Torosaurus Marsh, 1891

Lawson, Douglas A., 1976, Tyrannosaurus and Torosaurus, Maestrichtian Dinosaurs From Trans-Pecos, Texas, Journal of Paleontology 50 (1), pp. 158-164 : 161

Order Ornithischia

Suborder Ceratopsia

Family Ceratopsidae

Genus Torosaurus Marsh, 1891

Remarks.— There are three shapes to the frills of ceratopsia with fenestrated parietals, 1) triangular, found in Protoceratops , Chasmosaurus , and Pentaceratops , 2) figure-8, found in Brachyceratops , Monoclonius , Styracosaurus and presumably Pachyrhinosaurus , 3) broad frill with elliptical or circular fenestrae, found in Anchiceratops , Arrhinoceratops , and Torosaurus . Triceratops, except for a few “sports,” lacks fenestrae but is related to the last group. Considering the third group, in dorsal view Anchiceratops and Arrhinoceratops have rectangular frills with well developed epoccipitals. Torosaurus has a cardioid frill, has weakly developed epoccipitals, thinner parietals, and reduced vascular sulci. Since Torosaurus arose from an Arrhinoceratops- like form, a complete fossil record would show a gradation between forms and primitive torosaurs should be intermediate in character. Such a situation seems to exist in the case of Torosaurus utahensis .