Pentaceratops

Pentaceratops (five-horned face) is a genus of herbivorous ceratopsid dinosaur from the late Cretaceous Period of what is now North America. Pentaceratops fossils were first discovered in 1921. The genus was named in 1923 when its type species Pentaceratops sternbergii was described. Pentaceratops lived around 75-73 million years ago, its remains having been mostly found in the Kirtland Formation[1] in the San Juan Basin in New Mexico. Other dinosaurs which shared its habitat include Parasaurolophus cyrtocristatus, the pachycephalosaur Sphaerotholus, the armored dinosaur Nodocephalosaurus and the tyrannosauroid Bistahieversor. About a dozen skulls and skeletons have been uncovered, so that most bones are known. Of one very large specimen it is contested whether it belongs to Pentaceratops or represents a genus of its own: Titanoceratops. However, some subsequent researchers have considered Titanoceratops merely a large individual of P. sternbergii.[2]

Pentaceratops was about six meters (twenty feet) long, and has been estimated to have weighed around five tonnes. It had a short nose horn, two long brow horns, and long horns on the jugal bones. Its skull had a very long frill with triangular hornlets on the edge.

Discovery and species
The first exemplars were collected by Charles Hazelius Sternberg in the San Juan Basin in New Mexico. In 1921, Sternberg worked in commission of the Swedish Uppsala University and recovered at the Meyers Creek near the Kimbetoh Wash, in a layer of the Kirtland Formation, a skull and a rump, specimens PMU R.200 and PMU R.286 that he sent to paleontologist Carl Wiman. In 1922 Sternberg decided to work on his own account and discovered north of Tsaya Trading Post, in the Fossil Forest of San Juan County, a complete skeleton that he sold to the American Museum of Natural History. The museum then sent out a team headed by Charles Mook and Peter Kaisen to assist Sternberg in securing this specimen; subsequent digging by Sternberg in 1923 brought the total of AMNH specimens at four. The rump of the main specimen was discarded because it had insufficient value as a display.

The species was named and described by Henry Fairfield Osborn in 1923, as Pentaceratops sternbergii. The generic name means "five-horned face", derived from the Greek penta (πέντα, meaning five), keras (κέρας, horn) and -ops (ὤψ, face),[3] in reference to its two long epijugal bones, spikes which protrude out sidewards from under its eyes, in addition to the three more obvious horns as with Triceratops. Osborn obligingly gave it the specific name sternbergii honoring its discoverer as a veteran fossil hunter.[4] The name had been suggested to Osborn by William Diller Matthew; the specific epithet served as a consolation to the almost bankrupt Sternberg whose 1923 fossils were initially not acquired by the museum that had to use its 1923/1924 budget to process the finds of the great Asian expeditions by Roy Chapman Andrews.[5]

The holotype was the skull discovered by Sternberg in 1922, specimen AMNH 6325. It was found in a layer of the Fruitland Formation, dating from the Campanian, about seventy-five million years old. The other three AMNH specimens were AMNH 1624, a smaller skull; AMNH 1622, a pair of brow horns; and AMNH 1625, a piece of skull frill.

In 1930, Wiman named a second species of Pentaceratops: Pentaceratops fenestratus. It was based on Sternberg's 1921 specimens and the specific name referred to a hole in the left squamosal.[6] This was later considered to be the same species as, and thus a junior synonym of, Pentaceratops sternbergii, the hole being the likely effect of an injury.

In 1929 George Fryer Sternberg discovered specimen USNM V12002, a right squamosal. Pentaceratops proved to be a quite common fossil in the Fruitland and Kirtland formations. It has even been used as guide fossil: the appearance of Pentaceratops sternbergii in the fossil record marks the end of the Judithian land vertebrate age and the start of the Kirtlandian.[1] Subsequent finds include specimens MNA Pl. 1668, MNA Pl. 1747, NMMNH P-27468 and USNM 2416, partial skeletons with skull; YPM 1229, a skeleton lacking the skull; UALP 13342 and UKVP 16100, skulls; UNM B-1701, USNM 12741, USNM 12743, USNM 8604, SMP VP-1596, SMP VP-1488, SMP VP-1500 and SMP VP-1712, fragmentary skulls. Apart from the San Juan Basin finds, a juvenile specimen of Pentaceratops, SDMNH 43470, has been reported from the Williams Fork Formation of Colorado in 2006.[7]

Sometimes the identification of a specimen as Pentaceratops has proven to be highly contentious. In 1998 Thomas Lehman described OMNH 10165, a very large skull and its associated skeleton found in New Mexico in 1941, and presently on display at the Sam Noble Oklahoma Museum of Natural History, as being the largest Pentaceratops exemplar known, with the distinction of having produced the largest known skull of any land vertebrate.[8][9] However, in 2011, the skeleton was renamed as a separate genus: Titanoceratops.

Description
Pentaceratops is a large ceratopsid. Its known maximum size is dependent on the identity of specimen OMNH 10165. This specimen has a reconstructed length of 6.8 meters, a reconstructed skull length of 3.22 metres and a weight estimated by Lehman at 9877 kilograms.[8] The other specimens are smaller. Gregory S. Paul in 2010 estimated the body length at 6.4 meters and the weight at 4.7 tonnes.[11] Lehman calculated a composite maximal skull length for the smaller specimens of about 2.7 meters. Dodson estimated the body length at six meters, the skull length of AMNH 1624 at 2.3 meters. PMU R.200 has a length of 216 centimeters.[6]

The nose horn of Pentaceratops is small and pointing upwards and backwards. The brow horns are very long and curving strongly forwards. The somewhat upward tilted frill of Pentaceratops is considerably longer than that of Triceratops, with two large holes (parietal fenestrae) in it. It is rectangular, adorned by large triangular osteoderms: up to twelve episquamosals at the squamosal and three epiparietals at the parietal bone. These are largest at the rear corners of the frill, that are separated by a large U-shaped notch at the midline, a feature not recognized until 1981 when specimen UKVP 16100 was described.[12] Within the notch the first epiparietals point forwards. The very thick jugal and the squamosal do not touch each other, a possible autapomorphy.[10]

The torso of Pentaceratops is tall and wide. The rear dorsal vertebrae bear long spines from which perhaps ligaments ran to the front, to balance the high frill. The prepubis is long. The ischium is long and strongly curves forward. With the smaller specimens the thigh bone bows outwards.