners also may have also evolved different male and female body shapes.

Also, as Saitta pointed out to me, there is already some fossil evidence that male and female avian dinosaurs had distinct characteristics. "There has been a good case for sexual dimorphism in a fossil bird [called] Confuciusornis," he told me over email. Likewise, there is plenty of evidence that pterosaurs, which occupied the same airborne niches in the Mesozoic that birds inhabit today, also exhibited extreme variation between genders.

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But narrowing down sex differences in the non-avian dinosaurs is a lot tougher, because any number of factors can influence morphological diversity between these specimens. Aberrations can be chalked up to the presence of multiple species from the same family, or to different stages of growth, as with the ongoing debate over whether Triceratops really exists as a distinct species, or is simply the juvenile version of Torosaurus.

Saitta posited multiple possible alternate explanations for the divergent plate designs, and duly investigated each avenue. He ruled out the juvenile/adult hypothesis after CT scans and microscopic examination revealed mature bone tissue in both "morphs"—meaning that the plates were still different even when the individuals were clearly adults. "One morph is not the immature form of the other," he said in the study.

Saitta also eliminated the possibility that multiple species of Stegosaurus shared the same turf, arguing that there would be other skeletal differences between the specimens that demonstrate distinct niches. "The dimorphism is not a result of interspecific variation," he concluded.

The speculative male (left) and female (right) plates. Image: PLOS ONE/Evan Saitta

Furthermore, the way the two plate designs are shaped could be somewhat analogous to sexual dimorphism in modern animals. In the study, Saitta uses the example of cows. Though bovines are a far cry from Stegosaurus phylogenetically, they occupy a similar niche, and males and females have evolved horns for different purposes.

"In addition to both being large, quadrupedal herbivores, stegosaur plates and modern bovid horns are both composed of a bony core surrounded by a keratin sheath," explained Saitta in the paper.

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"Compared to females, males are typically expected to invest more energy into growing and maintaining their ornamentation," he said. "Applying the same reasoning garnered from studies of modern bovid horns to S. mjosi plates, the wide morph may represent the male while the tall morph could represent the female."

Male and female Stegosaurus body plans. Image: PLOS ONE/Evan Saitta

This find could genuinely be the first substantive evidence that male and female non-avian dinosaurs had distinct features. But even so, Saitta would like to hunt for more proof for sexual dimorphism in the fossil record.

"I think testing for sexual dimorphism in other species of dinosaurs, that have the proper context and a good fossil record, is certainly something to look forward to in the future," he told me.

Clearly, differentiating between dinosaur genders isn't as simple as looking up their skirts. Nevertheless, paleontologists like Saitta are painstakingly reconstructing the sexual biology of these long-lost creatures, painting a captivating picture of the flashy displays and powerful defenses of stegosaur lads and ladies. Maybe one day they'll even answer the ultimate Stegosaurus sex mystery: the mechanics of making love without impaling your partner.