| Monoclonius | |
|---|---|
 | |
| Nasal horn base and lectotype frill ofM. crassus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Family: | †Ceratopsidae |
| Subfamily: | †Centrosaurinae |
| Genus: | †Monoclonius Cope,1876 |
| Type species | |
| †Monoclonius crassus Cope, 1876 | |
Monoclonius (meaning "single sprout") is an extinctgenus of herbivorousceratopsiandinosaur found in theLate Cretaceous layers of theJudith River Formation inMontana, United States, and the uppermost rock layers of theDinosaur Park Formation inAlberta,Canada dated to between 75 and 74.6 million years ago.[1][2]
Monoclonius was first named byEdward Drinker Cope in 1876. Later, muchtaxonomic confusion was caused by the discovery ofCentrosaurus, a very similar genus of ceratopsian that is known from much better remains. Today, typicalMonoclonius specimens are usually believed to be juvenileCentrosaurus or subadults, in many cases of other genera such asCentrosaurus. Those specimens that remain under the nameMonoclonius are mostly too incomplete or immature to be confidently matched with adult specimens from the same time and place. This is especially true of thetype species,Monoclonius crassus. Therefore,Monoclonius is often considered anomen dubium now, pending further study.[2]
Monoclonius was Edward Drinker Cope's third named ceratopsian, afterAgathaumas andPolyonax. Several fossils were found by Cope, assisted by a youngCharles Hazelius Sternberg, in the summer of 1876 near theJudith River inChouteau County, Montana, only about a hundred miles (some 150 km) from the site of theBattle of the Little Bighorn, fought that June. The finds did not represent a single, let alone articulated, skeleton, but came from different locations. Together they included elements of most parts of the animal (only the feet were entirely missing), including the base part of a long nasal horn, part of the skull frill, brow horns, three fused cervical vertebrae, a sacrum, a shoulder girdle, an ilium, an ischium, two thighbones, a shinbone, a fibula and parts of a forelimb. Just two weeks after leaving Montana, Cope hastily described and named these finds on 30 October 1876 as thetype speciesMonoclonius crassus. Thespecific name means "the fat one" inLatin. Since the ceratopsians had not been recognised yet as a distinctive group, Cope was uncertain about much of the fossil material, not recognizing the nasal horn core, nor the brow horns, as part of a fossil horn. The skull frill he interpreted as anepisternum, an ossified part of the breastbone, and the fused cervicals he assumed to be anterior dorsals.[3]
Contrary to what was stated in most popular or technical science publications prior to 1992, the nameMonoclonius 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", from Greek μόνος,monos, "single", and κλωνίον,klonion, "sprout", in reference to the way its teeth grew compared to its relativeDiclonius ("double sprout"), which was named byEdward Drinker Cope in the same paper asMonoclonius. InDiclonius, Cope interpreted the fossils to show two series of teeth in use at one time (one mature set and one sprouting replacement set), while inMonoclonius, 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. This salient feature of the tooth, which specimen is now lost, almost certainly precludes it from being centrosaurine: it probably indeed is hadrosaurian and was by mistake associated with the rest of the type material.[4]
AfterOthniel Charles Marsh's description ofTriceratops in 1889, Cope reexamined hisMonoclonius specimen and realized thatTriceratops,Monoclonius, andAgathaumas represented a group of similar dinosaurs. In the same year he redescribedMonoclonius as having a large nasal horn and two smaller horns over the eyes and a largefrill, of which the parietal bone had been found with broad openings. In the same paper in which Cope examinedM. crassus, he also named three moreMonoclonius species. The first wasMonoclonius recurvicornis, meaning "with a recurved horn", based on specimen AMNH 3999, a short curved nasal horncore and two brow horns, that he had already reported in 1877 but not associated withM. crassus.[5] The second wasMonoclonius sphenocerus, the "wedge-horned" from Greek σΦηνός,sphènos, "wedge", based on specimen AMNH 3989, a 325 millimetres (12.8 in; 1.066 ft) long nasal horn, found by Sternberg in 1876 onCow Island in theMissouri. The third species wasMonoclonius fissus, "the split one", based on specimen AMNH 3988, apterygoid that Cope assumed to be a splitsquamosal.[6]
