Relative skull size evolution in Mesozoic archosauromorphs: potential drivers and morphological uniqueness of erythrosuchid archosauriforms

Abstract

Little is known about the large‐scale evolutionary patterns of skull size relative to body size, and the possible drivers behind these patterns, in Archosauromorpha. For example, the large skulls of erythrosuchids, a group of non‐archosaurian archosauromorphs from the Early and Middle Triassic, and of theropod dinosaurs are regarded as convergent adaptations for hypercarnivory. However, few investigations have explicitly tested whether erythrosuchid and theropod skulls are indeed disproportionately large for their body size, and whether this trend is driven by hypercarnivory. Here, we investigate archosauromorph relative skull size evolution, examining the scaling relationships between skull and body size of Palaeozoic and Mesozoic archosauromorphs using a robust phylogenetic framework and assessing the influence of potential drivers, such as taxonomy, diet, locomotory mode and inhabited biotope. Our results show that archosauromorph relative skull sizes are largely determined by phylogeny and that the other drivers have much weaker levels of influence. We find negative allometric scaling of skull size with respect to body size when all studied archosauromorphs are analysed. Within specific groups, skull size scales with positive allometry in non‐archosaurian archosauromorphs and, interestingly, scales isometrically in theropods. Ancestral reconstructions of skull–femur size ratio reveal a disproportionately large skull at the base of Erythrosuchidae and proportionately sized skulls at the bases of Theropoda, Carnosauria and Tyrannosauroidea. Relative skull sizes of erythrosuchids and theropods are therefore distinct from each other, indicating that disproportionately large skulls are not a prerequisite for hypercarnivory in archosauromorphs, and that erythrosuchids exhibit a bauplan unique among terrestrial Mesozoic carnivores.

Giant, swimming mouths: oral dimensions of extant sharks do not accurately predict body size in Dunkleosteus terrelli (Placodermi: Arthrodira)

“…It is rather unusual that in both of these cases the earlier-diversifying clade is characterized by disproportionately large heads or mouths (arthrodires and hyaenodonts), whereas their later ecological analogues (neoselachian sharks and carnivorans) have smaller mouths and heads. This phenomenon may not be restricted to fishes and mammals: erythrosuchids, which represent some of the earliest archosaurian experiments in macrocarnivory, also have disproportionately large heads compared to later predatory archosaurs such as dinosaurs and “rauisuchians” ( Bestwick et al, 2022 ). Large heads and mouths characterize a number of (mostly extinct) vertebrate clades, but their functional or evolutionary significance is unclear ( Bestwick et al, 2022 ).…”

“…This phenomenon may not be restricted to fishes and mammals: erythrosuchids, which represent some of the earliest archosaurian experiments in macrocarnivory, also have disproportionately large heads compared to later predatory archosaurs such as dinosaurs and “rauisuchians” ( Bestwick et al, 2022 ). Large heads and mouths characterize a number of (mostly extinct) vertebrate clades, but their functional or evolutionary significance is unclear ( Bestwick et al, 2022 ). It is possible these traits are a side-effect of early burst diversification.…”

Evaluating growth in Macrospondylus bollensis (Crocodylomorpha, Teleosauroidea) in the Toarcian Posidonia Shale, Germany

“…Bestwick et al . 2022): juveniles are represented by a total skull length up to 250 mm, subadults are characterized by a total skull length of between 250 and 450 mm, and adults are represented by a total skull length of more than 450 mm. All specimens were photographed with a Canon EOS 1300D and initial measurements were taken in millimetres (mm) using a fibreglass tape measure; measurements were then doublechecked in ImageJ (Schneider et al .…”

“…2021; Bestwick et al . 2022). For several extant taxa, multiple specimens were available; their values were respectively averaged.…”

“…Femur length has been used in other studies as a proxy for body length in the literature (Bestwick et al . 2022), therefore comparison with skull length will highlight evolutionary changes in skull length. To phylogenetically inform the regressions taking phylogenetic uncertainty into account, we used a series of time trees following the approach of Stubbs et al .…”