page_title: Torosaurus
section_title: Possible synonymy with Triceratops
hierarchical_section_title: Torosaurus / Possible synonymy with Triceratops
caption_attribution_description: 70 - 65 million years ago Late Cretaceous Item number: USNM 2412 This is the only known specimen of the Nedoceratops.
context_page_description: Torosaurus 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, though it is possible that the species range might extend to as far as 69 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 in length. From head to tail, Torosaurus is thought to have measured about 8 to 9 m long and weighed four to six tonnes. Torosaurus is distinguished from the contemporary Triceratops by an elongate frill with large 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, and instead resembled the earlier and more basal Triceratops horridus in having a short nose horn. Three species have been named, Torosaurus latus, T. gladius and T. utahensis. T. gladius is no longer considered a valid species, however.
context_section_description: A debate has sparked over the possibility that Torosaurus might be identical to Triceratops. In the Maastrichtian of Laramidia two closely related chasmosaurine genera shared the same habitat. The only discernible difference between them was the form of the frill. No Torosaurus juveniles are known but a considerable number of Triceratops juveniles have been found. Triceratops differs from other chasmosaurines in the retention as an adult of a juvenile trait: the short squamosals, a case of paedomorphosis. In 2009 John Scannella, investigating dinosaur ontogeny in the Hell Creek Formation of Montana, concluded that this situation could be best explained by the hypothesis that Triceratops and Torosaurus were growth stages of a single genus. The Torosaurus specimens would be fully mature individuals of Triceratops. Torosaurus would be a junior synonym of Triceratops, the latter name having priority.
  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.
  Scanella and Horner recognised that not all data were easily explained by their hypothesis. For these they advanced auxiliary hypotheses. One problem was that if Torosaurus were the normal last maturation phase of Triceratops, which phase they called the "toromorph", it would be expected that Torosaurus fossils were quite common, whereas in fact they are rather rare. This they explained by a high mortality of subadults and the possibility that old animals preferentially lived on heights, where erosion prevented fossilisation. A second problem was the size range of Torosaurus specimens which seems to suggest the existence of authentic Torosaurus subadults. Of these they claimed that the bone structure indicated a fully mature age, the size difference being the apparent result of individual variation. A third possible objection was the seeming lack of transitional forms between individuals with and without parietal holes; these fenestrae are always perfectly shaped, not like incipient perforations. To counter it, they pointed to specimen USNM 2412, the holotype of the contentious Nedoceratops, as an example of precisely such a transitional form. The problematic traits of this genus would simply reflect its being in the first stages of transforming into a "toromorph". A last problem was offered by the number of epoccipitals, the osteoderms on the frill edge. With Triceratops there are typically five epiparietals, including a midline osteoderm; with Torosaurus there are ten or twelve, a midline epiparietal being absent. Also the number of episquamosals on the side edge of the frill differs: five with Triceratops, six or seven with Torosaurus. This was explained by the assumption that the number of epocci
caption: Scanella & Horner saw Nedoceratops as an ontogenetic transitional form between Triceratops and Torosaurus
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