New Mexico Geological Society Annual Spring Meeting — Abstracts

It is difficult to test models of species-level evolution for dinosaurs because sample sizes are small, basic taxonomy is inadequate and the stratigraphic density of samples is low. Nevertheless, a case for the punctuated origin of the long-crested ceratopsian Torosaurus can be made.

The putative ancestor of Torosaurus is Pentaceratops, a long-crested ceratopsian known only from the Upper Cretaceous Fruitland and Kirtland Fromations in the San Juan Basin, New Mexico. Pentaceratops is known from one species, P. sternbergii (P. fenestratus) which ranges from late Campanian (Fruitland Formation) through late Maastrichtian (Naashoibito Member of Kirtland Shale) without undergoing any evident morphological change. P. sternbergii thus appears to represent an isolated, evolutionarily static species for five or more million years. In the San Juan Basin, Torosaurus first appears in the late Maastrichtian Naashoibito Member of the Kirtland Shale. Torosaurus is also known from temporally equivalent (or slightly younger?) strata in Texas (Javelina Formation), Utah (North Horn Formation), Wyoming (Lance Formation) and South Dakota (Hell Creek Formation). There are no morphological intermediates between Pentaceratops and Torosaurus.

The morphology and distribution in time and space of Pentaceratops and Torosaurus conforms well with a punctuated model of evolution. There is an evolutionarily static and isolated ancestral species (Pentaceratops) and a morphologically very distinct descendant species (Torosaurus). The temporal range of the ancestor and descendant overlap, and the descendant species achieved a much broader geographic distribution than its ancestor. However, a more detailed understanding of ceratopsian taxonomy and phylogeny is needed to further corroborate the case of punctuated evolution in Pentaceratops-Torosaurus.