Ceratopsians - The Horned, Frilled Dinosaurs

The Evolution and Behavior of Ceratopsian Dinosaurs

triceratops
Triceratops is the world's most famous ceratopsian dinosaur (Smithsonian Institution).

Among the most distinctive of all dinosaurs, ceratopsians (Greek for "horned faces") are also the most easily identified--even an eight-year-old can tell, just by looking, that Triceratops was closely related to Pentaceratops, and that both were close cousins of Chasmosaurus and Styracosaurus. However, this extensive family of horned, frilled dinosaurs has its own subtleties, and includes some genera you might not have expected.

(See a gallery of horned, frilled dinosaur pictures and profiles and a slideshow of famous horned dinosaurs that weren't Triceratops.)

Although the usual exceptions and qualifications apply, especially among early members of the breed, paleontologists broadly define ceratopsians as herbivorous, four-legged, elephant-like dinosaurs whose enormous heads sported elaborate horns and frills. The famous ceratopsians listed above lived exclusively in North America during the late Cretaceous period; in fact, ceratopsians may be the most "All-American" of dinosaurs, though some genera did hail from Eurasia and the earliest members of the breed originated in eastern Asia.

Early Ceratopsians

As stated above, the first horned, frilled dinosaurs weren't confined to North America; numerous specimens have also been discovered in Asia (most notably the area in and around Mongolia). Previously, as far as paleontologists could tell, the earliest true ceratopsian was believed to be the relatively small Psittacosaurus, which lived in Asia from 120 to 100 million years ago.

Psittacosaurus didn't look much like Triceratops, but close examination of this dinosaur's small, parrot-like skull reveals some distinctively ceratopsian traits. Recently, however, a new contender has come to light: the three-foot-long Chaoyangsaurus, which dates to the late Jurassic period (as with Psittacosaurus, Chaoyangsaurus has been pegged as a ceratopsian mostly because of the structure of its horny beak); another early genus is the 160-million-year-old Yinlong.

Because they lacked horns and frills, Psittacosaurus and these other dinosaurs are sometimes classified as "protoceratopsians," along with Leptoceratops, the oddly named Yamaceratops and Zuniceratops, and, of course, Protoceratops, which roamed the plains of Cretaceous central Asia in vast herds and was a favorite prey animal of raptors and tyrannosaurs (one Protoceratops fossil has been discovered locked in combat with a fossilized Velociraptor). Confusingly, some of these protoceratopsians coexisted with true ceratopsians, and researchers have yet to determine the exact genus of early Cretaceous protoceratopsian from which all later horned, frilled dinosaurs evolved.

The Ceratopsians of the Later Mesozoic Era

Fortunately, the story gets easier to follow once we reach the more famous ceratopsians of the late Cretaceous period. Not only did all these dinosaurs inhabit roughly the same territory at roughly the same time, but they all looked unnervingly alike, save for the differing arrangements of the horns and frills on their heads. For example, Torosaurus possessed two big horns, Triceratops three; Chasmosaurus' frill was rectangular in shape, while Styracosaurus' looked more like a triangle.

(Some paleontologists claim that Torosaurus was actually a growth stage of Triceratops, an issue that has yet to be conclusively settled.)

Why did these dinosaurs sport such elaborate head displays? As with many such anatomical features in the animal kingdom, they probably served a dual (or triple) purpose: horns could be used to fend off ravenous predators as well as to intimidate fellow males in the herd for mating rights, and frills could make a ceratopsian look bigger in the eyes of a hungry Tyrannosaurus Rex, as well as attract the opposite sex and (possibly) dissipate or collect heat. A recent study concludes that the main factor driving the evolution of horns and frills in ceratopsians was the need for members of the same herd to recognize each other!

Paleontologists divide the horned, frilled dinosaurs of the late Cretaceous period into two families.

"Chasmosaurine" ceratopsians, typified by Chasmosaurus, had relatively long brow horns and large frills, while "centrosaurine" ceratopsians, typified by Centrosaurus, possessed shorter brow horns and smaller frills, often with large, ornate spines projecting from the top. However, these distinctions shouldn't be taken as set in stone, since new ceratopsians are constantly being discovered across the expanse of North America--in fact, more certaopsians have been discovered in the U.S. than any other type of dinosaur.

Ceratopsian Family Life

Paleontologists often have a hard time distinguishing male from female dinosaurs, and they sometimes can't even conclusively identify juveniles (which may have been either the children of one genus of dinosaur or the full-grown adults of another). Ceratopsians, though, are one of the few families of dinosaurs in which the males and females can usually be told apart. The trick is that, as a rule, male ceratopsians had bigger frills and horns, while those of females were slightly (or sometimes significantly) smaller.

Oddly enough, the hatchlings of different genera of horned, frilled dinosaurs seem to have been born with pretty much identical skulls, only developing their distinctive horns and frills as they grew into adolescence and adulthood. In this way, ceratopsians were very similar to pachycephalosaurs (bone-headed dinosaurs), the skulls of which also changed shape as they aged. As you can imagine, this has led to a fair amount of confusion; an unwary paleontologist may assign two grossly different ceratopsian skulls to two different genera, when they were actually left by differently aged individuals of the same species.