PHYLOGENETIC SIGNALS IN PHYTOSAUR TOOTH ENAMEL MICROSTRUCTURE AND IMPLICATIONS FOR NEWARK SUPERGROUP PHYTOSAURS

Phytosaurs are an extinct clade of archosauriformes known from strata of Late Triassic age that appear similar to crocodiles. Some phytosaurs exhibit heterodonty, rendering it difficult to determine the taxonomic status of isolated teeth based on their gross morphology, meaning that isolated teeth are not considered useful in either taxonomic or phylogenetic studies. However, such teeth comprise much of their fossil record, including most Newark Supergroup occurrences. Others have shown that tooth enamel microstructure (EM, also called schmelzmuster) can have a phylogenetic signal in some reptiles, providing an alternative to gross morphology. Phytosaur EM has yet to be studied in detail, and previous attempts (n = 3 teeth) yielded inconclusive results other than recording great variation in enamel thickness. If there is a phylogenetic signal in features of the EM, it may be possible to use these features to assign isolated teeth to taxa with more certainty than through gross morphology alone. Also, in light of a recent study finding phytosaurs basal to crown-group archosaurs, EM features of phytosaurs may be plesiomorphic for Archosauria. We used teeth from multiple tooth positions from several localities of different ages, and therefore different heterodont taxa, from the Upper Triassic Chinle Group of the western USA to compare to teeth from the Newark Supergroup of North Carolina. Chinle taxa investigated include Angistorhinus, Smilosuchus, Machaeroprosopus, and Redondasuchus. Preliminary results show that enamel thickness ranges from 15um to 200um, with type U teeth (unserrated conical premaxillary teeth) having the least variation and type B teeth (maxilla teeth, low and blade-like with serrations) having the most. The teeth typically exhibit well defined columnar enamel, although Angistorhinus appears to have the least organized microstructure. Only one taxon (Redondasuchus) exhibited any parallel enamel, and a single tooth (of “Smilosuchus”) exhibited thin lines of incremental growth, but we have not seen evidence of any one taxon having distinct microstructural characteristics. In our subsequent studies we will refine our understanding of the microstructure with additional specimens and improved SEM imaging.