TOOTH ENAMEL MICROSTRUCTURE OF SELECTED ARCHOSAURS REPTILIA: ARCHOSAURIA FROM THE UPPER TRIASSIC CHINLE GROUP, WESTERN USA: TAXONOMIC AND EVOLUTIONARY SIGNIFICANCE

Within Amniota, tooth enamel microstructure yields much information on taxonomy, functional morphology, ontogeny, and phylogeny. We surveyed tooth enamel microstructural features in five archosaurs from the Upper Triassic Chinle Group: three heterodont phytosaurs, the crurotarsan Revueltosaurus callenderi, and the putative ornithischian Krzyzanowskisaurus hunti. The phytosaurs most likely represent the genera Angistorhinus, Rutiodon (=Smilosuchus), and Redondasaurus, taxa known from strata of Otischalkian (early-late Carnian), Adamanian (late-latest Carnian) and Apachean (latest Norian-Rhaetian) age, respectively. K. hunti is restricted to strata of Adamanian age and R. callenderi to strata of Revueltian (early-mid Norian) age. The sample size is small (14 teeth total), yet more comprehensive than in most similar studies. All taxa exhibit some form of columnar enamel, the derived condition in archosaurs. Columnar enamel is best expressed in Angistorhinus and Rutiodon, and less so in Redondasaurus, and columnar features are much better developed in the phytosaurs than in either R. callenderi or K. hunti, although the latter tend to have much thicker enamel (up to 130 µm), especially relative to their overall size. Lines of incremental growth (LIGs) are present to varying degrees in Angistorhinus, Redondasaurus, R. callenderi, and K. hunti. LIGs were not well developed in the phytosaurs but much more pronounced in the other taxa. There is no convincing taxonomic variation in enamel microstrucuture features in phytosaurs as intraspecific (positional and/or ontogenetic) variation is extensive. Phytosaur enamel is broadly similar to other xiphydont archosaurian carnivores in it its thickness (~15-115 µm; average 40-60 µm) and columnar structure. Similarities supporting a close relationship between R. callenderi and K. hunti include overall thickness and thickness trends, the poorly defined columnar units, and the expression of LIGs, although these similarities may also indicate that the two taxa convergently evolved a similar feeding strategy. Thick enamel and columnar (rather than parallel) enamel are associated with relatively durophagous diets, but not grinding mastication. The numerous LIGs imply a longer period of growth and slower replacement in these genera than in phytosaurs.