TOOTH ENAMEL MICROSTRUCTURE IN PHYTOSAURIA (REPTILIA:ARCHOSAURIA) AND ITS IMPLICATIONS FOR PHYLOGENY, ECOLOGY, AND BIOSTRATIGRAPHY

Phytosaurs are an extinct group of archosaurs known from Laurentian and Gondwanan strata of Late Triassic age that are superficially similar to crocodiles. Many phytosaurs are known to exhibit heterodonty, resulting in different taxa of phytosaurs having multiple types of teeth that appear similar. It is therefore difficult to determine the taxonomic status of isolated teeth without an associated skull. The past assignment of morphologically dissimilar isolated teeth to multiple taxa has caused confusion in light of the discovery of heterodonty, with the end result that isolated teeth are not considered useful in either taxonomic or phylogenetic studies of phytosaurs. However, much of the phytosaurian fossil record is composed of isolated teeth, so a significant amount of data is ignored because of an inability to categorize them with our current understanding. Studies of reptilian teeth demonstrate that there is often, but not always, a phylogenetic signal in tooth enamel microstructure. Phytosaur tooth enamel has not yet been studied in detail. If there is a phylogenetic signal to be found in the microstructure, it may become possible to use microstructural features of the enamel to assign isolated teeth to taxa with more certainty than can be done through gross morphology. One objective of this study is to determine if this is, in fact, possible. Another objective is to compare the microstructure found in different tooth types. We hope to use this to lend evidence to the hypothesized functions of different morphologies (e.g., puncturing, slicing, etc..). We are examining teeth from four localities of different ages in the Upper Triassic Chinle Group of the western USA. Our final objective is to compare the microstructure found in different ages to see if microstructure has changed in complexity through time. Preliminary studies show enamel that is approximately 100µm thick and possesses well-defined columnar enamel in at least two tooth types.