New Mexico Geological Society Annual Spring Meeting — Abstracts

During the spring of 1984, the New Mexico Museum of Natural History (NMMNH) collected a portion of a skeleton (NMMNH P-3698) from the Hall Lake Member, McRae Formation, exposed near the margin of Elephant Lake Reservoir (NMMNH locality 353) and referred it to Tyrannosaurus rex. The specimen included a left dentary, right prearticular, a partial right palatine, several loose teeth, and a hemal arch. Additional parts of the specimen were reported to remain at the site. However, the locality was drowned by rising lake levels and had been inaccessible for nearly 19 years.

During September, 2002, the NMMNH recovered additional portions of this specimen following a drop in lake level. Disarticulated bones were found clustered in a conglomerate; these include a partial right splenial, a partial right angular, a right articular, a right squamosal, a. right postorbital, portions of several isolated teeth, and several additional hemal arches. Diagnostic characters of the postorbital and palatine are the basis of our referral of the specimen to T. rex.

The Elephant Butte T. rex is significant for several reasons: 1) it represents the only specifically diagnostic dinosaur to be collected from the McRae Formation and supports a late Maastrichtian age for the Hall Lake Member; 2) it confirms previous identification of this specimen as T. rex and verifies the presence of T. rex in the southern Rocky Mountain region; and 3) it is the most complete T. rex from New Mexico and the southern United States. The specimen is far more complete than the single maxilla that has been referred to T. rex from the Javelina Formation of Big Bend Texas. Moreover, the referral of the Big Bend specimen to T. rex has been questioned though we agree with the original identification. T. rex has also been tentatively identified in the Naashoibito Member of the Kirtland Formation based on presence of isolated teeth, a partial dentary, a phalanx, and a partial metatarsal IV. Recent identification of T. rex from the North Hom Formation of Utah and the tentative identification of T. rex from the Denver Formation of Colorado show that T. rex was widespread in western North America at the end of the Cretaceous. T. rex has also been identified in upper Maastrichtian deposits of Wyoming, Montana, South Dakota, North Dakota, Alberta, and Saskatchewan. T. rex is the most widespread Mesozoic dinosaur species and the genus Tyrannosaurus (=Tarbosaurus) is also one of the most widespread as relatively complete and diagnostic remains are present in both Asia and western North America.

The pattern of tyrannosaur diversity during the late Cretaceous resembles that of other North American non-avian dinosaurs as a whole in that there is a decline in generic diversity approaching the end of the Cretaceous. This is due, in part, to a decrease in provinciality as faunas in western North America become increasingly more homogeneous. The distribution of T. rex does not reflect the general biogeographic division between north and south Laramidia as seen in other Late Cretaceous dinosaurian clades and in this sense T. rex is an unusual component of the late Maastrichtian fauna.