Watt, Emily C; (2025) Morphological evolution of the tetrapod lower jaw. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Among the myriad adaptations necessary for transitioning from water-based to landbased living, the tetrapod jaw successively modified to cope with feeding out of the water and living on land. Through the tetrapod clade, encompassing amphibians, reptiles and birds, and mammals, and their ancestors back to the water-to-land transition around 390Ma, the lower jaw has been modified in structure, shape, and function, and has contributed to the expansion and proliferation of tetrapods into new ecological niches. Jaw adaptations have allowed tetrapods to specialise in feeding ecology, through loss of jaw elements, reduction in muscle mass, alterations to the jaw joint, and changes in muscle structure. Considerable scholarship has focussed on the lower jaw, particularly in terms of function, at key transitional moments in tetrapod evolution, including the water-toland transition in early tetrapods, and the origination of the mammalian middle ear in mammaliamorph synapsids. Outside of these pivotal events, research typically focuses on morphology and function between closely related organisms (such as crocodilians), rather than presenting broad-scale patterns across classes or clades. This thesis aims to draw both strands of scholarship together, providing a comprehensive analysis of structural and morphological evolution across the entire tetrapod clade. Not only does it consider moments of pivotal change in tetrapod evolution, but it also examines macroevolutionary patterns of lower jaw evolution across the whole clade and within and between tetrapod subclades. This research provides the first macroevolutionary analysis of tetrapod lower jaw morphology and structure, using a vast newly compiled character dataet, and a new high-resolution 3D dataset, each spanning near family-level sampling of both fossil and extant tetrapod species. Using a phylogenetic framework alongside these vast morphological datasets, I quantify patterns and rates of structural and morphological variation of the lower jaw within and between subclades and through major transitions. Overall, this thesis finds pervasive simplification of the lower jaw across Tetrapoda and within major subclades. This simplification is present in both the lower jaw structure and the morphology; however, the two factors are not evolving in tandem. Considerable scholarship has focussed on the lower jaw, particularly in terms of function, at key transitional moments in tetrapod evolution, including the water-to-land transition in early tetrapods, and the origination of the mammalian middle ear in mammaliamorph synapsids. Outside of these pivotal events, research typically focuses on morphology and function between closely related organisms (such as crocodilians), rather than presenting broad-scale patterns across classes or clades. This thesis aims to draw both strands of scholarship together, providing a comprehensive analysis of structural and morphological evolution across the entire tetrapod clade. Not only does it consider moments of pivotal change in tetrapod evolution, but it also examines macroevolutionary patterns of lower jaw evolution across the whole clade and within and between tetrapod subclades. This research provides the first macroevolutionary analysis of tetrapod lower jaw morphology and structure, using a vast newly compiled character dataet, and a new high-resolution 3D dataset, each spanning near family-level sampling of both fossil and extant tetrapod species. Using a phylogenetic framework alongside these vast morphological datasets, I quantify patterns and rates of structural and morphological variation of the lower jaw within and between subclades and through major transitions. Overall, this thesis finds pervasive simplification of the lower jaw across Tetrapoda and within major subclades. This simplification is present in both the lower jaw structure and the morphology; however, the two factors are not evolving in tandem. The most variable extant clade in terms of lower jaw composition is Caudata, the newts and salamanders, likely driven by their complex life histories constraining the degree of structural simplification. By contrast, the most variable extant clade in terms of lower jaw morphology is Mammalia, hypothesised to be demonstrative of the inherent limitations of significant structural simplification, where variation is forced to be within the morphology rather than in both structure and shape. This research contributes novel fine-scaled analyses of the patterns of structural and morphological evolution of the lower jaw across and between tetrapod subclades. It collates two novel datasets, one discrete character-based, focussed on the presence and absence of lower jaw components, and the other a collection of 3D scans of lower jaws spanning Tetrapoda. This thesis also provides the first review of lower jaw composition spanning the entire clade, and highlights gaps in our knowledge of lower jaw structure, especially in extant anurans and birds. In so doing, this research provides new insight into the overwhelming simplification of the lower jaw occurring across all major tetrapod subclades. The most variable extant clade in terms of lower jaw composition is Caudata, the newts and salamanders, likely driven by their complex life histories constraining the degree of structural simplification. By contrast, the most variable extant clade in terms of lower jaw morphology is Mammalia, hypothesised to be demonstrative of the inherent limitations of significant structural simplification, where variation is forced to be within the morphology rather than in both structure and shape. This research contributes novel fine-scaled analyses of the patterns of structural and morphological evolution of the lower jaw across and between tetrapod subclades. It collates two novel datasets, one discrete character-based, focussed on the presence and absence of lower jaw components, and the other a collection of 3D scans of lower jaws spanning Tetrapoda. This thesis also provides the first review of lower jaw composition spanning the entire clade, and highlights gaps in our knowledge of lower jaw structure, especially in extant anurans and birds. In so doing, this research provides new insight into the overwhelming simplification of the lower jaw occurring across all major tetrapod subclades.

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