Integrating palaeontological and neontological perspectives to unravel the secrets of a third of vertebrate diversity

There are over 18,500 living species of acanthomorph (spiny-rayed fishes), and they represent half of all living fish species and a third of extant vertebrates. They inhabit countless ecological niches, they are incredibly diverse, and they have developed a number of specialised morphological innovations. The earliest acanthomorphs are known from the Cenomanian, but they were neither diverse not abundant during the Cretaceous period. Following the end-Cretaceous extinction event, the acanthomorphs radiated, and by the Eocene they were anatomically modern. However, our understanding of the relationships between most fossil and living acanthomorphs, and the macroevolutionary processes behind their rise to dominance in the Recent, are poorly known. In this thesis, I aimed to develop our understanding of the evolution of the acanthomorphs by combining palaeontological and neontological data. I undertook three separate projects to accomplish this. First, I studied the anatomy two fossil representatives (†Aipichthys pretiosus and †'Aipichthys' nuchalis) of a primitive acanthomorph lineage, the lampridomorphs. I then placed these taxa in a phylogenetic analysis to reveal the pattern of character evolution leading to the crown-group. Second, I aimed to understand the morpho- logical condition at the base of the acanthomorph tree by studying the relationships between two extinct groups (†Ctenothrissiformes and †Pattersonichthyiformes) that may have branched from the acanthomorph stem. Phylogenetic analyses utilising morphological and molecular data support a stem ctenosquamate placement for these taxa. Third, using a timescaled supertree, I studied the evolution of one of the major acanthomorph innovations, the protrusile upper jaw. The combination of palaeontological and neontological data provided deep insight into the evolution of the acanthomorphs, and allowed for the identification of patterns and processes that could not be identified from studying extant taxa in isolation.