Morphological and functional novelty within otomorphan fishes
Open Access Depositednot all instances of high diversity (i.e., taxonomic, ecological, or morphological diversity) are preceded by discrete, identifiable novelties, and not all novel traits lead to increased diversification. To better understand patterns of biodiversity and the limits of form and function, specifically within the teleost skull, it is important to investigate the evolutionary consequences of novelty across lineages of varying evolutionary diversity. This dissertation investigates morphological and functional novelty within Otomorpha, a large clade of bony fishes, with particular interest given to trophic innovations for microphagous feeding. Focusing on Gonorynchiformes, a species-depauperate yet morphologically diverse order within Otomorpha, this work characterizes cranioskeletal diversity with an emphasis on novel feeding mechanisms and structures. For decades, the phylogenetic relationship among gonorynchiforms has been highly debated, and the absence of a robust phylogenetic framework has hindered comparative studies. In chapter 1, I conduct a phylogenomic analysis of Gonorynchiformes, providing the most taxonomically inclusive and highest resolution phylogenetic hypothesis to date. I further provide the first quantitative analysis of three-dimensional skull shape disparity within Gonorynchiformes and compare the gonorynchiform morphospace with that of preliminary sampling of other ostariophysan fishes. I find that the gonorynchiforms occupy a broad morphospace including novel regions of ostariophysan trait-space. In chapter 2, I focus on the most bizarre of gonorynchiforms, Phractolaemus ansorgii, whose unique craniofacial anatomy renders it the only member of its order capable of feeding via premaxillary protrusion. Here, I use a suite of anatomical methods to characterize the feeding mechanism employed by P. ansorgii and present a new functional hypothesis for premaxillary protrusion based on a revision of the musculoskeletal anatomy. Using high-speed videography, I measure two performance metrics, speed and distance of the protrusion, the results of which reveal this to be a previously undescribed form of extreme jaw protrusion (distance of protrusion >30% of head length). In chapter 3, I investigate the epibranchial organ, a trophic structure associated with microphagy that has evolved independently within four orders of Otomorpha. Previous descriptions of epibranchial organs were made with varying degrees of anatomical resolution. Furthermore, early assessments of their evolutionary history (i.e., whether they are convergent or homologous structures within Otomorpha and teleosts more broadly) relied upon phylogenies we now know to be inaccurate. Here, I characterize the anatomical variation and histological structure of the epibranchial organ, identifying common functional components at the gross and microscopic level. With these findings, I established a new system for categorizing the anatomical diversity of epibranchial organs and assess the relationship between epibranchial organ morphology, phylogeny, and diet.
The evolution of novel traits has long been hypothesized to open previously inaccessible regions of the adaptive landscape, permitting new functional interactions between organisms and their environment and increasing the potential for diversification. Although novelty and diversification are conceptually linked, their relationship is not always straightforward
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