An Analysis of the Macroevolutionary Dynamics of Diet in Amniotes

Abstract

This doctoral thesis explores the macroevolution of diet in extant amniote lineages and two extinct lineages: early-branching synapsids and non-avian theropod dinosaurs. I collate relative dietary percentages for species- and genus-level taxa in four major amniote lineages (turtles, lizards, birds, and mammals). By applying a rigorous, transparent, and repeatable analytical framework and considering both extant and fossil species, this research enhances our understanding of the intricate interplay between diet, body size, and evolutionary history. In Chapter One, I employ principal components analysis and k-means cluster analyses on percentile data of different food sources in the dietary compositions of extant amniote taxa to delineate dietary guilds without relying on arbitrary diet classification thresholds. The key findings of this chapter are: (a) invertebrate material is the most widespread food source among different dietary guilds, and (b) the majority of species in all major amniote lineages include some animal-based food in their dietary composition, except mammals. In Chapter Two I use marginal maximum likelihood modelling to investigate the tempo and directionality of evolutionary transitions between dietary guilds in each major lineage of amniotes. The key findings of this chapter are: (a) invertivory, the ancestral condition of most amniote lineages, is the most evolutionary plastic feeding strategy, (b) evolutionary shifts between specialist herbivorous and faunivorous guilds follow multiple evolutionary pathways through a spectrum of generalist dietary strategies. Evolutionary reversions are frequent at all stages along these evolutionary pathways. In Chapter Three, I utilize linear modelling and analyses of variance to assess the correlation between diet and body size in each major amniote lineage in a phylogenetic comparative framework. The key findings of this chapter are: (a) neither individual food item percentages nor dietary guild memberships explain body size variance in aggregate among amniotes, however (b) invertivory and nectarivory are correlated with the lowest body sizes among amniotes. The synthesis of these results is a macroevolutionary pattern wherein evolutionary shifts occur out of ancestrally invertivorous lineages, through omnivorous and generalist feeding guilds, into specialised carnivorous and herbivorous descendants. Evolutionary shifts out of invertivory often conflate with increased body size.