Among the most distinctive of all dinosaurs, ceratopsians are also the most easily identified--even an eight-year-old can tell, just by looking, that Triceratops is closely related to Pentaceratops, and that both are close cousins to Chasmosaurus and Styracosaurus. However, the ceratopsian family has its own subtleties, and includes some genuses you might not expect. (See a gallery of ceratopsian pictures.)
Stay up to date on the latest dinosaur news--sign up for the free About.com dinosaur newsletter today!
Although there are the usual exceptions and qualifications, paleontologists broadly define ceratopsians as herbivorous, four-legged dinosaurs whose enormous heads sported elaborate horns and frills. The famous ceratopsians listed above lived exclusively in North America during the Cretaceous period; in fact, the ceratopsians may be the most "All-American" of all the dinosaurs.
Early Ceratopsians
The first ceratopsians weren't confined to North America; numerous specimens have also been found in Asia (most notably the area in and around Mongolia). As far as paleontologists can tell, the earliest true ceratopsian was 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 shows some distinctively ceratopsian traits.
Because it lacked horns and frills, Psittacosaurus is usually classified as a protoceratopsian, along with Leptoceratops, the oddly named Yamaceratops and Zuniceratops, and Protoceratops, the last of which roamed the plains of Cretaceous North America in vast herds. Confusingly, some of these protoceratopsians coexisted with ceratopsians, and researchers have yet to determine the exact genus of protoceratopsian from which all the ceratopsians evolved.
Later Ceratopsians
Fortunately, the story gets easier to follow once we reach the more famous ceratopsians. Not only did these dinosaurs all inhabit roughly the same territory at roughly the same time (the late Cretaceous), but they all looked unnervingly alike, except for the different arrangements of horns and frills on their heads. Torosaurus had two big horns, Triceratops three; Chasmosaurus' frill was rectangular in shape, while Styracosaurus' looked more like a triangle.
Why did these ceratopsians have such elaborate displays? As with many anatomical features produced by evolution, they probably served dual purposes: horns could be used to fend off ravenous predators as well as to intimidate fellow males for mating rights, and frills could make a ceratopsian look bigger in the eyes of a T. Rex, as well as attract the opposite sex and (possibly) dissipate or collect heat.
Ceratopsian Families
Paleontologists usually have a hard time distinguishing male from female dinosaurs, and sometimes can't even conclusively identify juveniles (are they the children of one genus, or the full-grown adults of another?) Ceratopsians, though, are one of the few families of dinosaurs whose males, females, and kids can usually be told apart.
The trick is that (as a rule) only male ceratopsians had huge frills and horns, while those of females were noticeably smaller (in this way, ceratopsians were similar to modern mammals like walruses). Also, ceratopsian babies were all born with pretty much identical skulls; they only developed their distinctive horns and frills as they grew.
Here's a list of the most notable ceratopsians and protoceratopsians; you can click on the links for more information.
Centrosaurus Like a unicorn, this ceratopsian only had one horn.
Chasmosaurus The only dinosaur that came with its own awning.
Leptoceratops One of the most primitive of all ceratopsians.
Pachyrhinosaurus This "thick-nosed lizard" roamed the North American forests.
Pentaceratops This "five-horned" herbivore really had only three.
Protoceratops A famous dinosaur with a very funky frill.
Psittacosaurus This dinosaur's noggin wouldn't have looked out of place on a parrot.
Styracosaurus Winner of the "most elaborate head display" competition.
Torosaurus This horned, frilled herbivore was a close cousin of Triceratops.
Triceratops The famous three-horned plant-eater.
Yamaceratops No, it didn't have a sweet potato for a head.
Zuniceratops This horned dinosaur was discovered by an eight-year-old boy.
