Anchiceratops

Anchiceratops Temporal range: Late Cretaceous,
Skull cast of TMP 1983.001.0001, Geological Museum (Copenhagen)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Ceratopsia
Family:	†Ceratopsidae
Subfamily:	†Chasmosaurinae
Genus:	†Anchiceratops Brown, 1914
Type species
†Anchiceratops ornatus Brown, 1914
Synonyms
Anchiceratops longirostris Sternberg, 1926

Anchiceratops (/ˌæŋkiˈsɛrətɒps/ ANG-ki-SERR-ə-tops; meaning "near Ceratops", derived from the Greek "anchi -/αγχι-" "near", "keras-/κερας-" "horn", "-ops/ωψ" "face") is a genus of herbivorous chasmosaurine ceratopsid dinosaurs. It contains the single species Anchiceratops ornatus, from the Late Cretaceous Period of western North America. About a dozen skulls of the species have been found.

Anchiceratops was about four to five metres long. Its skull featured two long brow horns and a short horn on the nose. The skull frill was elongated and rectangular, its edges adorned by coarse triangular projections.

Discoveries and species

Type specimen AMNH 5251

American paleontologist Barnum Brown named Anchiceratops in 1914, as he believed Anchiceratops was a transitional form closely related to both Monoclonius and Triceratops and intermediate between them, but closest in the development of the skull frill to the latter, hence the generic name meaning "near (ἄγχι) Ceratops". There is one valid species known today (Anchiceratops ornatus), whose name refers to the ornate margin of its frill. A second species was named Anchiceratops longirostris by Charles M. Sternberg in 1929, but this species is widely considered a junior synonym of A. ornatus today.

The first remains of Anchiceratops were discovered along the Red Deer River in the Canadian province of Alberta in 1912 by an expedition led by Barnum Brown. The holotype, specimen AMNH 5251, is the back half of a skull, including the long frill, and two other partial skulls, specimens AMNH 5259 (the paratype) and AMNH 5273, were found at the same time, which are now stored in the American Museum of Natural History in New York City. A complete skull was discovered by C.M. Sternberg at Morrin in 1924, specimen NMC 8535, and described as A. longirostris five years later. Another specimen, NMC 8547 (or CMN 8547) collected by Sternberg in 1925, lacks the skull but is otherwise the most complete skeleton known from any ceratopsid, preserving a complete spinal column down to the last tail vertebra. Sternberg's material is now housed in the Canadian Museum of Nature in Ottawa. NMC 8547 is displayed as a half-mount with the better preserved right side showing, and completed with a cast skull replica of NMC 8535. Other material has been found since, including one or two possible bonebed deposits in Alberta, but very little Anchiceratops material has been described.

NMC 8547 mounted at the Canadian Museum of Nature, completed with a skull cast of NMC 8535. NMC 8547 might represent a separate taxon

Most Anchiceratops fossils have been discovered in the Horseshoe Canyon Formation of Alberta, which belongs to the later part of the Campanian stage of the Late Cretaceous Period (Anchiceratops remains are known from the lower part of the formation, and range in age between 72.5-71 million years ago). Frill fragments found in the early Maastrichtian Almond Formation of Wyoming in the United States resemble Anchiceratops. However, brown horn fragments (specimens NMC 9590 and 10645) and frill pieces (specimina NMC 9813, 9814 and 9829) have been found from two localities in the older Oldman and Dinosaur Park Formations (late Campanian, 76.5-75 million years ago) with the characteristic pattern of points seen in Anchiceratops frills. These may represent early records of A. ornatus or possibly a second, related species. In 2012, Jordan Cole Mallon e.a. pointed out that Anchicertops ornatus as a species was exceptionally long-lived; other ceratopid species typically last a few hundred thousand years. Several possible explanations were given: a decreased competition by related species; less habitat fragmentation by the recession of the Western Interior Seaway; and a more generalist lifestyle.

Partial skull FMNH P15003 in the Field Museum

In 2012, Mallon concluded that many more Anchiceratops fossils had been collected than previously had been realised. These included the specimens TMP 1983.001.0001, a nearly complete skull of a juvenile; UW 2419, a nearly complete skull; ROM 802, a skull lacking the snout; FMNH P15003, the upper side of a skull lacking the snout; CMN 11838, a left skull frill; CMN 12-1915, frill fragments; and UALVP 1618, the rear edge of a frill. This larger number of fossils can be examined by statistical analysis to solve certain long-standing controversies about the genus. C.M. Sternberg had originally designated a smaller skull as the new species Anchiceratops longirostris, because of its size, and also its much more slender horns that point forwards instead of upwards. Later however, paleontologists had concluded that the size and form of this skull falls within the range of variation seen in A. ornatus and that it were probably a member of that species. It had also been proposed by Thomas Lehman in 1990 that Anchiceratops were a sexually dimorphic species, where the skull of A. longirostris actually represents a female. Other Anchiceratops skulls that are larger, more robust, and show much longer horns that point more vertically were proposed to represent the male. Preliminary statistical analysis by Mallon of the Anchiceratops specimens revealed that these variations in skull form do not actually fall into two distinct morphs, and more likely represent individual variation, forcing him to reject the hypothesis that there were two species. Likewise there was no proof of sexual dimorphism.

