Torosaurus

Torosaurus ("perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous ceratopsid dinosaur that lived during the late Maastrichtian stage of the Cretaceous period, between 68 and 66 million years ago. Fossils have been discovered across the Western Interior of North America, from Saskatchewan to southern Texas.

Torosaurus possessed one of the largest skulls of any known land animal. The frilled skull reached up to 2.77 metres (9.1 ft) in length. From head to tail, Torosaurus is thought to have measured about 8 to 9 m (26 to 30 ft) long[1][2] and weighed four to six tonnes. Torosaurus is distinguished from the contemporary Triceratops by an elongate frill with large openings (fenestrae), long squamosal bones of the frill with a trough on their upper surface, and the presence of five or more pairs of hornlets (epoccipitals) on the back of the frill.[3] Torosaurus also lacked the long nose horn seen in Triceratops prorsus, and instead resembled the earlier and more basal Triceratops horridus in having a short nose horn.[3] Three species have been named, Torosaurus latus, T. gladius and T. utahensis. T. gladius is no longer considered a valid species, however.

Recently the validity of Torosaurus has been disputed.[4] A 2010 study of fossil bone histology combined with an investigation of frill shape concluded that Torosaurus probably represented the mature form of Triceratops, with the bones of typical Triceratops specimens still immature and showing signs of a first development of distinct Torosaurus frill holes. During maturation, the skull frill would have been greatly lengthened and holes would have appeared in it.[5][6][7] In 2011, 2012 and 2013 however, studies of external features of known specimens have claimed that morphological differences between the two genera preclude their synonymy. The main problems are a lack of good transitional forms, the apparent existence of authentic Torosaurus subadults, different skull proportions independent of maturation and the assertion that hole formation at an adult stage is not part of a normal ceratopian maturation sequence.

Discovery and species
In 1891, two years after the naming of Triceratops, a pair of ceratopsian skulls with elongated frills bearing holes were found in southeastern Wyoming, Niobrara County, by John Bell Hatcher. Hatcher's employer, paleontologist Professor Othniel Charles Marsh, coined the genus Torosaurus for them.

The name Torosaurus is frequently translated as "bull lizard" from the Latin noun taurus or Spanish toro but much more likely is derived from the Greek verb τορέω (toreo, "to pierce, perforate").[11] The allusion is to the fenestrae or ("window-like") holes in the elongated frill, which have traditionally served to distinguish it from the solid frill of Triceratops. Much of the confusion over etymology of the name results from the fact that Marsh never explicitly explained it in his papers.

Two Torosaurus species have been identified:
 * T. latus Marsh, 1891 (type species). Latus means "the wide one" in Latin, in reference to the frill.
 * T. utahensis (Gilmore, 1946) Lawson, 1976

Another species was subsequently regarded as identical to T. latus:
 * T. gladius Marsh, 1891. Gladius is "sword" in Latin, in reference to the elongated shape of the squamosal.

T. latus was based on holotype YPM 1830, a partial skull. The holotype of T. gladius was specimen YPM 1831, a larger skull. Both fossils were found in the Lance Formation, dating from the Maastrichtian. Similar specimens found in Wyoming, Montana, South Dakota, North Dakota, Colorado, Utah and Saskatchewan have since been referred to Torosaurus. Those that can be identified with some certainty include: ANSP 15192, a smaller individual in South Dakota uncovered by Edwin Harris Colbert in 1944;[12] MPM VP6841, a partial skeleton with skull, now mounted at Milwaukee; SMM P97.6.1, a skull lacking the snout; and two partial skulls from the Hell Creek Formation reported in 2002: MOR 981, discovered in 1998, and MOR 1122, from 2001.[13] Fragmentary remains that could possibly be identified with the genus have been found in the Big Bend Region of Texas and in the San Juan Basin of New Mexico.[14] Paleontologists have observed that Torosaurus specimens are uncommon in the fossil record; specimens of Triceratops are much more abundant.

