ENAMEL MICROSTRUCTURE IN THE TRIASSIC ARCHOSAURS REVUELTOSAURUS CALLENDERI AND KRZYZANOWSKISAURUS HUNTI AND ITS IMPLICATIONS FOR PHYLOGENY

Reptilian tooth enamel microstructure can be used for interpreting concepts such as functional morphology and convergent evolution at many levels. We examined teeth from Revueltosaurus callenderi, a crurotarsan, and Krzyzanowskisaurus hunti, a possible basal ornithischian, both from the Upper Triassic Chinle Group in the southwestern USA. We examined a premaxillary tooth in longitudinal section and a maxillary/dentary tooth in transverse section from both taxa. To test the hypotheses that enamel yields data on functional morphology and/or phylogeny, we measured enamel thickness and microstructural features throughout, then compared them to each other and to all previously studied archosaurs. Teeth of both taxa bore relatively thick enamel for their size (<20 mm crown height). R. callenderi ranged from ~15 µm basally to ~130 µm apically in the premaxillary, and ~40-60 µm in the transverse section of the maxillary/dentary, with thickening along the denticles. K. hunti teeth ranged from ~18 µm basally to ~145 µm apically in longitudinal section and ~29-58 µm in transverse section. Both taxa have well-developed basal unit layers (BUL) and weakly developed columnar microstructure. The diverging pattern in K. hunti is better developed and exhibits a radial pattern within the columns, especially in transverse section. In general, parallel enamel is considered primitive and columnar derived in amniotes. Columnar enamel is assumed to be more resistant to fracture imposed by a diet requiring greater tooth strength with less emphasis on grinding. Well-developed lines of incremental growth were present in both taxa.

We used techniques similar to those of previous workers to facilitate comparisons with other taxa. R. callenderi and K. hunti have enamel microstructure grossly similar to that of several ornithischian taxa, and compare favorably to that of rauisuchids and tyrannosaurids in microstructure, but with more prominent incremental lines.

R. callenderi and K. hunti teeth are similar, but possess no unique synapomorphies that confirm the hypothesis that they are closely related; however, their convergence with numerous other taxa does not preclude that possibility.