In 2010 Mallon removed specimen NMC 8547 from the fossil material of Anchiceratops because of the few frill elements associated with it, it could not be determined with certainty whether they belonged to this taxon; from the same formation the related Arrhinoceratops is known which had a comparable frill surface structure. According to Mallon, C.M. Sternberg had in 1925 unproblematically referred the find to Anchiceratops because he had been unaware that Arrhinoceratops had been described in March of that year. If correct, this would mean that no unequivocal postcrania, elements behind the skull, of Anchiceratops are known.

Description

Restoration, with generalised chasmosaurine rump, not based on NMC 8547

Like other ceratopsids, A. ornatus was a quadrupedal herbivore with three horns on its face, a parrot-like beak, and a long frill extending from the back of its head. The two horns above the eyes were longer than the single horn on its snout, as in other chasmosaurines.

Anchiceratops was a medium-sized ceratopsid. If specimen NMC 8547 is not taken into account, no very exact estimations of the body length of Anchiceratops can be given. Some popular science book state that it approached 20 feet (6 m) in length. In 2010 Gregory S. Paul, on the assumption that specimen NMC 8547 represented Anchiceratops, estimated its length at 4.3 metres, its weight at 1.2 tonnes.

Anchiceratops frills are very distinctive. Rectangular in shape, the frill is edged by large epoccipitals, osteoderms in the form of triangular bony projections. These are exceptionally wide and coarse. Some of these epoccipitals are on the side of the frill, formed by the squamosal; these episquamosals vary between five and nine in number. The last episquamosal is very large, approaching the size of the three osteoderms per side on the rear edge of the frill, the epiparietals. Another characteristic feature is the pair of bony knobs located on either side of the midline, towards the end of the frill. These are pointing sideways and are very variable in form and size between individuals. The parietal bone, forming the rear edge and the middle of the frill, has smaller parietal fenestrae, window-like openings, than those seen in other chasmosaurines like Pentaceratops and Torosaurus. The frill has deep arterial grooves on both the upper and the underside.

Specimen NMC 8547, on which traditionally descriptions of the postcrania of Anchiceratops have been based, has many traits that are unique in the Chasmosaurinae. The vertebral column contains seventy-four vertebrae: ten of the neck, thirteen dorsals, twelve sacrals and thirty-nine caudals. Typically chasmosaurines have twelve dorsals, ten sacrals and up to forty-six tail vertebrae. Mallon presumed that the synsacrum, the fused vertebrae supporting the pelvis, had shifted to the rear. The neck of NMC 8547 is exceptionally long, with four syncervicals, fused anterior cervical vertebrae. Also the pelvis is very long. The tail is short. The forelimbs are very robust, with a large deltopectoral crest on the humerus, indicating a heavy musculature.

Phylogeny

Ceratopsidae Centrosaurinae Chasmosaurinae Chasmosaurus Mojoceratops Agujaceratops Utahceratops Pentaceratops Coahuilaceratops Kosmoceratops Vagaceratops Anchiceratops Arrhinoceratops Triceratopsini Ojoceratops Eotriceratops Torosaurus Nedoceratops Triceratops Theropod cladogram based on the phylogenetic analysis, conducted by Sampson et al.', in 2010.

Brown in 1914 assigned Anchiceratops to the Ceratopsia. In 1915, William Diller Matthew refined this to the Ceratopsidae. In 1990, Peter Dodson and Phil Currie placed it in the Chasmosaurinae.

Modern cladistic analyses usually recover Anchiceratops in a more advanced position, close to Arrhinoceratops, than Chasmosaurus. Mallon's study of 2012 concluded however, that Anchiceratops was more closely related to Chasmosaurus than to Triceratops.

Paleobiology

ROM 802, a skull in the Royal Ontario Museum

Anchiceratops is rare compared to other ceratopsians in the area, and usually found near marine sediments, in both the Horseshoe Canyon and Dinosaur Park Formations. This indicates that Anchiceratops may have lived in estuaries where other ceratopsids did not live. Flowering plants were increasingly common but still rare compared to the conifers, cycads and ferns which probably made up the majority of ceratopsian diets.

In 1959, Wann Langston jr suggested that Anchiceratops was a semi-aquatic species. The long snout would have served to allow the animal to cross deeper swamps walking, catching breath on the water surface; the heavy frill would have acted as a counterbalance to point the beak upwards. Later paleontologists tended to reject this notion, emphasising dinosaurs in general were land animals, but in 2012 Mallon again supposed a sem-aquatic lifestyle, like a modern hippopotamus, for at least NMC 8547. This would be an explanation for the robustness and extreme musculature of the limbs. Mallon admitted the small tail was not a swimming-organ.

In 1914 Brown suggested that the distinctive frill and horn form of Anchiceratops were caused by sexual selection and intra-species recognition, as he could not explain the differences between the taxa by a difference in defence function.