Abstract
Cranial morphology is remarkably varied in living amniotes and the diversity of shapes is thought to correspond with feeding ecology, a relationship repeatedly demonstrated at smaller phylogenetic scales, but one that remains untested across amniote phylogeny. Using a combination of morphometric methods, we investigate the links between phylogenetic relationships, diet and skull shape in an expansive dataset of extant toothed amniotes: mammals, lepidosaurs and crocodylians. We find that both phylogeny and dietary ecology have statistically significant effects on cranial shape. The three major clades largely partition morphospace with limited overlap. Dietary generalists often occupy clade-specific central regions of morphospace. Some parallel changes in cranial shape occur in clades with distinct evolutionary histories but similar diets. However, members of a given clade often present distinct cranial shape solutions for a given diet, and the vast majority of species retain the unique aspects of their ancestral skull plan, underscoring the limits of morphological convergence due to ecology in amniotes. These data demonstrate that certain cranial shapes may provide functional advantages suited to particular dietary ecologies, but accounting for both phylogenetic history and ecology can provide a more nuanced approach to inferring the ecology and functional morphology of cryptic or extinct amniotes.
Highlights
Cranial morphology is remarkably varied in living amniotes and the diversity of shapes is thought to correspond with feeding ecology, a relationship repeatedly demonstrated at smaller phylogenetic scales, but one that remains untested across amniote phylogeny
To what extent do these similarities result in morphospace overlap across a large taxonomic sample? Morphology is subject to a combination of selective, phylogenetic and developmental constraints, so when we claim that organisms have converged, how close is the correspondence and what role do factors such as phylogenetic history play in constraining the degree to which organisms can resemble each other? What are the limits of convergence and can similarities in shape alone be used to generate hypotheses about the dietary ecology of distantly related organisms?
The few studies that have compared skull shape across disparate taxonomic groups suggest that dietary ecology and feeding behaviour play a consistent role in determining cranial morphology
Summary
Cranial morphology is remarkably varied in living amniotes and the diversity of shapes is thought to correspond with feeding ecology, a relationship repeatedly demonstrated at smaller phylogenetic scales, but one that remains untested across amniote phylogeny. Skull shape disparity relates to a myriad of factors, including common ancestry ( phylogenetic history), developmental constraint, size, feeding ecology, locomotion and stochastic processes (e.g. genetic drift) [1,16,17,18,19,20,21,22,23] Studies examining these associations, frequently have a narrow phylogenetic focus, with few investigations comparing taxa across major tetrapod clades [2,9,13,24,25,26]. These repeated patterns of morphological convergence among distantly related animals may shed light on the ecology of extinct organisms, especially those that lack living relatives or possess skull shapes dissimilar to extant amniotes
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