Torosaurus (meaning "perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous chasmosaurine ceratopsian dinosaur which lived during the Maastrichtian Stage of the Cretaceous Period between 68 and 66 million years ago, although it is possible that the species range might extend back as far back as 69 million years ago.*Hicks, J.F., Johnson, K.R., Obradovich, J. D., Miggins, D.P., and Tauxe, L. 2003. Magnetostratigraphy of Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. II., Rocky Mountain Geology 38: 1-27. Fossils which have been definitively identified as Torosaurus or which might belong to it have been discovered across the Western Interior of North America, from as far north as Saskatchewan to as far south as Texas.
Measuring between and weighing around , Torosaurus possessed the longest skull of any known land animal, with the frilled skull reaching in length. Torosaurus is thought to have been the same size as the contemporary Triceratops, but is distinguished by an elongated frill with large oval shaped openings, long squamosal bones of the frill with a trough on their upper surface, and the presence of five or more pairs of hornlets on the back of the frill. Torosaurus possessed a short upright nose horn similar to that seen in Triceratops horridus. Two species of Torosaurus are presently considered valid: T. latus and T. utahensis. Torosaurus gladius is no longer considered a valid species, being now considered a synonym of T. latus.
In 2010, the validity of Torosaurus was disputed. A 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. 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 hole formation at an adult stage not being part of a normal ceratopsian maturation sequence. Consequently, newer research has favored the interpretation that Torosaurus is indeed a separate genus from Triceratops.
Discovery and species In 1891, two years after the naming of Triceratops, a pair of ceratopsian skulls with elongated frills bearing holes were found by John Bell Hatcher in Niobrara County in southeastern Wyoming. Hatcher's employer, paleontologist Professor Othniel Charles Marsh, coined the genus Torosaurus for them.O.C. Marsh, 1891, "Notice of new vertebrate fossils", The American Journal of Science, series 3 42: 265-269
The name Torosaurus is frequently mistranslated as "bull lizard" from the Latin noun taurus or the Spanish word toro, but is much more likely derived from the Greek verb (toreo), which means "to perforate".Dodson, P. The Horned Dinosaurs. Princeton Univ. Press (Princeton), 1996. The allusion is to the fenestrae, "window-like" holes, in the elongated frill, which have traditionally served to distinguish it from the solid frill of Triceratops. In 1896, Marsh explained: "The open perforations in the parietal, which have suggested the name Torosaurus, readily separate this genus from all the gigantic species hitherto known in the Ceratopsidae..." (pg. 216).
Two Torosaurus species have been identified: T. latus (type species). Latus means "the wide one" in Latin, in reference to the frill. T. utahensis () Lawson, 1976
Another species was subsequently regarded as identical to T. latus: T. gladius . Gladius is "sword" in Latin, in reference to the elongated shape of the squamosal.
T. latus was based on holotype YPM 1830, which is a partial skull. The holotype of T. gladius was specimen YPM 1831, which is an even larger skull. Both fossils were found in the Lance Formation of the late 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),Colbert, E.H. and J.D. Bump, 1947, "A skull of Torosaurus from South Dakota and a revision of the genus:, Proceedings of the Academy of Natural Sciences of Philadelphia, 99: 93–106 MPM VP6841 (a partial skeleton with a skull mounted at the Milwaukee Public Museum), 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).Farke, A., 2002, "A review of Torosaurus (Dinosauria: Ceratopsidae) specimens from Texas and New Mexico", Journal of Vertebrate Paleontology, 22: 52A Fragmentary remains that could possibly be identified as Torosaurus have been found in the Big Bend Region of Texas and the San Juan Basin of New Mexico.Lucas, S.G., Mack, G.H., Estep, G.W., 1998, "The Ceratopsian dinosaur Torosaurus from the Upper Cretaceous McRae Formation, Sierra County, New Mexico", New Mexico Geological Society Guidebook, 49th Field Conference, Las Cruces County II Paleontologists have observed that Torosaurus specimens are uncommon in the fossil record, with specimens of Triceratops being 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).C.W. Gilmore, 1946, "Reptilian fauna of the North Horn Formation of central Utah", United States Department of the Interior Geological Survey Professional Paper 210-C: 29-53 In 1976, it was renamed to Torosaurus utahensis by Douglas Lawson.D.A. Lawson, 1976, "Tyrannosaurus and Torosaurus, Maestrichtian dinosaurs from Trans-Pecos, Texas", Journal of Paleontology 50(1): 158-164 Review by Robert Sullivan et al. in 2005Sullivan, R. M., A. C. Boere, and S. G. Lucas. 2005. Redescription of the ceratopsid dinosaur Torosaurus utahensis (Gilmore, 1946) and a revision of the genus. Journal of Paleontology 79:564-582. left it as Torosaurus utahensis and somewhat older than T. latus. In 2008, Rebecca Hunt referred considerable additional material to this species. 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 a species of Triceratops.
