Triceratops

Triceratops is a genus of herbivorous ceratopsid dinosaur that first appeared during the late Maastrichtian stage of the late Cretaceous period, about 68 million years ago (mya) in what is now North America. It is one of the last known non-avian dinosaur genera, and became extinct in the Cretaceous–Paleogene extinction event 66 million years ago. The name Triceratops, which literally means 'three-horned face', is derived from the Ancient Greek words τρί- (tri-) meaning 'three', κέρας (kéras) meaning 'horn', and ὤψ (ōps) meaning 'face'. Triceratops has been documented by numerous remains collected since the genus was first described in 1889 by Othniel Charles Marsh. Specimens representing life stages from hatchling to adult have been found. As the archetypal ceratopsid, Triceratops is one of the most popular dinosaurs, and has been featured in film, postal stamps, and many other types of media. Bearing a large bony frill and three horns on the skull, and its large four-legged body possessing similarities with the modern rhinoceros, Triceratops is one of the most recognizable of all dinosaurs and the best known ceratopsid. It was also one of the largest, up to nine metres long and twelve tonnes in weight. It shared the landscape with and was probably preyed upon by Tyrannosaurus, though it is less certain that the two did battle in the manner often depicted in museum displays and popular images. The functions of the frills and three distinctive facial horns on its head have long inspired debate. Traditionally, these have been viewed as defensive weapons against predators. More recent interpretations find it probable that these features were primarily used in species identification, courtship and dominance display, much like the antlers and horns of modern species. Triceratops was traditionally placed within the 'short-frilled' ceratopsids but modern cladistic studies show it to be a member of the Chasmosaurinae which usually have long frills. Two species, T. horridus and T. prorsus, are considered valid today, from the seventeen species that have ever been named. Research published in 2010 concluded that the contemporaneous Torosaurus, a ceratopsid long regarded as a separate genus, represents Triceratops in its mature form. This view was immediately disputed with examination of more fossil evidence needed to settle the debate. Individual Triceratops are estimated to have reached about 7.9 to 9.0 m (25.9–29.5 ft) in length, 2.9 to 3.0 m (9.5 to 9.8 ft) in height, and 6.1–12.0 tonnes (13,000–26,000 lb) in weight. The most distinctive feature is their large skull, among the largest of all land animals. The largest known skull (specimen MWC 7584, formerly BYU 12183) is estimated to have been 2.5 metres (8.2 ft) in length when complete, and could reach almost a third of the length of the entire animal. A specimen of T. horridus named Kelsey measured 7.3 metres (24 ft) long with a 1.98 metres (6.5 ft) skull, stood about 2.3 metres (7.5 ft) tall, and was estimated by the Black Hills institute to weight nearly 6 tonnes (5.9 long tons; 6.6 short tons). A Triceratops 8 metres (26 ft) long has been estimated by Gregory S. Paul to have massed 9 tonnes (8.9 long tons; 9.9 short tons). Like all chasmosaurines, Triceratops had a large skull relative to its body size. The front of the head was equipped with a high toothless beak. The core of the top beak was formed by a special rostral bone. Behind it, the premaxillae bones were located, embayed from behind by very large circular nostrils. In chasmosaurines, the premaxillae met on their midline in a complex bone plate, the rear edge of which was reinforced by the 'narial strut'. From the base of this strut, a triangular process jutted out into the nostril. Triceratops differs from most relatives in that this process was hollowed out on the outer side. Behind the toothless premaxilla, the maxilla bore thirty-six to forty tooth positions, in which three to five teeth per position were vertically stacked. The teeth were closely appressed, forming a 'dental battery' curving to the inside. The skull bore a single horn on the snout, above the nostrils. In Triceratops, the nose horn is sometimes recognisable as a separate ossification, the epinasal. The skull also featured a pair of 'brow' or supraorbital horns approximately 1 m (3.3 ft) long, with one above each eye. The jugal bones pointed to below at the rear sides of the skull and were capped by separate epijugals. With Triceratops these were not particularly large and sometimes touched the quadratojugals. The bones of the skull roof were fused. By a folding of the frontal bones, a 'double' skull roof was created. In Triceratops, some specimens show a fontanelle, an opening in the upper roof layer. The cavity between the layers invaded the bone cores of the brow horns. At the rear of the skull, the outer squamosal bones and the inner parietal bones grew into a relatively short, bony frill, adorned with epoccipitals in young specimens. These were low triangular processes on the frill edge, representing separate skin ossifications or osteoderms. Typically, with Triceratops specimens there are two epoccipitals present on each parietal bone, with an additional central process on their border. Each squamosal bone had five processes. Most other ceratopsids had large parietal fenestrae, openings, in their frills, but those of Triceratops were noticeably solid, unless the genus Torosaurus represents mature Triceratops individuals. Under the frill, at the rear of the skull, a huge occipital condyle, up to 106 millimetres in diameter, connected the head to the neck. The lower jaws were elongated and met at their tips in a shared epidentary bone, the core of the toothless lower beak. In the dentary bone, the tooth battery curved to the outside to meet the battery of the upper jaw. At the rear of the lower jaw, the articular bone was exceptionally wide, matching the general width of the jaw joint.