Bardua, Carla Michelle; (2019) Morphological evolution and modularity of the amphibian skull. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Lissamphibia, the only extant, non-amniote tetrapod clade, are morphologically incredibly diverse. However, to date, studies of morphological evolution, phenotypic integration (covariation) and modularity (the division of a structure into sets of integrated traits) have concentrated overwhelmingly on amniotes. In this thesis I quantified cranial morphological variation across two lissamphibian clades, with representative specimens from every extant genus (caecilians) and family (frogs). Shape was captured in detail, using a high-dimensional surface-based geometric morphometric approach, to test alternative models of cranial organisation and reconstruct cranial evolution across caecilians and frogs. I found both frog and caecilian crania are highly modular, and the pattern of cranial integration is strongly conserved across the clades. Of particular interest is the highly integrated, fast-evolving jaw suspensorium region of both frogs and caecilians, suggesting feeding mechanics may be driving cranial evolution in these clades. In addition, ecology exerts a stronger influence on morphology than developmental strategy for both clades. Fossorial, semi-fossorial, and aquatic species are the most disparate and fastest-evolving among frogs, while aquatic caecilian species are the fastest-evolving for that clade. Ossification sequence timing significantly influences integration, evolutionary rate, and disparity across frogs, and there is no simple relationship between integration and evolutionary rate or disparity. Finally, to extend the study of morphological evolution into deep time, I investigated influences on cranial morphology for fossil and extant frogs from the Early Cretaceous to the Recent. Given the extremely dorso-ventrally compressed nature of fossil frogs, I collected two-dimensional cranial outline data for 42 fossil and 93 extant frogs. Phylogeny exerts the strongest influence on cranial morphology, with allometry and developmental strategy acting as only weak influences on cranial outlines. This thesis represents a significant advance in the study of cranial modularity and morphological evolution across frogs and caecilians, unrivalled in shape description, in taxonomic sampling, and in the in-depth exploration of hypotheses of modularity and macroevolutionary patterns.

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