Torosaurus utahensis (Gilmore), 1976

Lawson, Douglas A., 1976, Tyrannosaurus and Torosaurus, Maestrichtian Dinosaurs From Trans-Pecos, Texas, Journal of Paleontology 50 (1), pp. 158-164 : 162-163

Torosaurus utahensis (Gilmore)

new comb.

Text-fig. 5 a, b

Arrhinoceratops? utahensis Gilmore, 1946 , U. S. Geol. Survey Prof. Paper 210-C, p. 42, Pl. II, fig. 1.

Holotype. — USNM 15583 . A right squamosal, quadrate, quadratojugal, postorbital, supraorbital horn core, postfrontal, lacrimal, jugal, and epijugal. Type locality. — West side of North Horn Mountain, Manti National Forest , Emery County, Utah.

Referred material.— USNM 15875 (Paratype), a right squamosal and the posterior part of a parietal; USNM 16573 , part of a left parietal; TMM 41480-1 , posterior part of right parietal. Horizon. — Lower half of the North Horn Formation; lower third of the Tornillo Group; Maestrichtian Age.

Revised diagnosis.— Torosaurus utahensis can be distinguished from T. latus by the more anteriorly placed supraorbital horns, and the presence of straight, diagonal and longitudinal vascular sulci on the parietals. Distinct epoccipitals are present only on the anterolateral edge of the squamosal. The squamosal of T. utahensis is proportionally shorter than in T. latus .

Description. —The right parietal, TMM 41480-1 , is extremely thin, reaching a thickness of 24 mm at the midline of the frill, and in a zone within 150 mm of the posterior edge. The bone thins gradually toward the posterior margin of the frill and 166 mm from the parietal-squamosal suture. The frill thins to a broken edge only 5 mm thick approximately 280 mm from the midline. There is a sharp sagittal ridge running along the interparietal bar. The bar has a triangular cross-section; its ventral surface is flat. Subdued undulations are present along the curved posterior edge of the frill, but distinct epoccipitals are lacking. Vascular sulci are developed on the posteromedial area on the dorsal surface of the frill. The remaining surfaces appear relatively smooth, exhibiting broadly separated, straight, diagonal and longitudinal, wide and deep vascular sulci on the dorsal and ventral surfaces.

Discussion. —The squamosal of the type of Torosaurus utahensis ( USNM 15583 ) and the parietal and squamosal of USNM 15785 possess shallow vascular sulci on the dorsal surface. Both the Texas and Utah specimens exhibit a few straight, diagonal and longitudinal, broad and deep vascular sulci on an otherwise relatively smooth parietal. Both specimens have a slightly undulating posterior border that lacks distinct epoccipitals. The greatest thickness of the frill of the North Horn specimen ( USNM 15583 ) is 18 mm approximately 40 mm anterior to the posterior edge. From there, the frill thins to 30 mm at the posterior border of the fenestra (Gilmore, 1946). The parietal thickens slightly within 25 mm of its posterior edge. The similarity in the topology of these parietals leaves little doubt that they are from members of the same species.

Although Gilmore questionably assigned the Utah ceratopsian to the genus Arrhinoceratops , close examination of the type A. brachyops ( ROM 5135 ; Parks, 1925) shows a number of differences. Unlike Torosaurus utahensis , Arrhinoceratops brachyops possesses deep reticulate sculpturing of the frill. Unlike the parietal of A. brachyops which thickens to 30 mm about 40 mm from the posterior edge, the parietal of Torosaurus utahensis does not exceed 17 mm in a comparable zone. The ornamentation on the posterior edge of the frill of Arrhinoceratops is much exaggerated compared to that of Torosaurus utahensis . In dorsal view, the outline of the frill in the two genera is quite different—rectangular in Arrhinoceratops and cardioid in Torosaurus .

The frill of Torosaurus latus is fairly smooth on both the dorsal and ventral surfaces and has small marginal undulations as in Torosaurus utahensis . The vascular sulci, less than 2 mm in diameter on the dorsal surface of the frill in T. latus (YPM 1830), are more pronounced in the area near the midline. The ventral surface of Torosaurus latus (ANSP 15192) exhibits a significantly deep and extensive network of vascular sulci as does the paratype of Torosaurus utahensis . The squamosal of T. utahensis ( USNM 15583 ) is proportionally shorter than that of T. latus .

The supraorbital horns of T. utahensis ( USNM 15583 ) arise directly in front of the anterior edge of the orbits, but the greater part of the horn arises behind the posterior edge of the orbit as in T. latus . The horns of Torosaurus give an appearance of being extensions of the skull because the dorsal edge of the horns may continue up from the squamosals with little change in slope. The anterior edge of the horns may lie in front of or above the posterior edge of the orbits ( Colbert and Bump, 1947). The rapidly tapering horns of Torosaurus curve dorsally at the tip, whereas the relatively blunt horns of Arrhinoceratops taper more gradually, starting from a base which lies over the orbits.

It should be pointed out that the characteristics used to distinguish T. utahensis from T. latus might actually be those that separate young and old of the same species. In USNM 15583 all the bones of the skull were unfused as in young individuals. The frill is thinner in the Texas and Utah specimens than in those from Wyoming and South Dakota. The distance between parietal-squamosal sutures at the posterior edge of the frill is smallest in the two southern specimens. Even though the squamosal of T. utahensis is proportionally shorter than T. latus , it is approximately 945 mm long in USNM 15583 compared to 790 mm (ANSP 15192), 1240 mm (YPM 1830), and 1430 mm (YPM 1831) in T. latus ( Colbert and Bump, 1947) . But geographic distribution and lithologic association of the Torosaurus specimens adds credence to the recognition of two species; T. utahensis being the southern species that lived hundreds of kilometers from the coast and T. latus the northern species that lived near the coast.

Figures

Text-fig. 5—Torosaurus utahensis (Gilmore) n. comb., TMM 41480-1, right parietal, a, dorsal view, b, ventral view. x 1/8.