Torosaurus utahensis was originally described as Arrhinoceratops utahensis by Charles Whitney Gilmore in 1946, based on specimen USNM 15583, a frill fragment from Emery County, Utah.[15] In 1976 it was renamed Torosaurus utahensis by Douglas Lawson.[16] Review by Robert Sullivan et al. in 2005[17] left it as Torosaurus utahensis and somewhat older than T. latus. In 2008 Rebecca Hunt referred considerable additional material to this species.[18] However, subsequent studies suggested it may well be either Arrhinoceratops or a new genus, as dinosaurs from the northern Hell Creek formation and southern "Alamosaurus fauna" rarely overlap and were probably separated by a geographic barrier. Research has not yet been published on whether T. utahensis should be regarded as a new genus or, as has been suggested for T. latus, the mature growth stage of some species of Triceratops.

Description
The individuals referred to Torosaurus are all large, comparable to the largest Triceratops specimens. Due to the elongated frill, especially the skull length is considerable. Hatcher estimated the skull of YPM 1830 at 2.2 metres, of YPM 1831 at 2.35 metres.[19] In 1933 Richard Swann Lull increased this to 2.4 metres and 2.57 metres respectively.[20]Based on this, Torosaurus was seen as having the longest skull of any known land animal. However, in 1998 Thomas Lehman claimed that a Pentaceratops specimen possessed a partial skull that would have been 2.9 metres long in life.[21] This was again doubted by Nicholas Longrich who in 2011 named this exemplar as a separate genus Titanoceratops and concluded its skull had been reconstructed as too long.[22] Furthermore, in 2006 Andrew Farke had pointed out that the new skulls described by him were on average even longer than Hatcher's original two: MOR 1122 has a length of 252 centimetres and MOR 981 of 277 centimetres.

Farke in 2006 established some diagnostic traits of Torosaurus. The frill is extremely long in comparison to the remainder of the skull. The rear, parietal, edge of the frill bears ten or more epiparietals, triangular osteoderms. A midline epiparietal is absent; likewise no osteoderm straddles the parietal-squamosal boundary. The parietal bone is thin. It is pierced by parietal fenestrae in the form of circular or transversely oval openings. The parietal bone is about 20% wider than long. Farke identified a single trait in which T. latus differed from both Triceratops horridus and T. utahensis: its squamosal bore a conspicuous ridge on the edge with the parietal combined with a deep longitudinal trough parallel to it.[23]

Farke pointed out that the known Torosaurus specimens are rather variable. The orbital "brow" horns are sometimes large and curved to the front, as with MOR 981, sometimes short and straight as shown by MOR 1122 and ANSP 15191. Also the position of these horns differs: often they are located directly on top of the eye socket but with YPM 1831 they originate at the rear edge of the orbit. Likewise there is a variation in the form of the nose horn. YPM 1831 and to a lesser extent YPM 1830 have a straight upright nasal horn but MOR 981, ANSP 15192 and especially MOR 1122 at most possess a low bump. The frill too differs. ANSP 15192 and YPM 1830 have a shield curving upwards at the rear, but the frill of YPM 1831 is nearly flat, though this could be an artefact of restoration. The frill of YPM 1831 is also heart-shaped, with a clear mideline notch, whereas the rear edge of the other specimens is straight. The frill proportions are quite variable: with YPM 1831 the length-width ratio is 1.26 but MOR 981 has a shield 2.28 times longer than wide. The number of epiparietals is difficult to assess as most fossils seem to have lost them. MOR 981 and MOR 1122 have ten and twelve epiparietals respectively. YPM 1831 has been restored with a fontanelle in the skull roof, which possibly is authentic. Farke also concluded that the degree of variability did not exceed that shown by related genera.[23]

Farke stressed that, apart from the frill, no systematic differences could be found between Torosaurus and Triceratops. All Torosaurus specimens are similar in that they lack a truly long nasal horn and a horizontal arterial groove at the front base of that horn, but Triceratops fossils with the same combination of traits are not uncommon.[23] Hunt in 2008 concluded that T. utahensis, contrary to T. latus but similar to Triceratops, possessed a midline epiparietal.