Tooth enamel microstructure in North American Phytosauria (Diapsida:Archosauriformes): Implications for biogeography and ecology of a Late Triassic clade of crocodylian-like predators

Abstract

Teeth can provide important insight into diet and evolution of extinct vertebrates. Tooth enamel microstructure records functional and phylogenetic signals beyond the gross morphology of the dentition. Here, we provide the first systematic sampling of phytosaur tooth enamel to address questions of intra-and interspecific variation, and thus taxonomic identification, biogeographic connectivity, and heterodonty. We sampled 23 phytosaur teeth from five localities throughout the American Southwest and one locality from the Newark Supergroup of North Carolina. These teeth probably represent five heterodont genera and are tentatively assigned to Angistorhinus, Smilosuchus, Machaeroprosopus, Redondasaurus, and "Rutiodon". We used scanning electron microscopy to examine their enamel microstructure from transverse, longitudinal, and tangential cross-sections. All sampled teeth are composed of columnar enamel ranging in thickness from 20 to 150 µm, typically 50-100 µm, across all genera. In phytosaurs from the western US, lines of incremental growth (LIGs) are rare, whereas in the Newark Supergroup phytosaur "Rutiodon", LIGs are abundant and welldeveloped. Although phytosaur tooth enamel microstructure is not useful for the taxonomic assignment of isolated teeth, it can be used to differentiate phytosaurs from different basins and lends support to the hypothesis that western and eastern North American phytosaurs are taxonomically distinct. The posterior blade-like teeth of heterodont phytosaurs are consistently composed of proportionately thicker enamel (10-14 µm thicker than anterior teeth of comparable size in heterodont phytosaurs), implying a greater degree of force on these teeth during food processing. Combined with independent measures of diet, enamel microstructure can help refine dietary hypotheses during the Triassic archosauriform radiation.

A diverse diapsid tooth assemblage from the Early Triassic (Driefontein locality, South Africa) records the recovery of diapsids following the end-Permian mass extinction

“…The use of CT to visualize the morphology of replacement teeth can help in cases where erupted teeth are missing from sockets. Other methods of dietary interpretation for teeth should be used to confirmed ecological interpretations of morphotypes, like OPCR [ 27 ], microwear [ 71 ], and enamel microstructure [ 72 , 73 ].…”

A new phytosaur species (Archosauriformes) from the Upper Triassic of Jameson Land, central East Greenland

“…The difference between "weakly" to "strong heterodonty" was not clearly explained until Hungerbühler (2000), who described in detail the dentition of the phytosaur Nicrosaurus kapffi. This has been used since then (Spielmann and Lucas, 2012;Datta et al, 2021) to help determine species, and even invoked to determine the feeding behavior (Hungerbühler, 2000;Hoffman et al, 2021). The different levels of heterodonty can differentiate phytosaurs based upon "tripartite dentition" (tip of the snout, premaxilla, and maxilla), or "bipartite dentition" (tip and post-tip of the snout; Hungerbühler, 2000).…”