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Abstract Actinopterygii is a major extant vertebrate group, but limited data are available for its earliest members. Here we investigate the morphology of Devonian actinopterygians, focusing on the lower jaw. We use X‐ray computed tomography (XCT) to provide comprehensive descriptions of the mandibles of 19 species, which span the whole of the Devonian and represent roughly two‐thirds of all taxa known from more than isolated or fragmentary material. Our findings corroborate previous reports in part but reveal considerable new anatomical data and represent the first detailed description for roughly half of these taxa. The mandibles display substantial variation in size, spanning more than an order of magnitude. Although most conform to a generalized pattern of a large dentary and one or two smaller infradentaries, XCT data reveal significant differences in the structure of the jaw and arrangement of teeth that may be of functional relevance. We report the presence of a rudimentary coronoid process in several taxa, contributed to by the dentary and/or infradentaries, as well a raised articular region, resulting in a mandible with an offset bite and that functions as a bent level arm. Among the most striking variation is that of tooth morphology: several taxa have heterodont dentary teeth that vary in size and orientation, and multiple variations on enlarged, whorl‐like and posteriorly‐oriented anterior coronoid dentition are observed. We use these new data to revise morphological characters that may be of phylogenetic significance and consider the possible functional implicationds of these traits. The observed variation in mandible form and structure suggests previously unappreciated functional diversity among otherwise morphologically homogenous Devonian ray‐finned fishes. 

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.