You might imagine that a dinosaur as huge and distinctive as Triceratops would have a relatively uncomplicated fossil history. Well, you'd be wrong: when the first scattered remnants of Triceratops were unearthed in the late 19th century, paleontologists had no idea what they were dealing with, and wound up taking one and one-half steps back for every two step forwards in the process of identifying this dinosaur.
As with any good story, this one has a dramatic prologue. In 1872, the famous paleontologist Edward Drinker Cope examined the hip and rib bones of a huge fossil specimen discovered in Wyoming's Lance Formation. Cope named this creature Agathaumas sylvestris, or "marvelous forest dweller," but lacking a head (or other characteristic structure) he was unsure exactly what kind of dinosaur he was dealing with. Sadly for his legacy, he tentatively decided that it was a hadrosaur; if he had guessed correctly, we might today be referring to ceratopsians as "agathaumids!"
How Was Triceratops Named?
To Edward Drinker Cope's great chagrin, the honor of naming Triceratops went to his arch-rival, Othniel C. Marsh--but not before this eminent paleontologist made a couple of wrong turns himself. In 1887, Marsh examined a fragment of skull, complete with horns, that had been discovered in Colorado--and promptly concluded that it belonged to a megafauna mammal of the Pliocene epoch, which he dubbed Bison alticornis. The next year, a few more mysterious fragments found their way into Marsh's study, which he assigned to the dinosaur genus Ceratops ("horned face"), while still believing that the previous fossil belonged to a giant bison.
Unlike other paleontologists, whose mistakes have consigned them to the fringes of science history, Marsh was in a position to reverse his own errors. In 1888, Marsh was sent yet a third specimen, from Wyoming's Lance Formation (whence Cope's Agathaumas had come); he assigned this, as well as the Bison alticornis specimen, to Ceratops. Shortly afterward, he changed his mind and assigned the Lance formation specimen to a new genus he named Triceratops; it remained to others to file the Bison alticornis specimen under Triceratops as well. (Ceratops survives as a genus, but is today considered a nomen dubium.)
How Many Triceratops Species Are There?
When Othniel C. Marsh named Triceratops, he identified only one species: Triceratops horridus, meaning "rough three-horned face" (and not "horrible" or "horrid," as this species name is often mistranslated). As new Triceratops fossils were discovered over the next few decades, belonging to individuals of various genders and growth stages (male, female, teenager, adult, etc.), slight differences in the shapes of the skulls impelled paleontologists to create no less than 15 additional species, ranging from Triceratops albertensis to Triceratops sulcatus (this last perpetrated by no less an authority than Marsh himself).
It was only in the late 20th century that paleontologists managed to pare down this profusion of Triceratops species. One influential paper, published in 1986, argued that the only valid species was the original Triceratops horridus, while a later researcher made a good case that a second species, Triceratops prorsus, also remained valid. The first paper appealed, reasonably enough, to the fact that the large modern mammals (like giraffes and elephants) in any given area usually come in only one or two variants, and it would be overly generous of nature to endow Triceratops with a dozen species, all living in the same time and and the same place.
How Many Ceratopsian Dinosaurs Were There?
Paleontologists have been debating the genus and species classification of Triceratops for over a hundred years. The controversy extends beyond the Triceratops genus itself; so many ceratopsian dinosaurs have been identified and named in the past few decades that some people wonder if we truly understand the various growth stages of these horned, frilled herbivores.
In 2009, paleontologist John Scannella--supported by his friend and mentor Jack Horner--argued that the ceratopsian genus Torosaurus was invalid, consisting of fossils that actually belonged to extremely old-aged Triceratops adults. (Of course, the popular media immediately got the gist of the argument wrong, blaring headlines like "Researcher Claims Triceratops Wasn't a Dinosaur"). Recently, various parties have argued that the old ceratopsian genus Nedoceratops should actually be assigned to Triceratops, as should new genera like Ojoceratops and Tatankaceratops.
This continuing argument about growth stages extends beyond ceratopsians to another family of dinosaurs that sported thick, heavy, distinctive skulls--the pachycephalosaurs, or "boneheads." Around the same time as the Triceratops/Torosaurus controversy, Jack Horner published a paper arguing that the genera Dracorex and Stygimoloch actually consisted of Pachycephalosaurus individuals in different life stages. Clearly, when it comes to ornately skulled dinosaurs, there's still a lot we don't know!