History of discovery
The paper that described Nedoceratops was originally part of O. C. Marsh's magnum opus, his Ceratopsidae monograph. Unfortunately, Marsh died (1899) before the work was completed, and John Bell Hatcher endeavored to complete the Triceratops section. However, he died of typhus in 1904 at the age of 42, leaving the paper still uncompleted. It fell to Richard Swann Lull to complete the monograph in 1905, publishing Hatcher's description of a skull separately and giving it the name Diceratops hatcheri; Diceratops means "two horned face."
Since the Diceratops paper had been written by Hatcher, and Lull had only contributed the name and published the paper after Hatcher's death, Lull was not quite as convinced of the distinctiveness of Diceratops, thinking it primarily pathological. By 1933, Lull had had second thoughts about Diceratops being a distinct genus and he put it in a subgenus of Triceratops: Triceratops (Diceratops) hatcheri, including T. obtusus; largely attributing its differences to being that of an aged individual.
Because the Diceratops name was already in use for a hymenopteran (Foerster, 1868), Andrey Sergeevich Ukrainsky gave the animal its current name Nedoceratops in 2007. Unaware that Ukrainsky had already renamed the animal, Octávio Mateus coined another new name for it in 2008, Diceratus. Diceratus is now considered a junior synonym of Nedoceratops.
Nedoceratops means "insufficient horned face". The "nedo" is derived from the Russian prefix meaning "insufficient". The suffix common among ceratopsians, "ceratops", means "horned face". It was named in reference to its lack of a nasal horn.
The nearly complete skull known as "USNM 2412" is the only fossil of Nedoceratops. Like Hatcher's other Triceratops skulls, it was found in eastern Wyoming, i.c. in 1891 in Niobrara County near Lightning Creek. Superficially, it resembles that of Triceratops, but on closer examination, it differs: there is just a rounded stump where the nasal horn should be and the occipital (brow) horns stand almost vertically. Compared to other Triceratops skulls, it is slightly larger than average (1.8m in longest length), but its face is rather short. There also are large holes in the frill, unlike other Triceratops skulls known. Some of these may be pathological, others seem to be genetic. Paleontologist Andrew Farke suggested that the holes in the were from the result of "accidental gorging" from another dinosaur. However, this issue is still up for debate.
The following traits of USNM 2412 have been tentatively proposed by Andrew Farke as distinguishing Nedoceratops from Triceratops: It lacks a recognizable nasal horn. It has almost vertical postorbital horncores. It has small parietal fenestrae (lacking in Triceratops).
The type species is Nedoceratops hatcheri. Nedoceratops belonged to the Ceratopsia (the name is Greek for "horned faces", Keratopia), a group of herbivorous dinosaurs with parrot-like beaks which thrived in North America and Asia during the Cretaceous Period, which ended roughly 66 million years ago. All ceratopsians became extinct at the end of this era.
Several authors have suggested that Nedoceratops may be directly ancestral to Triceratops, or perhaps its nearest relative. An ongoing debate concerns the status of Triceratops, Torosaurus, and Nedoceratops. In a series of publications, John B. Scannella and John R. Horner (2010 and 2011) and claimed that the USNM 2412 skull (i.e., of Nedoceratops) belonged to a "young adult" Triceratops. Evidence for this hypothesis included the shapes of the epoccipital and squamosal bones, and a neck frill (parietal bone) that had "incipient" openings (contrasting with no openings in subadult Triceratops and large openings in adult Triceratops formerly assigned to Torosaurus). The authors were of the opinion that the nasal horn of the USNM 2412 skull could have been lost when the animal was alive or when it became fossilized., and that all three "genera" actually represent different aged individuals of Triceratops. In 2012, Farke proposed a counter argument, and suggested that the bone surface texture and shape of the horns of Nedoceratops indicate an "old adult". A follow-up study by Leonardo Maiorino and colleagues in 2013 using morphometrics found support for Triceratops and Torosaurus being distinct, valid taxa, with Nedoceratops occupying variable positions with respect to the other two but generally outside the range of variation, concluding that "the size of USNM 2412 is a plausible intermediate, but the shape is not."