Torosaurus
Torosaurus | |
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Specimen nicknamed "Tiny", Denver Museum of Nature and Science | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | †Ornithischia |
Clade: | †Ceratopsia |
Family: | †Ceratopsidae |
Subfamily: | †Chasmosaurinae |
Tribe: | †Triceratopsini |
Genus: | †Torosaurus Marsh, 1891 |
Type species | |
†Torosaurus latus Marsh, 1891
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Other species | |
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Synonyms | |
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Torosaurus (meaning "perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous chasmosaurine ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous period, between 68 and 66 million years ago, though it is possible that the species range might extend to as far back as 69 million years ago.[1] Fossils have been discovered across the Western Interior of North America, from as far north as Saskatchewan to as far south as Texas.
Torosaurus possessed the largest skulls of any known land animal, with the frilled skull reaching 2.77 metres (9.1 ft) in length. Torosaurus is thought to have been the same size as the contemporary Triceratops,[2][3][4] 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.[5] Torosaurus also lacked the long nose horn seen in Triceratops prorsus. It instead resembled the earlier and more basal Triceratops horridus, thanks to having a short nose horn.[5] Three species have been named so far: Torosaurus latus, T. gladius, and T. utahensis. T. gladius is no longer considered a valid species, however.
In 2010, the validity of Torosaurus was disputed.[6] 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.[7][8][9] 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.[5][10][11] Consequently, it is still heavily debated whether Torosaurus truly is an adult Triceratops or a separate genus, though most researchers currently favor the latter interpretation.
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.[12]
The name Torosaurus is frequently translated as "bull lizard" from the Latin noun taurus or the Spanish word toro, but is much more likely derived from the Greek verb τορέω (toreo), which means "to perforate".[13] 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. 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
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
Description
The individuals referred to Torosaurus are all large, comparable to the largest Triceratops specimens. Due to the elongated frill, the skull length is especially considerable. Hatcher estimated the skull of YPM 1830 at 2.2 m (7.2 ft) and that of YPM 1831 at 2.35 m (7.7 ft).[21] In 1933, Richard Swann Lull increased this to 2.4 m (7.9 ft) and 2.57 m (8.4 ft), respectively.[22] 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 2.9 m (9.5 ft) long in life.[23] 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.[24] 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 2.52 m (8.3 ft) and MOR 981 has a length of 2.77 m (9.1 ft).[25] The well preserved skull of the Torosaurus specimen nicknamed "Adam", first put on display in 2023, is the largest known at 3 m (9.8 ft) long.[26][27]
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
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.[25]
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.[25] In 2008, Hunt concluded that T. utahensis, contrary to T. latus, but similar to Triceratops, possessed a midline epiparietal.[20]
Classification
Below is a ceratopsid cladogram based on the phylogenetic analysis conducted by Sampson et al. in 2010:[28]
In 1891, Marsh placed Torosaurus in the Ceratopsidae family of Ceratopsia (Greek: "horned faces"),[12] a group of herbivorous dinosaurs with parrot-like beaks that thrived in North America and Asia during the Jurassic and Cretaceous Periods.
Torosaurus has, with its long frill, traditionally been classified in
Possible synonymy with Triceratops
A debate has sparked over the possibility that Torosaurus might be identical to Triceratops. In the
In 2010, Scanella and
Scanella and Horner recognised that not all data were easily explained by their hypothesis. For these, they advanced
Scannella and Horner's conclusions have not been unanimously accepted. Several experts, though admitting the possibility that the "toromorph" hypothesis is correct, have denied that this is probable. The hypothesis was directly challenged by a 2011 paper by Andrew Farke and a 2012 paper by Nicholas Longrich. In 2011, Farke redescribed the problematic
The same year, Scanella and Horner responded to some of Farke's critique. They admitted that USNM 2412, in view of its pathologies, was not an ideal candidate for a transitional form, but stressed that, apart from swellings, the holes in its frill were also bordered by granular and thinning bone. Taking all the evidence into consideration, they thought it much more likely that Nedoceratops represented a diseased individual of Triceratops than a genus of its own. They also pointed to Triceratops specimens showing the precise combination of veined, granular, and young striated bone that Farke had considered to be improbable. The idea that the thin areas on Triceratops frills were muscle attachment sites was rejected by them because the bone at these points did not show the rugose surface typical for such an attachment. For the difference in the number of epoccipitals, they offered two additional explanations. The osteoderm tips of old individuals might have eroded during life in such a way that each osteoderm gave the impression of two being present. In this way, the normal number of five or six Triceratops epiparietals could have doubled to ten or twelve, precisely the amount seen with the Torosaurus specimens. Alternatively, the Torosaurus latus specimens, having been found in older layers, might, in a process of anagenesis, represent an early stage of Triceratops evolution. The oldest specimen that can be dated, MOR 1122, has twelve epiparietals, while the younger MOR 981 possesses ten, seeming to indicate an evolutionary sequence in which the number of epiparietals gradually decreased.[32]
