Ontogeny within a convergent evolutionary context

Abstract

Morphological innovations and the evolution of these key features have long been primary targets of paleobiological investigations; yet not all morphologies are entirely novel, and focus on convergent morphologies (e.g., longirostry, limblessness, wings) provide a unique opportunity to investigate the micro- and macroevolutionary processes that drive disparity and diversification. Convergent evolution currently is hypothesized to occur when distantly related clades occupy similar ecological niches, resulting in lineages evolving similar morphologies; therefore, studies of convergence can also be used to investigate the relationship between environment, form, and function. Even though there are multiple studies to assess the extent of convergence between similar morphologies, investigations on how convergent morphologies develop (i.e., through growth) are less common and may illuminate convergent evolutionary processes. Afterall, an organism only exhibits the anatomies and morphologies that it does because of the ontogenetic (= growth) processes that form them. The fauna of the Triassic Period (~252–201.5 Ma) were shown to have evolved body plans that were converged upon by crocodylians and dinosaurs after the Triassic-Jurassic Mass Extinction (~201.5 Ma) and represent an opportunity to investigate the ontogenetic processes that form convergent morphologies, especially in organisms that are still alive today (i.e., crocodylians). The Middle and Late Triassic phytosaurs are a cosmopolitan clade of large, longirostrine, semi-aquatic archosaur-relatives whose overall morphologies were later converged upon by crocodylians, but the processes that shaped that convergence remain understudied. My PhD dissertation aims to conduct a detailed qualitative and quantitative study of phytosaur ontogeny and addresses the following research questions: (1) How do the crania and postcrania change through ontogeny in a single phytosaur genus, 'Redondasaurus'? (2) How do traits related to rostral morphologies (i.e., dental alveolus dimensions) change throughout growth across phytosaurs, and how do those features compare to those of their convergent counterparts, crocodylians? (3) How do the bone microstructural properties of an inferred hatchling phytosaur compare to those from other earlier diverging archosaur-relatives and their convergent counterparts, crocodylians? Ultimately, these questions will add valuable insight into the growth dynamics at the base of the archosaur tree, new or unexpected growth trajectories that have yet to be considered in the clade, and whether convergent growth trajectories are tied to convergent morphologies in distantly related clades. My first chapter aims to provide the qualitative foundation for understanding phytosaur cranial ontogeny in the only known size series across all of Phytosauria: 'Redondasaurus' bermani from the Coelophysis Quarry in New Mexico. This ontogenetic series reveals that the youngest ontogenetic stages of 'Redondasaurus' possess morphologies more commonly associated with earlier diverging phytosaur taxa, suggesting that some heterochronic processes have influenced the evolution of phytosaur crania. My second chapter quantitatively tests the qualitative observations from Chapter 1 and integrates postcranial data via osteohistological analyses by producing an ontogram (=cladistic analysis of ontogeny) that shows that the two known species of 'Redondasaurus' could be synonymized because all morphological differences between the species are ontogenetically variable characters. In my third chapter, I test the ontogenetic (i.e., allometric) trajectories of dental alveoli between phytosaurs and extant crocodylians to determine whether the relationships between rostral shapes and dental characteristics are shared between these two clades, given their hypothesized shared ecological and dietary niches. This analysis shows that ontogenetic trajectories are variable between these two groups and that crocodylian and phytosaur rostra and dental features were subjected to variable ontogenetic influences. Lastly, my fourth chapter describes the osteohistological properties in the smallest known phytosaur femur within an evolutionary context and reveals that phytosaurs do not exhibit the fast-to-slow growth signal typically observed across Archosauria and instead, appear to exhibit size-dependent growth. Phytosaurs have long been considered "crocodile-like"; however, my dissertation reveals nuanced differences in the development and evolution of these two convergent clades suggesting that their respective evolutionary processes are unique despite forming convergent morphologies.