VARIATION IN PHYTOSAUR (DIAPSIDA:ARCHOSAURIFORMES) TOOTH ENAMEL MICROSTRUCTURE AND POSSIBLE IMPLICATIONS FOR PHYTOSAUR ECOLOGY

Phytosaurs are an extinct clade of archosauriforms known from Upper Triassic deposits across Pangea. Isolated phytosaur teeth comprise much of their fossil record, but are not considered taxonomically useful, largely because all possess the same basic ziphodont tooth morphology, even if some taxa are heterodont. Past studies demonstrate that tooth enamel microstructure (EM, or schmelzmuster) can have a phylogenetic signal in some reptiles, providing an alternative to gross morphology to identify isolated teeth. Previous studies of phytosaur EM (n = 3 teeth) were inconclusive, other than documenting great variation of enamel thickness (~20-150µm). We sampled teeth from multiple tooth positions of heterodont taxa from various localities of different ages, from the Chinle Group of the western USA and the Newark Supergroup of North Carolina, to see if we could use EM features to discriminate taxa. Sampled Chinle taxa include, from stratigraphically lowest to highest: Angistorhinus, Smilosuchus, Machaeroprosopus, and Redondasaurus; the Newark phytosaur is tentatively assigned to Rutiodon. We found that enamel thickness ranges from 15-200µm, 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 Chinle teeth usually exhibit well defined columnar enamel, though Angistorhinus appeared to have the least organized microstructure. The Newark teeth exhibit more poorly defined columnar enamel. Only one taxon (Redondasaurus) exhibited any parallel enamel, and a single Chinle tooth (of “Smilosuchus”) exhibited thin lines of incremental growth (LIGs), but the Newark teeth exhibit numerous, well defined LIGs. Column width varied from 5-37µm, with no obvious differences in column thickness between taxa or by tooth position. Therefore, we have not seen any one genus with distinct (autopomorphic) EM characteristics, but there does appear to be some variation between taxa of different basins. This indicates that phytosaur EM may not reflect phylogeny as much as indicate ecological differences between phytosaurs in different geographic ranges.