Why Did the Largest Carnivorous Dinosaurs Rock Flashy Head Ornaments?

Earth has been home to some phenomenally terrifying predators over the ages, but few are as universally feared and admired as meat-loving dinosaurs like Tyrannosaurus rex. Belonging to the theropod family, from which modern birds evolved, these Mesozoic giants were the largest carnivores ever to walk on land, measuring dozens of feet long and tipping the scales at over 40,000 pounds in some cases.

The mind-boggling scale of these creatures has made them popular as movie monsters and museum centerpieces, but little is known about the evolutionary forces that propelled them to such gargantuan dimensions.

Now, new research published on Tuesday in Nature Communications identifies a virtually unexplored correlation between theropod body mass and the presence of flashy cranial "ornaments," such as crests, knobs, or horns.

According to the study, 20 of the 22 largest theropod dinosaur species that left behind well-preserved skulls displayed some form of bony ornamentation on their heads. Moreover, ornamented theropods that weighed over 1,000 kilograms (2,203 pounds) experienced accelerated giantism, meaning they grew larger at about 20 times the rate of unornamented theropod species, suggesting there is a link between extravagant headgear and theropod size.

"It shows that there is an overall trend, and that we should go back and reconsider what the ornament itself is doing for the theropod," Terry Gates, a paleontologist based at North Carolina State University and lead author of the study, told me over the phone.

Many animal clades have evolved dramatic cranial ornaments, from the antlers and horns of ungulates to the flamboyant casques of cassowaries. These visually stunning accoutrements are often used as signals to attract mates, or as inbuilt weapons against sexual rivals.

But the macroevolutionary effect of head ornamentation on body mass has hardly been explored. "One of the big reasons that nobody has been able to detect it is that the fossil record has not been included in studies about modern taxa," Gates said. "Without the context of the fossil record, you're not able to see these long term trends."

As one of the world experts on duck-billed dinosaurs, another group of giant animals that evolved ostentatious cranial ornaments, Gates is interested in fleshing out the evolutionary role these headpieces play across many ancient and modern clades.

"I am currently looking at galliformes—birds like turkeys, chickens, curassows, and quails," he said, adding that "we need to keep in perspective what giantism is for a modern day chicken. It's nowhere close to a T. rex."

Indeed, though chickens are relatives of the mighty tyrannosaurs, it's fair to say that they are not quite so physically imposing (unless you have alektorophobia). That said, "the preliminary findings seem to show that the largest individuals have this bony ornamentation on their heads and small individuals do not," according to Gates.

Likewise, smaller theropods referenced in the study, such as the Maniraptoriformes clade from which birds evolved, were less likely to have cranial ornamentation. The team reasoned that these animals might have relied more on feather displays for sexual signalling, rather than investing energy in frills, crests, or horns observed in their larger relatives.

Pursuing this relationship between ornamentation and giantism further would not only shed light on larger evolutionary trends, it could provide clues about the environments and ecological niches these extinct animals inhabited.

For instance, Gates and his co-authors suggested that large ornamented theropods may have inhabited unforested, open habitats where their attention-grabbing headpieces would be most conspicuous to mates and rivals. Given that ornaments can place demands on their host bodies, such as requiring strengthened vertebrae to support increased head weight, they may also yield interesting information about dinosaur anatomy.

"Can we start to discern what's going on in the way the bodies operate by the style and size of ornamentation?" Gates asked. "You're going to have a possible coupling of many different anatomical systems."

It may even be possible to decipher the color or patterning of theropod head ornaments using the same methods used to unlock intricate details about feather and scale color in other dinosaurs. As if it hasn't been exciting enough to reimagine these iconic carnivores with vibrant feathered plumage, we may also be on the brink of understanding their tantalizing and bizarre cranial bling.

"I think we're finding things that are very surprising," Gates said. "There seems to be some amazing stuff preserved that we never even imagined."

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