Around 150 million years ago, it might not have been that difficult to look at a Stegosaurus and tell whether it was a male or a female. Today, however, it's a big challenge. Researchers find themselves staring at piles of bones, often unable to tell whether they're looking at multiple animals, different species, or one individual with a number of variations. Because of that, researchers seem to agree that there's been no convincing evidence so far for sex-based differences — like the mane on a male lion, a phenomenon scientists call "sexual dimorphism" — in any dinosaur, despite those differences being common among modern animals.
"It's a feature we'd expect to see."
Despite those difficulties, a researcher at the University of Bristol now says that he's found the "best evidence yet" for sexual dimorphism in one species of Stegosaurus. In a paper published this afternoon in PLOS One, Evan Saitta describes differences in the plates running along the spine of the Stegosaurus mjosi that may differentiate the species' males and females. "There's simply no other explanation" for the different plates, Saitta tells The Verge.
In the work, performed while he was an undergraduate at Princeton University, Saitta identified two types of plates: one that was narrow and tall and another that was shorter but much wider. "By ruling out other possible explanations, I can say with some confidence that I think it is sexual dimorphism that's causing the two distinct types of plates," he says. The males, he posits, likely had the wider plates, potentially as a way to attract mates.
To land on sexual dimorphism, Saitta first had to rule out quite a few other alternate possibilities — possibilities that papers in the past have failed to address. Those include whether these plates could have come from Stegosaurus of different ages, from different species of Stegosaurus, or from common variations within the species that would occur regardless of sex. Having eliminated those possibilities using aging techniques and observations of multiple specimens, Saitta settled on sexual dimorphism as the reason for the differences. "Male and female sexual dimorphism is the only thing that's left," he says. "It makes sense, too. It's a feature we'd expect to see."
"I would not accept this as evidence of sexual dimorphism."
While sexual dimorphism is certainly a possible explanation, other researchers aren't quite as convinced as Saitta. Kevin Padian, a paleontologist and professor of integrative biology at University of California, Berkeley who wasn't involved in the study, says that he sees "no evidence" of a distinction between the two plate types that Saitta identifies. "Without better association documented, or better skeletochronology, you can’t tell," Padian writes in an email to The Verge. "At any rate I would not accept this as evidence of sexual dimorphism, or of dimorphism at all."
Basically, Padian sees Saitta's study as falling victim to the same gaps in research as prior reports on sexual dimorphism in dinosaurs. He doesn't see the study as being able to rule out the many alternate possibilities for why these differences exist. "There is no convincing evidence of how many animals are present, how the bones and plates may be associated, whether the animal(s) in question may be fully grown, or what the absolute age of any specimen is," Padian writes.
Another paleontologist contacted by The Verge, who was not involved in the study and did not want to publicly criticize the work, agreed that there were issues with the findings. He said that the study presents a good case for dimorphism, but that it isn't necessarily sexual — it could still turn out to represent other differences, such as distinctions between species.
If dimorphism is present, it could be for other purposes
Saitta disagrees with their conclusions, writing in an email to The Verge that it sounded like Padian "looked for anything that might counter my claims without actually reading through the whole of the evidence." (Padian says that the study's supplemental material doesn't change the story.) "Determining sexual dimorphism is about ruling out other possibilities," Saitta writes. "I stand by my statement that these stegosaurs represent the best evidence yet for sexual dimorphism, as did my peer reviewers, since other possibilities have been tested for."
Albert Prieto-Marquez, an evolutionary biology researcher who is also at the University of Bristol but was not involved in the study, reiterates that idea in an email to The Verge. "This is a rare case among dinosaurs in which sexual dimorphism could be shown to be the hypothesis that best fits the morphological data in the fossils," he writes.
Concerns with the paper only underscore how difficult it remains to identify sexual dimorphism in dinosaurs. That is — if it existed at all. It's possible that dinosaurs' ornamentation, like the plates on the back of a Stegosaurus, were actually used for other purposes, such as identifying separate species, rather than identifying males and females within a species.
"Some people were suggesting that maybe it was for species recognition and all that," Saitta says. "I think that's a generalization. Simply looking at living animals today, the presence of sexual dimorphism is so widespread ... that it's unbelievable to think that no dinosaurs were sexually dimorphic. I just don't see how that's possible."