MORPHOMETRIC ANALYSIS OF TEETH OF CROSBYSAURUS HARRISAE (REPTILIA:ARCHOSAURIFORMES) FROM THE LATE TRIASSIC (ADAMANIAN), CHINLE GROUP, SOUTHWESTERN U.S.A

The Late Triassic archosauriform Crosbysaurus harrisae Heckert is known from multiple dissociated, yet distinctive, teeth found in Upper Triassic strata in vertebrate microfossil assemblages in Texas, New Mexico, and Arizona. It is an index taxon of the Adamanian (Late Triassic: ?latest Carnian-?early Norian) land vertebrate faunachron. Specimens are small (≤ 2.5 mm in height), but possess multiple comparatively large denticles bearing subdenticles on both the mesial and distal sides. Teeth we assign to C. harrisae include shorter maxillary/dentary(?) teeth as well as taller teeth we tentatively interpret as premaxillary. The absence of early ornithischian fossils, as well as the ambiguious identity of C. harrisae, poses a problem for the further understanding of the evolution of such forms. We compared Crosbysaurus teeth to other taxa known from the Triassic (including pterosaurs, silesaurs, ornithischian dinosaurs, and cynodontid mammals) to test possible relationships. The teeth exhibit similarities with ornithischian dinosaurs, but, as others have noted, lack a true cingulum. Understanding the morphology as well as the identity of these teeth is important to understand the evolution of Late Triassic tetrapods and possibly the clade Dinosauria. Morphometric analysis of teeth (n = 14) included measurements of the fore aft base length (FABL), crown height (CH), basal width (BW), and number of primary denticles on both mesial and distal sides. The intercarinal angle between mesial and distal carinae is ~180˚ and suggests a narrow and elongated jaw. Preliminary results include the following: CH mean = 1.81 mm (standard deviation – SD = 0.64 mm); FABL mean = 1.48 mm (SD = 0.43 mm); BW mean = 0.69 mm (SD = 0.11 mm). Procrustes transformation of three landmarks were made to compare the shapes of the teeth to one another using thin plate splines and Fourier shape analysis. Fourier shape uses trigonometric summations to create ellipses which encapsulate the overall shapes of the teeth on the mesial, distal, and occlusal sides.