Tooth enamel microstructure of Revueltosaurus and Krzyzanowskisaurus (Reptilia: Archosauria) from the upper triassic chinle group, USA: Implications for function, growth, and phylogeny

Abstract

Tooth enamel microstructure can carry significant phylogenetic, ontogenetic, and functional information within amniotes. Here we provide the first descriptions of the tooth enamel microstructure of two Late Triassic taxa, the crurotarsan Revueltosaurus callenderi Hunt and the putative ornithischian Krzyzanowskisaurus hunti (Heckert), which some consider closely related. To test the hypotheses that enamel thickness corresponds to function and/or phylogeny we analyzed the enamel of each at various scales, measuring enamel thickness and examining microstructural features throughout both longitudinal and cross-sectional thickness using previously established techniques to facilitate comparisons. Both taxa possess thick (up to ~150 μm) enamel for their size (< 20 mm crown height). Enamel in R. callenderi ranged from ~5-152 μm across a premaxillary tooth in longitudinal section, and ~42-92 μm in a maxillary/dentary tooth transverse section. K. hunti enamel thickness was ~18-155 μm longitudinally and ~29-75 μm transversely. Both also had well-developed basal unit layers (BUL) and weakly developed columnar microstructure. Well-developed lines of incremental growth (LIG) are present in both taxa, through which the columnar enamel grades into parallel crystallite enamel. Their enamel microstructure is therefore grossly similar to that of several ornithischian taxa, especially ankylosaurs, with which they are strongly convergent, and also compares well to rauisuchids and tyrannosaurids. The relatively unique combination of microstructural characteristics in the schmelzmuster of R. callenderi and K. hunti supports the hypothesis that they are closely related, but does not conclusively preclude a different taxonomic placement for K. hunti so we retain its separate generic designation. © Society for Vertebrate Paleontology January 2013.