Dietary constraints of phytosaurian reptiles revealed by dental microwear textural analysis

Abstract

Phytosaurs are a group of large, semi‐aquatic archosaurian reptiles from the Middle–Late Triassic. They have often been interpreted as carnivorous or piscivorous due to their large size, morphological similarity to extant crocodilians and preservation in fluvial, lacustrine and coastal deposits. However, these dietary hypotheses are difficult to test, meaning that phytosaur ecologies and their roles in Triassic food webs remain incompletely constrained. Here, we apply dental microwear textural analysis to the three‐dimensional sub‐micrometre scale tooth surface textures that form during food consumption to provide the first quantitative dietary constraints for five species of phytosaur. We furthermore explore the impacts of tooth position and cranial robustness on phytosaur microwear textures. We find subtle systematic texture differences between teeth from different positions along phytosaur tooth rows, which we interpret to be the result of different loading pressures experienced during food consumption, rather than functional partitioning of food processing along tooth rows. We find rougher microwear textures in morphologically robust taxa. This may be the result of seizing and processing larger prey items compared to those captured by gracile taxa, rather than dietary differences per se. We reveal relatively low dietary diversity between our study phytosaurs and that individual species show a lack of dietary specialization. Species are predominantly carnivorous and/or piscivorous, with two taxa exhibiting slight preferences for ‘harder’ invertebrates. Our results provide strong evidence for higher degrees of ecological convergence between phytosaurs and extant crocodilians than previously appreciated, furthering our understanding of the functioning and evolution of Triassic ecosystems.

Dental microwear texture analysis along reptile tooth rows: complex variation with non-dietary variables

“…Furthermore, DMTA has been shown to be effective at discriminating diets between species or populations, even with low sample sizes [12][13][14]. DMTA therefore provides a valuable tool for providing robust insight into the levels of niche partitioning and resource competition for a huge range of modern taxa [6,9,[14][15][16][17][18], and for reconstructing extinct food webs from the diets of fossil taxa [19][20][21][22][23][24][25][26][27][28].…”

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

“…However, recent tooth enamel microwear analysis of phytosaurs did not find evidence of material processing differences for teeth in different sections of the jaw (Bestwick et al, 2021). Instead, the differences between the posterior (type B) and the anterior + middle teeth (types C, I, U) are hypothesized to be the result of differential forces during food processing, potentially by side-to-side shaking of the head or the "death roll" of extant crocodylians (Bestwick et al, 2021), though those authors also note that these ideas have not been explicitly tested in phytosaurs. A simpler explanation is that the Type B teeth experience greater pressure, generating greater bite force, than more anterior teeth since jaws are Type 3 levers with Type B teeth closer to the fulcrum.…”

“…This would parallel the role of carnassial teeth in carnivoran mammals, another heterodont predatory clade with blade-shaped teeth in the back of the jaw, although in carnivorans the carnassial teeth function with tooth-tooth occlusion, not evidenced in phytosaurs (Greaves, 1983). However, recent tooth enamel microwear analysis of phytosaurs did not find evidence of material processing differences for teeth in different sections of the jaw (Bestwick et al, 2021). Instead, the differences between the posterior (type B) and the anterior + middle teeth (types C, I, U) are hypothesized to be the result of differential forces during food processing, potentially by side-to-side shaking of the head or the "death roll" of extant crocodylians (Bestwick et al, 2021), though those authors also note that these ideas have not been explicitly tested in phytosaurs.…”

“…Phytosaurs are often reconstructed as semiaquatic predators with variation in dietary specialization based upon overall skull morphology (Hunt, 1989) and heterodont dentition (Hungerbühler, 2000;Datta et al, 2020;Bestwick et al, 2021). It was not until recently that these hypotheses have been explicitly tested.…”

“…These heterodont teeth are proposed to be used for a carnivorous diet, and with a primary function of, respectively: grabbing, piercing, immobilizing, and slicing (Hungerbühler, 2000). Although recent phytosaur enamel microwear work did not find support for different teeth or taxa specializing in distinct food processing strategies, there were textural differences between the different tooth positions along the jaw, indicating differential pressure loading (Bestwick et al, 2021).…”