Biology

Study Finds The Biggest, Sexiest Dinosaurs Had Horned Heads

A new study suggests dinosaurs with elaborate cranial ornaments got larger 20 times faster than their less blessed relatives

Biology
Models of Tyrannosaurus rex from "Walking With Dinosaurs" — Illustration: Tara Jacoby
Sep 29, 2016 at 5:41 PM ET

If a dinosaur species wanted to get as big as possible as fast as possible, it didn’t just need access to lots of food and wide-open spaces. As it turns out, it also needed a big, sexy crest on the top of its head. New analysis of the fossil record reveals dinosaur species with some kind of cranial ornament evolved to giant body sizes 20 times faster than other therapods, taking as little as four to six million years to become relatively big.

Paleontologists already knew that crests, horns, and other bony bumps likely helped dinosaurs look sexy and powerful for social displays, much as antlers do for deer and moose today. Now, North Carolina State paleontologist Terry Gates and his colleagues made this new discovery by looking at more than a hundred fossils from therapods, a group of mostly carnivorous dinosaurs that includes Tyrannosaurus rex and velociraptors.

He noticed that most smaller raptors lacked the headgear that distinguished a lot of their bigger cousins, and he was curious to find out whether there really was a link between body size and head ornamentation. As it turns out, the connection with bony bumps isn’t so much with a species being gigantic — therapod species without ornamentation can also reach massive sizes — as it is to do with how fast a species became gigantic. But why should such ornamentation turbo-charge the evolution of a dinosaur species?

“That’s tough, because there are several ways this growth could come about,” Gates told Vocativ. “It could be, number one, because having ornamentation could have social repercussions, in which the animals that had the horns were in competition.” If dinosaurs with horns or other bumps were constantly fighting for dominance, it figures the bigger dinosaurs would tend to do better and more often live to pass on their genes to the next generation. This possibility imagines a kind of evolutionary arms race, in which generation after generation of fighting for dominance would select for the biggest dinosaurs over and over, until the entire species had swelled to giant size in the name of better mating displays. It’s almost like the sexier the dinosaurs got, the bigger dinosaurs got — not that that is Gates’s working theory.

Another possibility is considerably less violent. According to Gates, it’s possible that a disproportionate number of the bumpy-headed dinosaurs species happened to live in open habitats like deserts or grasslands, which are more favorable to growth in body size than cramped forest environments. Most likely, both factors played a role in driving this ultra-fast growth in these dinosaur species, but it will take considerably more data than paleontologists currently possess to know precisely what drove this rapid evolution.

Still, the role of head ornamentation for mating and competition may well be a key piece of the puzzle. Let’s compare ornamentation with another, later therapod adaptation: feathers. After all, birds — dinosaurs’ main living descendants — are still very happy to use feathers in their own sexual displays. For whatever reason, ornamentation seemed to unlock massive growth potential in therapods in a way that feathers generally didn’t. Gates explained that, while a few feathered raptors did grow to giant sizes, many species remained small, despite living in open habitats that would seemingly encourage such growth.

There are advantages to reaching the kind of sizes that bumpy-headed therapods did at such record speeds, including a more efficient metabolism and an extended lifespan. But even those perks may well have come up short in comparison with what feathers offered in favor of head ornaments. “You can lose your feathers and then you gain brand new feathers with the next molt,” said Gates, pointing out bumps wouldn’t just grow back if a dinosaur suffered an accident. “[Feathers] are bright, and they’re shiny. You can raise them up, you can lower them down. They’re much more flexible, they’re lighter weight … There’s lots of good reasons not to have bony cranial ornaments.”

Ultimately, Gates and other researchers are looking at a big paleontological puzzle, and they don’t yet have all the pieces they need to solve it. This new research gets them closer, but Gates said that much more research of dinosaur society will be necessary to tease out the complicated relationships between adaptations and functions that we’re seeing here — and it’s not exactly easy studying societies that haven’t existed for millions of years. For now, we know there are some sort of links between head ornamentation, body size, the habitats dinosaurs lived in, and their sexy social displays, but it’s still only a guess which factors drove which adaptations, and which were happy accidents of evolution.

“Trying to figure out the purpose of these structures in animals that have been dead since the Mesozoic is extremely difficult,” said Gates, who explains that structures often evolve for one purpose but are coopted for use in different ways, adding another wrench into evolutionary study.

“Evolution is not as simple as we originally thought.”