In 2012, Longrich investigated the problem by applying the principle of
Longrich also suggested some additional objections to the "toromorph" hypothesis. There are no transitional forms known regarding the number of epiparietals. Also, it is hard to see how their number could have increased, as they occupied the full frill edge and would, in a metaplastic growth process, simply have increased their size and the remainder of the frill. The proposed splitting of osteoderms by erosion has only been established with episquamosals, never with epiparietals. Torosaurus has a squamosal that is thickened at the inner side and concave at the outer surface, while the Triceratops squamosal is concave at the inner side and flat on top. Transitional forms are unknown. The Torosaurus squamosal is also, independent from absolute size, much more elongated. Longrich pointed out that when Torosaurus and Triceratops specimens are combined to create a single growth sequence, as Scanella and Horner had done, Torosaurus specimens ANSP 15192 and YPM 1831 were outliers relative to the
In 2013, Farke and
The hypothesis that the Torosaurus latus specimens might represent a "toromorph" phase of Triceratops maturation has raised the question of whether the second Torosaurus species, Torosaurus utahensis, is a "toromorph" as well. This issue has been complicated by the lack of good fossil material, as most specimens consist of isolated bones. T. utahensis was largely referred to Torosaurus because of elongated squamosals, indicating a long frill. The number of epiparietals and the size, location, or even existence of parietal fenestrae are unknown. Researchers have claimed that distinct juvenile Torosaurus have been excavated from a bonebed in the Javelina Formation of Big Bend National Park, basing their identification as Torosaurus cf. utahensis on their proximity to an adult with a characteristic Torosaurus parietal.[20] Scanella and Horner concluded that only future finds could solve this problem. They suggested that this taxon, which extends the Torosaurus range southwards of that of Triceratops, might represent a separate chasmosaurine genus or a third Triceratops species. Farke's 2013 morphometric study was inconclusive on this point, with T. utahensis morphospace falling in between Triceratops and Torosaurus latus and not well separated from either.[11]
In 2022, Mallon et al. argued that two specimens found in Canada's Frenchman and Scollard Formations, EM P16.1. (at Eastend Historical Museum in Saskatchewan) and UALVP 1646 (at the University of Alberta), are subadults and can be referred to Torosaurus, this indicating that it is a valid taxon. The same study also noted that Torosaurus indeed lived during the Late Maastrichtian.[34]
See also
References
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- ^ a b c Maiorino L., Farke A.A., Kotsakis T., Piras P.,2013, "Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs", PLoS ONE 8(11): e81608. doi:10.1371/journal.pone.0081608
- ^ a b O.C. Marsh, 1891, "Notice of new vertebrate fossils", The American Journal of Science, series 3 42: 265-269
- ^ Dodson, P. The Horned Dinosaurs. Princeton Univ. Press (Princeton), 1996.
- ^ 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
- ^ Farke, A., 2002, "A review of Torosaurus (Dinosauria: Ceratopsidae) specimens from Texas and New Mexico", Journal of Vertebrate Paleontology, 22: 52A
- ^ 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
- ^ 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
- ^ D.A. Lawson, 1976, "Tyrannosaurus and Torosaurus, Maestrichtian dinosaurs from Trans-Pecos, Texas", Journal of Paleontology 50(1): 158-164
- ^ Sullivan, 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.
- ^ a b c Hunt, Rebecca K. and Thomas M. Lehman. 2008. Attributes of the ceratopsian dinosaur Torosaurus, and new material from the Javelina Formation (Maastrichtian) of Texas. Journal of Paleontology 82(6): 1127–1138.
- ^ Hatcher, J.B., Marsh O.C. and Lull, R.S., 1907, The Ceratopsia, Monographs of the United States Geological Survey 49: 1-198
- ^ Lull, R.S., 1933, A revision of the Ceratopsia or horned dinosaurs, Memoirs of the Peabody Museum of Natural History 3(3): 1-175
- ^ 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
- ^ Nicholas R. Longrich, 2011, "Titanoceratops ouranous, a giant horned dinosaur from the Late Campanian of New Mexico", Cretaceous Research, 32: 264-276
- ^ a b c d e Farke, A. A. "Cranial osteology and phylogenetic relationships of the chasmosaurine ceratopsid Torosaurus latus", pp. 235-257. In K. Carpenter (ed.). Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Indiana Univ. Press (Bloomington), 2006.
- ^ "This Torosaurus skeleton is the largest horned dinosaur ever discovered in the world". museumofevolution.com. Retrieved June 20, 2023.
- ^ "Largest dinosaur skull ever found on display in Denmark". Wisconsin State Journal. June 13, 2023. Retrieved July 1, 2023.
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- ^ Scannella J., 2009, "And then there was one: synonymy consequences of Triceratops cranial ontogeny", Journal of Vertebrate Paleontology 29: 177A
- ^ "New Analyses Of Dinosaur Growth May Wipe Out One-third Of Species". Sciencedaily.com. October 31, 2009. Retrieved August 3, 2010.
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- ^ Horner, J.R., Lamm, E-T., 2011, "Ontogeny of the parietal frill of Triceratops: a preliminary histological analysis", Comptes Rendus de l’Academie des Sciences Paris série D 10: 439–452
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