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 Cretaceousperiod, 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 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 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. Two valid species have been named, Torosaurus latus  and T. utahensis.

Recently the validity of Torosaurus has been disputed. 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. 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 authenticTorosaurus 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 seq

uence.

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"). 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.

Classification
In 1891, Marsh placed Torosaurus in the Ceratopsidae family of the Ceratopsia (Greek: "horned faces"),  a group of omnivorous? dinosaurs with parrot-like beaks which thrived in North America and Asia during the Jurassic and Cretaceous Periods.

Torosaurus has, with its long frill, traditionally been classified in the Chasmosaurinae. It was seen as a late member of a line descending from Anchiceratops or Arrhinoceratops. It was thus placed in a different branch from Triceratops which well into the 1980s was seen as a member of the Centrosaurinae because of its short frill. However, in the 1990s exact cladistic analysis showed that both genera were chasmosaurines. Recent analyses invariably show a close relationship between Torosaurus and Triceratops.

Debate about a possible identity with 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 Torosaur juveniles are known but a considerable number of Triceratops juveniles. 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 ofMontana, concluded that this situation could be best explained by the hypothesis that Triceratops and Torosaurus were growth stages of a single genus. TheTorosaurus specimens would be fully mature individuals of Triceratops. Torosaurus would be a junior synonym of Triceratops, the latter name having priority.

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

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 asTriceratops, 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 e xpected 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 preferably lived on heights, where erosion prevented fossilisation. A second problem was the size range ofTorosaurus 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 hornlets, 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 hornlets increased during maturation. Also it was pointed out that both number and position of the osteoderms are variable with Triceratops as shown by specimen MOR 2923, having six hornlets but lacking a midline one.