In 1895, for financial reasons, Cope was forced to sell a large part of his collection to theAmerican Museum of Natural History. This included hisMonoclonius specimens that thus received AMNH inventory numbers. TheM. crassus fossils were catalogued as AMNH 3998. AlthoughJohn Bell Hatcher had been one of Marsh's workers and therefore in the 'Yale Camp' of theBone Wars, the rivalry between Cope and Marsh, after the death of both he was invited to complete Marsh's monograph on theCeratopsia also using Cope's material. Hatcher was very critical of Cope's collecting methods. Cope rarely identified specimens in the field with precise locations and often ended up describingcomposites, rather than single individuals. Hatcher reexamined the presumedtype specimen ofM. crassus and concluded it in fact represented several individual animals and thus was a series ofsyntypes. Therefore, he selected one of these as thelectotype, the name-bearing fossil, and chose the distinctive left parietal, forming the dorsal part of the neck frill. The severalsquamosals, sides of the frill, in the collection could not be associated to this lectotype and he did not believe that Cope's orbital horn (catalogued under a different number) belonged to it. This analysis was eventually, after Hatcher had deceased also, published byRichard Swann Lull in 1907.[7]
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In the years after Cope's 1889 paper, it appears that there was a tendency to describe any ceratopsid material from the Judith River beds asMonoclonius. The first dinosaur species described from Canada were ceratopsians, in 1902 byLawrence Lambe, including three new species ofMonoclonius based on fragmentary skulls. Two of these,Monoclonius belli andMonoclonius canadensis, were later seen as two species within separate genera:Chasmosaurus belli andEoceratops canadensis. The third,Monoclonius dawsoni, of which the epithet honouredGeorge Mercer Dawson, was based on a partial skull, specimen NMC 1173. To this species a parietal was referred, specimen NMC 971.[8] However, in 1904, Lambe decided that this parietal represented a different species and genus that he namedCentrosaurus apertus.[9]
With newer specimens collected by Charles H. Sternberg, it became accepted thatCentrosaurus was distinctly separate fromMonoclonius, at least by Lambe. This was challenged in a 1914 paper byBarnum Brown who reviewedMonoclonius andCentrosaurus, dismissing most of Cope's species, leaving onlyM. crassus. Comparing the parietals ofMonoclonius andCentrosaurus, he concluded that any differences were caused by the fact that theM. crassus lectotype had been that of an old animal and damaged by erosion. This would mean that the two were synonymous, with the nameMonoclonius having priority. In the same paper he named another species:Monoclonius flexus, "the curved one", based on specimen AMNH 5239, a skull found in 1912 and featuring a forward curving nasal horn.[10] In 1915, Lambe answered Brown in another paper — the review of the Ceratopsia in which Lambe established three families — transferringM. dawsoni toBrachyceratops andM. sphenocerus toStyracosaurus. This leftM. crassus, which he considered non-diagnostic, largely due to its damage and the lack of a nasal horn. Lambe ended the paper by referring Brown'sM. flexus toCentrosaurus apertus, the type species ofCentrosaurus.[11] The next round fell in 1917 to Brown in a paper on Albertan centrosaurines, which, for the first time, analyzed a complete ceratopsian skeleton, specimen AMNH 5351 found by him in 1914, which he namedMonoclonius nasicornus ("with the nose-horn"). In the same paper he described yet another species,Monoclonius cutleri, the epithet honouringWilliam Edmund Cutler, based on specimen AMNH 5427, a headless skeleton featuring skin impressions.[12]
The matter bounced back and forth, over the next few years, until R.S. Lull published his "Revision of the Ceratopsia", in 1933. Although, unlike the 1907 monograph, it has relatively few illustrations, it attempted to identify and locate all ceratopsian specimens then known. Lull described another almost complete specimen from Alberta: AMNH 5341, presently exhibited as YPM 2015 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, and referred it to aMonoclonius (Centrosaurus) flexus. Lull had decided thatCentrosaurus was a junior synonym ofMonoclonius, but distinct enough to deserve subgeneric rank; he therefore also created aMonoclonius (Centrosaurus) apertus.[13]Charles Mortram Sternberg, son of Charles H. Sternberg, in 1938 firmly established the existence ofMonoclonius-type forms in Alberta — no further specimens had come from Montana since 1876 — and claimed that differences justified the separation of the two genera.Monoclonius-types were rarer and found in earlier horizons thanCentrosaurus-types, seemingly indicating that the one would be ancestral to the other.[14] In 1940 C.M. Sternberg named another species:Monoclonius lowei. The specific name honoured his field assistantHarold D'acre Robinson Lowe fromDrumheller who had worked six field seasons, during the 1925-1937 period, with him across southern Alberta, with other work inManitoba andSaskatchewan.[15] He created yet another combination in 1949, renamingBrachyceratops montanensis intoMonoclonius montanensis, a change today no longer accepted.[16] In 1964Oskar Kuhn renamedCentrosaurus longirostris intoMonoclonius longirostris.[17] In 1987Guy Leahy renamedStyracosaurus albertensis intoMonoclonius albertensis;[18] in 1990Thomas Lehman renamedAvaceratops lammersi intoMonoclonius lammersi.[19] Both names have found no acceptance.
