Tyrannosaurid teeth from the Lomas Coloradas Formation, Cabullona Group (Upper Cretaceous) Sonora, México

• Six unknown isolated theropod teeth were collected in the state of Sonora, Mexico.
• Morphological and statistical analyses assigned the specimens to the Tyrannosauridae.
• The discriminant function analysis classified four of the six teeth as Tyrannosaurus.
• These specimens represent the most southern record of this genus in North America.

The Lomas Coloradas Formation (Cabullona Group, Upper Cretaceous) in the state of Sonora, Mexico, has yielded a great diversity of continental vertebrates, especially dinosaurs. In this study we describe, analyze and illustrate six theropod teeth (ERNO specimens) that were found isolated and surface collected. Identification of the specimens is based upon the methodology provided by Smith (2005), Smith et al. (2007) and Smith et al. (2005). The results showed that the ERNO teeth are comparable to those of tyrannosaurid dinosaurs and some of them probably correspond to a new taxon. Their referral to the Tyrannosauridae family is supported by the presence of semi-conical, laterally compressed crowns with an ovoid cross-sectional base; slightly offset carinae with chisel-shaped denticles that are wider labio-lingually than longer proximo-distally; and the presence of enamel wrinkles at the base of some denticles on the labial surface. These wrinkles are not prominent adjacent to the serrations but they take the form of high relief deep enamel bands across the labial and lingual crown faces. Statistical principal component analysis (PCA) and discriminant function analysis (DFA) corroborated the taxonomically assignation of these teeth into this family. Particularly, the DFA analysis yielded very interesting results. This analysis classified ERNO 8549, 8550, 8551 and 8552 specimens as belonging to Tyrannosaurus, so they represent the most southern record of this genus in Western North America. Finally, the misclassification of ERNO 005 and ERNO 006 specimens remains puzzling. It probably was the result of the presence of juvenile individuals.