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.

[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.

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.

[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".

[15] 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.

[20] 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.

[24] 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.

[26] 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.

[20] Below is a ceratopsid cladogram based on the phylogenetic analysis conducted by Sampson et al. in 2010:[29] Centrosaurinae Chasmosaurus Mojoceratops Agujaceratops Utahceratops Pentaceratops Coahuilaceratops Kosmoceratops Vagaceratops Anchiceratops Arrhinoceratops Ojoceratops Eotriceratops Torosaurus Nedoceratops Triceratops 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.

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.

[30] 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.

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".

They explained this by a high mortality of subadults and the possibility that old animals preferentially lived on heights where erosion prevented fossilization .

Of these, they claimed that the bone structure indicated a fully mature age, with the size difference being the apparent result of individual variation.

Farke pointed out that the irregular holes in the Nedoceratops frill, far from piercing thinning bone, were surrounded by thick swellings.

Likewise, with Ceratopia in general, the formation of holes in the frill is not related to age, as even the youngest individuals often possess the parietal fenestrae.

Farke explained the thin bone areas on the frill of Triceratops, the purported location of incipient holes, to be muscle attachment sites.

Finally, Farke pointed out that specimen YPM 1831, despite its enormous size, was apparently not yet fully-grown, as shown by its unfused sutures and smooth bone texture.

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.

They also pointed to Triceratops specimens showing the precise combination of veined, granular, and young striated bone that Farke had considered to be improbable.

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 small individual ANSP 15192 was a relatively young adult, as shown by the lack of fusion of the snout bones.

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 considered the claim that the thin areas on Triceratops frills were precursors of parietal fenestrae, as the strongest proof of a transitional phase.

Longrich foresaw that Scanella and Horner would respond to his second test of their hypothesis by claiming that its results were caused by individual variation.

According to Longrich, the importance of this factor was limited however: e.g. the size difference between ANSP 15192 and YPM 1831 had better been explained by sexual dimorphism, the former possibly being a young adult female and the latter being a subadult male.

They concluded that Torosaurus latus skulls throughout maturation retained a different form from T. horridus and T. prorsus, the last two species showing an overlapping in their proportions.

Farke and Maiorino admitted that the low number of Torosaurus specimens reduced the reliability of these results, but concluded that Torosaurus and Triceratops were separate taxa, though allowing for the possibility of anagenesis, i.e. the several taxa forming a single chronospecies line of descent, given the lack of good stratigraphic data.

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.

(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.