In light of these problems and the recognition of OMNH 10131 as an adult Daspletosaurus, we consider Aublysodon to be a nomen dubium. A partial skull and skeleton of a Late Maastrichtian tyrannosaurid (LACM 23845) was referred to Albertosaurus lancensis by Molnar (1980) and later to A. megagracilis by Paul (1988). Pending personal examination of the material, we conservatively regard the specimen to represent a subadult T. rex on the basis of its relatively small size, the narrow frontal process of the nasals (a character that typifies Daspletosaurus and Tyrannosaurus), and the absence of a lacrimal cornual process, a character specific to Tyrannosaurus. Therefore, we consider T. rex to be the only tyrannosaurid present in late Maastrichtian sediments of western North America. Diagnostic T. rex fossils have been collected from Alberta (e.g., TMP 81.6.1), Montana (e.g., AMNH 5027), Wyoming (e.g., AMNH 5117), South Dakota (e.g., BHI 3033), Texas (e. g., TMM-41431), and New Mexico (NMMNH P-3698). A disconformity between the top of the De-na-zin Member of the Kirtland Formation and the overlying Naashoibito Member likely represents a six-million year gap in the sedimentary record of the San Juan Basin (l/Villiamson, 1998). Tyrannosaurid fossils collected from the Naashoibito Member are most comparable to T. rex, indicating a late Maastrichtian age for the unit. Tyrannosaurid material is represented by the complete crown of a maxillary or dentary tooth (NMMNH P—13000; Fig. 4I), tooth fragments (NMMNH 13-28367, NMMNH P-28369), cranial and postcranial fragments (NMMNH P-7199), and a pedal phalanx (AMNH 5882; Fig. 4A-F). The basal crown length (45.6 mm) of NMMNH P-13000 is greater than the largest Campanian tooth in our sample (34.0 mm, CMN 8506) and is more comparable to the size of T. rex (46_.7 mm, NMMNH P-3698). Also, the denticle densities are comparable to T. rex (see Table 4). We therefore agree with Lucas et al. (1987) that this specimen is referable to cf. T. rex. NMMNH P—7199 includes fragments of a left dentary and a vertebra, but these are too weathered and incomplete to permit identification. However, the tooth fragments associated With NMMNH P—7199 have 7.5-8.5 denticles per 5 millimeters, densities that are comparable with T. rex teeth (see Table 5). A partial skeleton (NMMNH P-3698; Fig. 4G-H, J-K, 10 J-M) of T. rex from the MCRae Formation unequivocally establishes the presence of this taxon in New Mexico. The presence of A. sarcophagus in Alberta between 71 and 69 Ma suggests that T. rex was absent from North America during the early Maastrichtian. If true, then the arrival of T. rex into North America was a devastating event for the endemic forms living on the western North American peninsula of Asiamerica (Russell, 1995), inwhich all forms were replaced. This faunal turnover suggests that, at least during the Campanian, a significant biogeographic barrier existed between western North America and Central Asia that allowed endemic forms to evolve in mutual isolation until the late Maastrichtian or at least prevented the eastward dispersal of Central Asian forms. The presence of T. rex in New Mexico indicates that the state was not excluded from the late Maastrichtian faunal turnover in western North America. T. rex has been found further south in the Big Bend region of western Texas (Lawson, 1976; contra Carpenter, 1990). Its presence in the American Southwest undermines the notion of northern and southern faunal realms (sensu Lehman, 1981, 1987) in western North America during the late Maastrichtian. In summary, the succession of tyrannosaurids during the late Campanian and late Maastrichtian is comparable to that of the northern Rocky Mountain region, except that the late Campanian forms are distinct at the species level.

Published as part of Carr, T. D. & Williamson, T. E., 2000, A review of Tyrannosauridae (Dinosauria, Coelurosauria) from New Mexico, pp. 113-146 in New Mexico Museum of Natural History and Science Bulletin 17 on pages 134-135, DOI: 10.5281/zenodo.3373655