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Monoclonius (meaning "single sprout") was a ceratopsian dinosaur from the Judith River Formation of Late Cretaceous Montana and Canada. It is often confused with Centrosaurus, a similar genus of ceratopsian (some think the two may even be identical, of a different age or gender). Monoclonius was described by Edward Drinker Cope in 1876. All Monoclonius specimens are now believed to be juveniles or subadults, in many cases of other genera such as Centrosaurus.
Contrary to popular belief, the name Monoclonius does not mean "single horn" or refer to its distinctive single nasal horn. In fact, the genus was named before it was known to have been a horned dinosaur, and had previously been considered a "hadrosaur." The name in fact means "single sprout", in reference to the way its teeth grew compared to its relative Diclonius ("double sprout"), which was named by Edward Drinker Cope in the same paper as Monoclonius. In Diclonius, Cope interpreted the fossils to show two series of teeth in use at one time (one mature set and one sprouting replacement set), while in Monoclonius, there appeared to be only one set of teeth in use as a chewing surface at any one time, with replacement teeth growing in only after mature teeth had fallen out.
Monoclonius was Edward Drinker Cope's third named ceratopsian (after Agathaumas and Polyonax). The type specimen was found in the summer of 1876 in Montana, only about 100 miles from the site of the Battle of the Little Bighorn that June. Although it was not an articulated skeleton, Cope recovered most of the animal (only the feet were entirely missing), including skull material and the base part of a long nasal horn. Since the ceratopsians were still unknown, Cope was uncertain about much of the skull material, not recognizing the horn core as part of a fossil horn.
After O. C. Marsh's description of Triceratops in 1889, Cope reexamined his Monoclonius specimen and realized that Triceratops, Monoclonius, and Agathaumas represented a group of similar dinosaurs. In the same paper that Cope examined M. crassus, he also named three more Monoclonius species. He described Monoclonius as having a large nasal horn and two smaller horns over the eyes and a large frill ( parietal) with broad openings.
Later, John Bell Hatcher (one of Marsh's workers and therefore in the ' Yale Camp' of the Bone Wars), in continuing Marsh's monograph on the Ceratopsidae, derided Cope's collecting methods. Cope rarely identified specimens in the field with precise locations and often ended up describing composites, rather than single individuals. Hatcher reexamined the type specimen of M. crassus and the only skull remain that he could positively assign to this specimen was the left half of the parietal (the dorsal part of the neck frill). He could not assign any of the several squamosals (side of the frill) in the collection to the type specimen and did not believe that Cope's orbital horn (catalogued under a different number) belonged to it.
In the years after Cope's 1889 paper, it appears that there was a tendency to describe everything from the Judith River beds as Monoclonius. The first dinosaur species described from Canada were ceratopsians in 1902 by Lawrence Lambe, including three new species of Monoclonius based on fragmentary skulls.
In 1904, Lambe described Centrosaurus, based on a second specimen (a skull in better condition than the first) that he had attributed to Monoclonius dawsoni in 1902. With newer specimens collected by Charles H. Sternberg, it became clear that Centrosaurus was distinctly separate from Monoclonius, at least to Lambe. In a 1914 paper, Barnum Brown reviewed Monoclonius and Centrosaurus, dismissing most of Cope's species, leaving only M. crassus. Comparing Monoclonius to Centrosaurus, he determined that the M. crassus specimen had been that of an old animal and damaged by erosion and that the two were synonymous. In 1915, Lambe answered Brown in another paper (this is the review of Ceratopsia in which Lambe established three families), transferring M. dawsoni to Brachyceratops and M. sphenocerus to Styracosaurus. This left M. crassus, which he considered non-diagnostic, largely due to its damage and the lack of a nasal horn. Lambe ended the paper by attributing Brown's M. flexus to Centrosaurus apertus (the type species of Centrosaurus). The next round fell to Brown in a paper on Albertan centrosaurines, which, for the first time, analyzed a complete ceratopsian skeleton, which he named Monoclonius nasicornus (he contributed to the confusion even more by describing yet another species, M. cutleri).
The matter bounced back and forth, over the next few years, until Richard Swann Lull published his "Revision of Ceratopsia", in 1933. Although, unlike the beautifully illustrated 1907 monograph, it has relatively few illustrations, it is known for the attempt to identify and locate all ceratopsian specimens then known. Lull described another specimen from Alberta (YPM 2015; Monoclonius (Centrosaurus) flexus) and decided that Centrosaurus was a junior synonym of Monoclonius, perhaps distinct enough to deserve subgeneric rank. (This specimen is exhibited at Yale's Peabody Museum in an unusual way: the left half shows the skeleton, but the right side is a reconstruction of the living animal.) Charles M. Sternberg, son of Charles H. Sternberg, in 1940 firmly established the existence of Monoclonius-type forms in Alberta (no further specimens have come from Montana since 1876) and showed that differences justified the separation of the two genera. Monoclonius-types are rarer and found in earlier horizons than Centrosaurus-types, seemingly indicating that the one is probably ancestral to the other.
Aside from fossils that were classified in Monoclonius because of confusion about the genus (see above), Monoclonius specimens are now generally recognized as juveniles or subadults. In some cases the adult form is an already-known species, but in others the adult may not yet be known to science.
- Monoclonius crassus Cope 1876 [AMNH 3998]
- M. albertensis (Lambe, 1913/Leahy, 1987); included with Styracosaurus albertensis.
- M. apertus (Lambe, 1904/Kuhn, 1964); included with Centrosaurus apertus.
- M. belli (Lambe, 1902); included with Chasmosaurus belli.
- M. canadensis (Lambe, 1902); included with Chasmosaurus canadensis.
- M. cutleri (Brown, 1917); back half of skeleton with some skull fragments, included with Centrosaurus apertus.
- M. dawsoni (Lambe, 1902; including Brachyceratops dawsoni and Centrosaurus dawsoni), included with Centrosaurus apertus.
- M. fissus Cope, 1889; isolated pterygoid (Cope identified it as a squamosal); nomen nudum .
- M. flexus (Brown, 1914); included with Centrosaurus apertus.
- M. longirostris (Sternberg, 1940/Kuhn, 1964); included with Centrosaurus apertus.
- M. lowei (Sternberg, 1940); a large, somewhat flattened, skull, apparently that of a subadult (sutures are not completely closed). Sternberg pointed out the resemblances of this specimen to Brachyceratops. The species was named in honour of Harold D'acre Robinson Lowe from Drumheller, AB. Lowe was a field assistant to C.M. Sternberg and worked six field seasons (during the 1925-1937 period) with him across southern Alberta, with other work in Manitoba and Saskatchewan.
- M. montanensis (Gilmore, 1914); included with Brachyceratops montanensis.
- M. nasicornis (Brown, 1917); included part with Centrosaurus apertus and part with Styracosaurus albertensis (Dodson believes this is actually the female of Styracosaurus)
- M. recurvicornis Cope, 1889; braincase, 3 horns and isolated fragments; nomen nudum included with Ceratops recurvicornis.
- M. sphenoceras Cope, 1890; nasal horn and premaxilla; nomen nudum including Agathaumas sphenoceras, A. monoclonius and Styracosaurus sphenoceras).
Diet and ecology
Monoclonius, like all Ceratopsians, was a herbivore. During the Cretaceous, flowering plants were "geographically limited on the landscape", so it is likely that this dinosaur fed on the predominant plants of the era: ferns, cycads and conifers. It would have used its sharp Ceratopsian beak to bite off the leaves or needles.