Ecological Influences on the Evolution of Skull Morphology in Triassic Archosaurs

Archosaurs are one of the most successfully diverse groups of vertebrates widely studied in palaeobiology. Previous studies have previously investigated the skull morphology within Archosauria, with many conclusions around how they evolved. This study investigates the evolution of skull morphology within Archosauria, with a focus on the orbit and external naris fenestrae. By examining 152 species of archosauromorphs from the Triassic Period, this research investigates how ecological traits of diet, locomotion, and biotope influence skull morphology with the use of phylogenetic comparative methods and morphometric analyses.Ancestral state reconstructions indicate that the orbit and external naris sizes when compared to skull size evolved through different selective pressures, suggesting stabilizing selection. However, when comparing the eye orbit to the external naris sizes, the best fitting model was the Early Burst model, which reflects a rapid initial diversification.There is a positive allometric relationship between skull size and both the orbit and external naris sizes. There was little difference between the orbit size relative to skull size for the diet types. The biotope had the most influence on the orbit size, reflecting the relationship between visual acuity and the habitat that a species occupies. Aerial species exhibited the largest external naris size, likely reflecting the adaptation of weight loss in the skull for flight. Herbivores and omnivores had the largest external naris, suggesting a likely increase in olfactory and respiratory adaptations. Locomotion had the least influence on the orbit and external naris sizes, with there being little difference between the traits.Comparisons with extant archosaurs and work on previous studies of extinct archosaurs reinforce these results, showing that skull morphology underwent distinctive evolutionary pressures, forging the diversity of archosaurs. This research enhances the understanding of adaptive radiation, specifically in Triassic archosaurs, further demonstrating the influences of ecological factors on skull morphology. This study contributes to a broader understanding of vertebrate evolution by demonstrating how morphological traits can evolve in response to ecological pressures.