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. Fossils 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 largest skulls 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 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. 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.
thumb|Marsh's original illustrations of the skulls of T. latus and its synonym T. gladius
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". 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 () <small>Lawson, 1976</small>
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), 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). 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. 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). In 1976, it was renamed to Torosaurus utahensis by Douglas Lawson. Review by Robert Sullivan et al. in 2005 left it as Torosaurus utahensis and somewhat older than T. latus. In 2008, Rebecca Hunt referred considerable additional material to this species. 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 . In 1933, Richard Swann Lull increased this to and , respectively. 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. 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. 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 .
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 ovalur 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.
In 1891, Marsh placed Torosaurus in the Ceratopsidae family of Ceratopsia (Greek: "horned faces"),
left|thumb|According to the "toromorph" hypothesis, Triceratops subadults (A, Triceratops prorsus holotype YPM 1822) would have gotten longer frills with holes as shown by B, Torosaurus latus specimen ANSP 15192
thumb|left|The end phase would have consisted of an enormously large and flat frill as exemplified by specimen YPM 1831 (A), its size shown by comparison to ANSP 15192 (B), an early adult
In 2010, Scanella and Jack Horner, Scannella's mentor at Montana State University, published research on the growth patterns in thirty-eight skull specimens (twenty-nine of Triceratops, nine of Torosaurus) from the Hell Creek formation. They concluded that Torosaurus indeed represents the mature form of Triceratops. Horner stressed that the frill of ceratopsian skulls consisted of metaplastic bone. A characteristic of metaplastic bone is that it can lengthen and shorten over time, extending and resorbing to form new shapes. Significant development is seen even in those skulls already identified as Triceratops, Horner observed, "where the horn orientation is backwards in juveniles and forward in adults". Approximately 50% of all subadult Triceratops skulls have two thin areas in the frill that correspond with the placement of the "holes" in Torosaurus skull frills, which are surrounded by mature granular bone, suggesting that these developed to offset the weight that would otherwise have been added as maturing Triceratops individuals grew longer frills. Horner made this part of a larger argument that, in general, many purported dinosaur species might have been growth stages of other known species. With old Triceratops individuals, the frill would have begun to lengthen considerably, causing it to flatten and widen at its rear edge. At the same time, parietal fenestrae would have appeared, resulting in the typical chasmosaurine frill shape.
thumb|upright|Specimen ANSP 15192 might, according to Longrich, be a young female adult
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.
left|thumb|Scanella & Horner saw [[Nedoceratops as an ontogenetic transitional form between Triceratops and Torosaurus]]
In 2012, Longrich investigated the problem by applying the principle of falsification. From any valid scientific hypothesis, predictions can be derived by which it can be tested. Longrich argued that the "toromorph" hypothesis implied three such predictions. Firstly, if Torosaurus were identical to Triceratops, their fossils should be found in the same locations. In fact, their geographical ranges do not perfectly coincide. In the very north, no Torosaurus fossils have been found, while from the south, only Torosaurus utahensis is known. However, this situation could be an artefact of the relative scarcity of Torosaurus remains and imperfect sampling. Longrich therefore concluded that the hypothesis was corroborated by the first prediction. Secondly, the hypothesis predicted that all Torosaurus specimens would be adults, while no Triceratops specimens would be very old. According to Longrich, this last point had not yet been established. Admittedly, in 2011, Horner had published a histological study showing that all Triceratops specimens investigated possessed a subadult bone structure, but the sample had been too small to allow for a valid generalisation to all Triceratops fossils. To better test the prediction, Longrich proposed a list of twenty-four external skull traits, by which specimens could be checked regarding their level of skull element fusion and their maturation. Thirty-six specimens were investigated upon applying these criteria. It transpired that the fusion typically took place in a certain sequence, providing additional information about their age. Indeed, by these criteria, most Torosaurus specimens were very old. However, there were two exceptions. The small individual ANSP 15192 was a relatively young adult, as shown by the lack of fusion of the snout bones. The youngest specimen was YPM 1831, with an unfused snout, epijugal, and occipital condyle. Furthermore, it had lost all of its frill osteoderms because they apparently had not been fused yet, while the frill edge had the external appearance of growing, young bone. On the other hand, Longrich found that ten of the Triceratops skulls investigated had attained the same level of maturation as the most aged Torosaurus specimens. Longrich concluded that the test of the second prediction refuted the hypothesis. The third prediction was that transitional forms could be found between Torosaurus and Triceratops. Longrich considered the claim that the thin areas on Triceratops frills were precursors of parietal fenestrae, as the strongest proof of a transitional phase. However, he pointed out that these structures differed in position. The Triceratops depressions are partly located on the squamosal, while the Torosaurus holes are fully surrounded by the parietal. Furthermore, the depressions are bordered by much thicker bone, while the Torosaurus holes are surrounded by thin bone. Longrich concluded that the hypothesis failed regarding the third prediction. Being refuted in two of the three predictions, the hypothesis should be rejected.
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. 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.
See also
- Timeline of ceratopsian research
References
- Dodson, P. (1996). The Horned Dinosaurs. Princeton University Press, Princeton, New Jersey, pp. xiv-346
External links
- Kids Zone - Torosaurus
- Dinosaurier Web - Torosaurus
- Chart showing Triceratops/Torosaur growth and development (New Scientist)