Paleontologists Probe the Bonehead that Foreshadowed Domed Dinos

This striking skull shape evolved at least twice. But what was its purpose?

Computer image of Triopticus skull overlaid on the field site where it was found. (Stocker and colleagues)
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Pachycephalosaurs are among the most famous of all dinosaurs, set apart from their relatives by the thickened domes of bone atop their skulls. But it turns out that these saurians were just copycats. In a study by Virginia Tech paleontologist Michelle Stocker and colleagues released in Current Biology today, there was an even earlier animal that pioneered the bonehead look over a hundred million years before pachycephalosaurs came onto the evolutionary scene.

The fossil, named Triopticus primus, has a bit of a circuitous history. The bones were uncovered by a Works Progress Administration crew digging in the 230 million year old Otis Chalk of west Texas during the late 1930s and early 1940s. Whoever excavated the fossil didn’t know what they had found. The specimen, consisting of a skull roof and braincase, went quietly to rest in the collections of the University of Texas at Austin. It wasn’t until 2010 that, while poking through the museum collections, Stocker and her colleagues rediscovered the rock-encased specimen and had another look.

What they found was a shocker. Even though there wasn’t much of the animal, when cleaned of the encasing sediment it had an anatomy unlike any other creature of its time. The skull roof was thick and bulbous, with a large hole in the middle for the parietal eye—an opening present in some animals that detects light. (Triopticus means “three vision” for how much the hole resembles an eye socket.) “One of the most interesting features of Triopticus is the thickened skull roof, just like what we see in the distantly-related pachycephalosaur dinosaurs from over 100 million years later,” Stocker says.

But Triopticus was no dinosaur. Stocker and her colleagues found that this lion-sized animal was something more archaic, categorized as an archosauriform—an early member of the larger group that includes crocodiles, dinosaurs and their relatives.

This was an oddball among oddballs. “None of the close relatives that we have for Triopticus in the Triassic have a similar structure to their heads,” Stocker says. Nor is it clear why the animal evolved such a distinctive skull. “It’s difficult for us to say what this domed morphology would have been for or what would have encouraged the evolution of this structure,” she says. While paleontologists debate whether the copycat pachycephalosaurs used their skulls for display, combat or something else, why Triopticus pioneered this look is a mystery.

Nevertheless, the discovery of this dome-headed animal highlights a curious pattern in the Age of Reptiles. During the time Triopticus, in the Triassic period, dinosaurs had evolved but were small, rare, and did not yet rule the land. Ancient and strange relatives of crocodiles were far more prominent, and they presaged what many dinosaurs would look like millions of years later down the line. 

There were heavily-armored herbivores called aetosaurs that resembled the later ankylosaurs, fierce carnivores called rauisuchids that looked like smaller versions of Tyrannosaurus, bipedal croc-cousins called shuvosaurids that seem similar to ostrich mimic dinosaurs, and now the more archaic Triopticus, setting the thick-skulled look long before pachycephalosaurs would do the same. Many of the shapes dinosaurs had evolved by the Cretaceous, crocodile cousins had already tried out millions of years earlier in the Triassic.

“The Triassic Period may have been a time of experimentation with respect to body plans,” Stocker says. Reptiles were bouncing back after the world’s worst mass extinction, with evolution able to generate new forms into a relatively open field of possibilities. The Age of Reptiles may have replayed this on a smaller scale. Many of the dinosaur-like croc cousins, as well as Triopticus and its kin, were largely wiped out by another mass extinction at the end of the Triassic, giving dinosaurs the opportunity to flourish and just happen to evolve along similar lines. Just as fashions get recycled a few decades after the first appear, so evolution can make what’s old new again.

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