New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent discoveries of diagnostic tyrannosaur specimens from late Campanian and late Maastrichtian strata of New Mexico coupled with a robust phylogeny of Tyrannosauroidea shed light on their biogeographic distribution in western North America.

Three incomplete tyrannosauroid skeletons collected from late Campanian beds of the San Juan Basin can be referred to two genera and species. The smallest skeleton is a juvenile of a new species of Daspletosaurus. Two large partial skeletons, presumably adults, are of a new genus of basal tyrannosauroid. The late Campanian tyrannosauroid fauna is similar to that of synchronous deposits in the Northern Rocky Mountain Region, in which sympatric genera are present (e.g., Daspletosaurus and Albertosaurus of the Dinosaur Park Formation).

Late Maastrichtian tyrannosaurids include incomplete isolated cranial and postcranial elements and isolated teeth from the Naashoibito Member, Kirtland Formation of the San Juan Basin, northwestern New Mexico that can be tentatively referred to Tyrannosaurus rex and a partial skeleton of an adult T. rex from the Hall Lake Member, McRae Formation at Elephant Butte Reservoir, south-central New Mexico. The occurrence of a single tyrannosaurid in the late Maastrichtian of New Mexico is congruent with the rest of western North America, in which T. rex is the only giant top predator.

The tyrannosauroid fauna of New Mexico differs from that of the Northern Rocky Mountain Region in the presence of a basal tyrannosauroid. Geographic isolation, in part, may account for this occurrence. However, a detailed examination of the historical biogeography of other Late Cretaceous dinosaur taxa of western North America is required to test this hypothesis. The presence of the basal genus and the new species of Daspletosaurus indicates that tyrannosauroid diversity in the late Campanian was higher than previously thought, which may be expected to increase as new fossils are sampled throughout Eurasia and North America. This diversity suggests that high rates of speciation may characterize the evolution of dinosaurs in western North America during the Late Cretaceous.