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 to Chasmosaurus and Styracosaurus. However, the ceratopsian family has its own subtleties, and includes some species you might not have expected. (See a gallery of ceratopsian pictures.)
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 all the dinosaurs, though some species did hail from Eurasia.
As stated above, 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 the relatively small Psittacosaurus, which lived in Asia from 120 to 100 million years ago. Psittacosaurus didn't look much like the much later Triceratops, but close examination of this dinosaur's small, parrot-like skull reveals some distinctively ceratopsian traits. In the past few years, though, a new contender has come to the fore: 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 distinct structure of its horny beak); another contender is the 160-million-year-old Yinlong.
Because it lacked horns and frills, Psittacosaurus is sometimes classified as a "protoceratopsian," along with Leptoceratops, the oddly named Yamaceratops and Zuniceratops, and, of course, Protoceratops, which roamed the plains of Cretaceous North America in vast herds and was a favorite prey animal of raptors and tyrannosaurs. Confusingly, some of these protoceratopsians coexisted with ceratopsians, and researchers have yet to determine the exact genus of early Cretaceous protoceratopsian from which all later ceratopsians evolved.
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 different arrangements of horns and frills on their heads. For example, Torosaurus had 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 have such elaborate head displays? As with many such anatomical features, they probably served a dual (or triple) purpose: 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 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 was the need for members of the same herd to recognize each other!
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 the children of one genus 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) only male ceratopsians had huge frills and horns, while those of females were significantly smaller.
Oddly enough, ceratopsian hatchlings of different genera 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 at different life stages. As you can imagine, this has led to a fair amount of confusion; an unwary paleontologist may assign two grossly different skulls to two different genera, when they were actually left by differently aged individuals of the same species.
Here's a list of the most notable ceratopsians and protoceratopsians; just click on the links for more information.
Achelousaurus Might this have been a growth stage of Pachyrhinosaurus?
Agathaumas The first ceratopsian dinosaur ever discovered.
Agujaceratops It was once classified as species of Chasmosaurus.
Ajkaceratops The first ceratopsian ever to be discovered in Europe.
Albalophosaurus One of the few dinosaurs ever to be discovered in Japan.
Albertaceratops The most basal "centrosaurine" yet identified.
Anchiceratops This dinosaur had a distinctively shaped frill.
Archaeoceratops Possibly the smallest ceratopsian that ever lived.
Arrhinoceratops This ceratopsian was named for its "missing" nose horn.
Auroraceratops A close relative of Archaeoceratops.
Avaceratops This ceratopsian is represented by a single juvenile.
Bravoceratops This ceratopsian was recently discovered in Texas.
Centrosaurus Like a unicorn, this ceratopsian only had one horn.
Cerasinops A small ceratopsian of the late Cretaceous.
Chaoyangsaurus An early ceratopsian of the late Jurassic period.
Chasmosaurus The only dinosaur that came with its own awning.
Coahiluaceratops This Mexican ceratopsian made its debut in 2010.
Coronosaurus This "crown lizard" was once classified as a species of Centrosaurus.
Diabloceratops This "devil-horned face" is a new addition to the ceratopsian family.
Diceratops Was this two-horned dinosaur really a specimen of Triceratops?
Eotriceratops This "dawn Triceratops" was recently discovered in Canada.
Gryphoceratops A tiny ceratopsian of Cretaceous North America.
Helioceratops A recently announced Asian ceratopsian.
Hongshanosaurus This early ceratopsian is known by two skulls.
Judiceratops The earliest Chasmosaurus ancestor yet identified.
Koreaceratops Did this ceratopsian like to go swimming?
Kosmoceratops This ceratopsian had a bizarre, downward-folding frill.
Leptoceratops One of the most primitive of all ceratopsians.
Liaoceratops A tiny ceratopsian of early Cretaceous Asia.
Magnirostris This ceratopsian had an unusually big beak.
Medusaceratops Named after the Greek goddess who had snakes for hair.
Microceratops Probably the smallest ceratopsian that ever lived.
Mojoceratops This ceratopsian had a heart-shaped frill.
Nasutoceratops This dinosaur had horns like a modern steer.
Ojoceratops A very close relative of Triceratops.
Pachyrhinosaurus This "thick-nosed lizard" roamed the North American forests.
Pentaceratops This "five-horned" dinosaur really had only three.
Prenoceratops A close relative of Leptoceratops.
Protoceratops A famous dinosaur with a very funky frill.
Psittacosaurus This dinosaur's noggin wouldn't have looked out of place on a parrot.
Rubeosaurus A ceratopsian dinosaur from the Two Medicine Formation.
Sinoceratops A rare ceratopsian from late Cretaceous China.
Spinops This ceratopsian was named 100 years after its bones were found.
Styracosaurus Winner of the "most elaborate head display" competition.
Titanoceratops The biggest of all the horned, frilled dinosaurs.
Torosaurus This horned, frilled herbivore was a close cousin of Triceratops.
Triceratops The famous three-horned plant-eater.
Udanoceratops The largest ceratopsian to run on two legs.
Unescoceratops Named after the United Nation's UNESCO.
Utahceratops Guess which state this dinosaur was discovered in?
Vagaceratops This big-frilled dinosaur was closely related to Kosmoceratops.
Xenoceratops This "alien horned face" was announced in 2012.
Xuanhuaceratops An early ceratopsian of the late Jurassic.
Yamaceratops No, it didn't have a sweet potato for a head.
Yinlong This "hidden dragon" was an early ceratopsian.
Zuniceratops This horned dinosaur was discovered by an eight-year-old boy.