- GeoRef, Copyright 2004, American Geological Institute.
A dinosaur skull, from the upper Hell Creek Formation. Late Cretaceous in age, collected in 1964 in southwestern North Dakota is displayed on the University of North Dakota campus as Triceratops prorsus Marsh. This identification is reconsidered in light of work by Ostrom and Wellnhofer (1986) and Lehman (1990), who suggested that the range of morphological variation exhibited by latest Cretaceous ceratopsian skulls can be accommodated within one species of Triceratops. T. horridus (Marsh), the type species. The skull is thought to be from a mature female, as indicated by forwardly inclined, wide-set, supraorbital horncores, characters chosen by Lehman as indicators of gender from an association of characters taken as sex-related. Dimensions of the skull are given. Based on detailed cladistic and morphometric analyses of many skulls, Forster (1996b) accepted two species of Triceratops as valid, T. horridus and T. prorsus. Consideration of her studies suggests that the skull is correctly identified. Reoccupation of the collecting site allowed its more accurate location, based upon newly available topographic maps. Twenty-nine years after this skull was removed, little evidence of the access road or excavation remains at the site.