Abstract

The holotype of the ceratopsid dinosaur Torosaurus (=Arrhinoceratops?) utahensis (Gilmore, 1946) consists of a right squamosal, jugal, quadrate, quadratojugal, epijugal, lacrimal, and postorbital horncore/orbital region. Some elements previously described by Gilmore (1946), notably the epoccipitals and parietals, were not originally included, so they cannot be considered part of the holotype. Associated elements (lower jaws and others), which may pertain to the holotype, are described for the first time; they, too, are not formally considered part of the type material, but they provide additional information regarding the osteology of this rare chasmosaurine.

Torosaurus utahensis differs from T. latus (type species) in having a squamosal that is shorter and squared-off at its distal end and an unusually expanded horncore base that lies above and anterior to the orbit. In contrast, T. latus has unusually long, attenuated triangular squamosals and a more restricted horncore base. The otic notch is more open in T. utahensis than T. latus. The genus Torosaurus is distinguished from other chasmosaurine genera by a combination of characters including a broad, thin, sheetlike parietal with relatively small, nearly circular fenestrae and broad median parietal bar; convex posterior margin of parietal; and relatively straight postorbital horncores that are oval (elliptical) in cross section.

Bona fide records of T. latus from Montana, South Dakota, and Wyoming are from strata of Lancian (late Maastrichtian) age. Previous reports of Torosaurus from the Naashoibito Member of the Ojo Alamo Formation (Lehman, 1981, 1985, 1996) in the San Juan Basin and the McRae Formation (Lucas et al., 1998), New Mexico, as well as the single Torosaurus record from Saskatchewan (Tokaryk, 1986), are based on specimens that can at best be identified as Chasmosaurinae genus indeterminate, because they lack derived features of the taxon. Putative Torosaurus specimens from the Big Bend region of Texas (Lawson, 1976; Lehman, 1996) are also considered as indeterminate chasmosaurines. All records of Torosaurus are Maastrichtian in age, but records of T. utahensis appear to be older than those of T. latus.

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