Why Did Dinosaurs Have Horns? It May Not Have Been Simply for Defense

Horned dinosaurs represent the peak of prehistoric fashion. From the relatively simple arrangement of the famous “three-horned face” Triceratops or the elaborate splash of ornaments on the skull of Styracosaurus, dinosaurs in the ceratopsid family were decorated with all manner of spikes, hooks and other pointy accessories. But what were once primarily thought of as prehistoric weapons have taken on a new gloss, as scientists discover that the dinosaurs themselves may have recognized other members of their species by their horns—and may have even found them sexy.

Once represented by only a handful of species, the number of known ceratopsids has risen into the dozens. Each species is distinguishable by its own arrangement of horns on the brows, nose and the expanse of bone jutting out from the back of the skull called the frill. The latest to be introduced, the 78-million-year-old Lokiceratops, had long horns angling outward over each eye and a set of pointed spikes along the margin of the rounded frill, including two downward-curved horns at the top. And just as paleontologists have keyed in to the horn arrangements to identify Lokiceratops as a new species, they are solving the prehistoric puzzle of why such elaborate structures evolved.

When horned dinosaurs were still relatively new to science, paleontologists looked at the reptiles’ impressive skulls and saw armaments. “The first concepts of horned dinosaurs definitely focused on the horns as potential weapons against predators or in fights with members of their own species,” says Raymond M. Alf Museum of Paleontology researcher Andrew Farke. In a 1905 paper, for example, paleontologist Richard Swann Lull opined that Triceratops must have charged at foes with its horns tilted down, while species with longer nose horns, such as Alberta’s Centrosaurus, must have thrusted at enemies with their noses. The horns were thought of primarily in terms of attack and defense.

But even as experts have recognized that some species, such as Triceratops, really did lock horns, most ceratopsids had horn arrangements that don’t seem optimized to fight off tyrannosaurs or battle each other. For some, they might have served to intimidate predators that wanted to take a bite. But, importantly, the horns and frills played multiple roles in the lives of the animals. “There is a broader scope in functionality of both horns and frills rather than a shift from thinking it is only one function to another,” says University of Pennsylvania paleontologist Ali Nabavizadeh.

The glut of new ceratopsids has underscored the social importance of prominent horns. “Display functions are entirely likely, especially given the wonderfully enormous variation across ceratopsids,” Nabavizadeh says. At least 20 new ceratopsid species have been named over the past 15 years, most of them from a relatively narrow span of Cretaceous time. Between 70 million and 80 million years ago, especially, multiple horned dinosaur species evolved in what’s now western North America and overlapped in time. If you were to travel from prehistoric Alaska to Mexico around 75 million years ago, you would encounter strikingly different horned dinosaur species as you moved south, sometimes two or three living in the same habitats.

One possibility for the strikingly differing horns is that specific arrangements allowed overlapping horned dinosaur species to tell each other apart. Having a long nose horn and no brow horns, or long brow horns with smaller ornaments around the frill, could have helped horned dinosaurs spot each other on the landscape. Paleontologists have also found that horn shapes changed as dinosaurs grew up, perhaps offering a way for dinosaurs of similar maturity to recognize each other and form groups.

The function of dinosaur horns do not necessarily reveal why they evolved in the first place, however, and paleontologists have been arguing over the point since the 1970s. One tantalizing possibility is that ceratopsid horns evolved so many shapes because of sexual selection, with social behavior molding the anatomy of dozens of species.

Detecting the influence of sexual selection in dinosaurs is a fraught task. One telltale hallmark that sexual selection has been at work is what experts know as sexual dimorphism, or consistent anatomical differences between different sexes such as size, tooth length or coloration. Sexual dimorphism, then, indicates that consistent differences between sexes of the same species must have come from interactions between individuals of that species—such as antlers involved in competition for mates or long, gaudy feathers as a sign of health to prospective mates. Paleontologists haven’t found a conclusive instance of sexual dimorphism in dinosaurs yet. Then again, the sexes of some living bird and reptile species only differ in color, the sounds they make or other features that don’t readily fossilize. And sexual selection can still influence the anatomy of a species even if it doesn’t create differences down to the bones.

“The way the horns and frills grow and evolve are most consistent with sexual selection being pretty important,” Farke says. When baby ceratopsids hatched from their eggs, their horns were smaller and sometimes even different shapes from those of the adults. Juvenile Triceratops fossils, for example, have tiny horns over each eye that change their angle as they grow into the long, forward-pointing horns of the adults. The fact that the horns change through the youth and adolescence of the dinosaurs to a final, mature arrangement suggests that the horns were important social signals and not simply defensive weapons that stayed static through the dinosaurs’ lives.

Ultimately, social factors likely shaped the different displays of horns and frills that ceratopsids wore even as the structures were still useful in other ways. What could impress another Medusaceratops in one context could be used to shield the neck against an attacking tyrannosaur in another. “Animals use their heads any way they want to,” Farke says, “and even if a horn is primarily the result of sexual selection, it’s still often a handy as a weapon.”

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.