Masticatory mechanisms, dental function, and diet in Triassic trilophosaurids (Reptilia: Allokotosauria)

Abstract

Documenting evidence of feeding behavior in extinct vertebrates is crucial to understanding trophic structure and stability of ecosystems over periods of Earth's history. Direct evidence of trophic interactions in the fossil record is rare, but proxies like dental microwear enable testable hypotheses of feeding behavior in extinct taxa. Here we present new evidence on the masticatory apparatuses and feeding behaviors of Late Triassic (Norian) trilophosaurid reptiles from southwestern North America based on Scanning Electron Microscope (SEM) observation and quantitative analysis of dental microwear in isolated teeth and dentigerous elements. Trilophosaurids are a Late Triassic archosauromorph group known from Europe and North America with distinctive labiolingually expanded teeth that are usually tricuspid; this clade is hypothesized to be herbivorous based on qualitative comparisons of their dentition to those of living taxa. Our study is among the first to infer their diet within a quantitative comparative framework. Analysis of pit-to-scratch ratios (a proxy for the hardness/toughness of dietary intake) indicate dietary disparity between the three examined trilophosaurid taxa, possibly driven by differing plant preferences. Analysis of scratch orientations, caused by contact between opposite teeth during jaw closure (a proxy for the direction of masticatory movements), may suggest derivation of a masticatory mechanism including labiolingual movement in Trilophosaurus, differing from the orthal jaw closure mechanism ancestral to archosauromorphs that is likely conserved in the sister group to Trilophosauridae, the Azendohsauridae. Variance in dental microwear patterns between the sampled taxa suggests potential herbivore niche space partitioning amongst North American trilophosaurids during the Norian. Furthermore, microwear evidence for complex masticatory movement in trilophosaurids may suggest previously unknown craniomandibular specializations, highlighting the need for reexamination of trilophosaurid craniomandibular morphology and jaw closure mechanics.