During the 1990s, the relation betweenMonoclonius andCentrosaurus was still contentious. There were three relevant possibilities. The first was that, as Barnum Brown had concluded in 1914,Monoclonius crassus was a valid species and identical toCentrosaurus apertus. In that caseCentrosaurus would be ajunior synonym andMonoclonius would have priority. The second was that, as Lambe had thought,Monoclonius crassus was anomen dubium, a species based on fossil material that was so indistinct that no other material could justifiably be associated with it. In that case, the nameMonoclonius could be disregarded andMonoclonius species other thanM. crassus — if notnomina dubia ornomina nuda themselves — would have to be referred to other genera. The third possibility was that bothMonoclonius andCentrosaurus were valid and thus separate.
The last position was from 1990 defended byPeter Dodson who claimed that specimen AMNH 3998, theM. crassus lectotype, differed from theCentrosaurus apertus holotype in having a very thin parietal close to the skull frill edge. That this was not simply a matter of individual variation would be proven by the fact thatM. lowei had a comparably thin frill.[20] However, in 1997Scott Sampson and colleagues concluded that theM. crassus lectotype and all comparableMonoclonius specimens referred tonomina dubia because they all represented juveniles or subadult individuals, as could be seen from their juvenile long-grained bone structure. In some cases the adult form is an already-known species, but in others the adult may not yet be known to science. Most centrosaurine species would thus have a "Monoclonius" phase in theirontogeny, which would explain why such specimens can be found from a wide range in time and space.[21]
In 1998 Dodson andAllison Tumarkin argued that the bone structure could also be explained byspecies-specificpedomorphosis, the retention by adults of juvenile traits. This would be proven by the fact that the holotype of M. lowei, specimen NMC 8790, possessed an interparietal bone, at 609 millimetres in length the longest of any centrosaurine specimen known. The second longest, specimen NMC 5429 ofCentrosaurus apertus, is only 545 millimetres long, showing NMC 8790 was not likely a subadult.[22] However, in 2006Michael Ryan concluded that theM. lowei holotype was an exceptionally large subadult after all, as shown by a third epiparietal,osteoderm on the frill edge, just beginning to develop, and skull sutures which are not completely closed.Monoclonius crassus was seen as anomen dubium.[2]
The developing consensus thatMonoclonius crassus is anomen dubium implies that the genus is in principle constrained to thistype species,M. crassus, and in fact to the lectotype frill recovered from theJudith River Formation ofMontana; even the other material of the AMNH 3998 inventory number cannot justifiably be referred to it. Most of the other historicalMonoclonius species have been referred to other genera or are generally seen asnomina dubia ornomina nuda.
Numerous other species have been assigned to the genusMonoclonius in the past, most of which have been either re-classified into other genera or are currently considered synonyms of previously named species.
In 1897, artistCharles R. Knight paintedAgathaumas sphenocerus for Cope. Knight based the painting on the partial skull of the species, which preserved a large nasal horn, andMonoclonius recurvicornis, which preserved small horns over the eyes.A. sphenoceros was originally referred to the genusMonoclonius and later toStyracosaurus, whileM. recurvicornis is a possibly a valid species but has yet to receive a new genus. The body was based on a more complete skeleton of the speciesTriceratops prorsus that had been described and illustrated by O.C. Marsh in 1896. The body armor depicted in the illustration was likely based on the misidentified squamosals ofPachycephalosaurus for the larger spikes, and the smaller armor based on the dermal scutes ofDenversaurus collected inLance, Wyoming by Marsh's crews in the 1890s. At the time,Monoclonius,Agathaumas, andTriceratops were all thought to be close relatives that differed mainly in the arrangement of the horns and the presence of openings in the frill.[28] This painting was later used as basis for a modelAgathaumas in the 1925 filmThe Lost World.[29]
Monoclonius was later reconstructed (based on specimens now classified asCentrosaurus) forPhil Tippett's short filmPrehistoric Beast (1984). The following year (1985), the shots used onPrehistoric Beast were used again in the television documentaryDinosaur!, directed byRobert Guenette. On April 6, 2011, theTippett Studio had published on its YouTube official channel a digital restoration of thePrehistoric Beast short.[30]
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