A new study suggest that relatives of Triceratops may have intimidated rivals and scored mates with their frilly headwear.
The findings are the latest in a long-running controversy over why ceratopsians—the dinosaur group containing Triceratops—lugged around massive, bony frills on their heads. The puzzle has sparked nearly as many explanations as there are fossils, such as the now-discredited notions that they acted as defensive armor, or even as temperature-regulating radiators.
“Ceratopsians are a real sod,” says David Hone of the Queen Mary University of London, lead author of the study, published January 14 in Palaeontologia Electronica. “Almost every hypothesis for their frills and horns is plausible in maybe a couple of species.”
Hone and his colleagues’ efforts to demystify the frill led them to fossils of Protoceratops andrewsi, a sheep-sized cousin of Triceratops that lived in modern-day Mongolia more than 70 million years ago.
Measurements of 37 specimens confirm that as the dinosaur’s body grew, its head frill mushroomed in size compared with the rest of the body, suggesting that it became more important later in life. The study also suggests similar teenage growth spurts for P. andrewsi’s tail and fang-like tooth.
Some modern animals go through similar growing pains, developing exaggerated features to settle social scores—or to woo mates and repel rivals, an evolutionary imperative called sexual selection.
“Every time we look at how the [facts] line up, the only hypothesis that stands up is something like sexual selection or social dominance,” says Hone.
But Hone hasn’t convinced everyone.
“They are guessing,” says Kevin Padian of the University of California, Berkeley. “They don’t tell us how they could be wrong.”
But it’s hard to know whether dinosaurs would have had a hard time identifying their own species, says Andy Farke of the Raymond M. Alf Museum of Paleontology, who previously has collaborated with Hone. “Often, we have one particular species in one particular area. It’s not like there are 30 different species of horned dinosaurs living alongside each other.”
Padian and Horner also take umbrage at Hone’s mention of sex, since paleontologists have struggled to tell male and female dinosaurs apart from their skeletons alone—making it all but impossible, they argue, to infer anything about how dinosaurs wooed one another.
But studies of modern animals suggest that sex and socializing can spur exaggerated features, even if males and females look the same. For instance, male and female black swans both sport curly feathers and use the feathers to pick mates and settle social squabbles.
“Don’t forget: most of our sexual selection literature comes from birds, which [are] really just an offshoot of the dinosaurs,” says Ken Kraaijeveld of VU University Amsterdam in the Netherlands, who studies how living animals including black swans evolve shared, flashy features. “I don’t think it’s too farfetched to assume that the behaviors that play a big role in birds would be important in dinosaurs, too.”
But unlike biologists, paleontologists are constrained to dinosaurs’ bones, complicating efforts to determine how teenage Triceratops may have used its awkwardly ballooning headgear.
“We can’t go into the wild and observe these things,” says Farke. “It’s difficult to believe that [the frills] are not being used in display and mating—but we can’t test it for certain.”
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