Description
The individuals referred to Torosaurus are all large, comparable to the largest Triceratops specimens, measuring long and weighing . Due to the elongated frill, the skull length is especially considerable. Hatcher estimated the skull of YPM 1830 at and that of YPM 1831 at .Hatcher, J.B., Marsh O.C. and Lull, R.S., 1907, The Ceratopsia, Monographs of the United States Geological Survey 49: 1-198 In 1933, Richard Swann Lull increased this to and , respectively.Lull, R.S., 1933, A revision of the Ceratopsia or horned dinosaurs, Memoirs of the Peabody Museum of Natural History 3(3): 1-175 Based on this, Torosaurus was seen as having the longest skull of any known land animal. In 1998, however, Thomas Lehman claimed that a Pentaceratops specimen possessed a partial skull that would have been long in life.Lehman, T.M., 1998, "A gigantic skull and skeleton of the horned dinosaur Pentaceratops sternbergi from New Mexico: Journal of Paleontology, 72(5): 894-906 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.Nicholas R. Longrich, 2011, "Titanoceratops ouranous, a giant horned dinosaur from the Late Campanian of New Mexico", Cretaceous Research, 32: 264-276 Furthermore, in 2006, Andrew Farke had pointed out that the new skulls described by him were even longer on average than Hatcher's original two. MOR 1122 has a length of and MOR 981 has a length of . The well preserved skull of the Torosaurus specimen nicknamed "Adam", first put on display in 2023, is the largest known at long. T. utahensis is estimated to have a possible maximum skull length of .
In 2006, Farke established some diagnostic traits of Torosaurus. The frill is extremely long in comparison to the remainder of the skull. The rear edge of the frill bears ten or more epiperietals, or triangular osteoderms. A midline triangular osteoderm is absent. Likewise, no osteoderm straddles the parietal-squamosal boundary. The parietal bone is thin and pierced by parietal fenestrae in the form of circular or transversely ovular openings. The parietal bone is about 20% wider than it is 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.
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, or sometimes short and straight, as shown by MOR 1122 and ANSP 15191. Also, the position of these horns differs, as they are often located directly on top of the eye socket. With YPM 1831, however, 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 possess a low bump at most. The frill differs too, as 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 midline 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 it is wide. The number of triangular osteoderms 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 is possibly authentic. Farke also concluded that the degree of variability did not exceed that shown by related genera.
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 said horn, but Triceratops fossils with the same combination of traits are not uncommon. In 2008, Hunt concluded that T. utahensis, contrary to T. latus, but similar to Triceratops, possessed a midline epiparietal.
Classification
Below is a ceratopsid cladogram based on the phylogenetic analysis conducted by Sampson et al. in 2010:
In 1891, Marsh placed Torosaurus in the Ceratopsidae family of Ceratopsia (Greek: "horned faces"), a group of herbivorous dinosaurs with parrot-like beaks that thrived in North America and Asia during the Jurassic and Cretaceous Periods.