Evolution of modular morphology in actinopterygian pectoral fins

With a generally conserved function for locomotion, paired fins have an extensive evolutionary history that has resulted in highly disparate morphologies in a wide diversity of taxa. Two-dimensional geometric morphometrics offers a means to quantitatively describe disparity and changes in the shape of pectoral fins. Actinopterygians lend themselves well to a study of this nature due to their prolific diversity and prevalence in the fossil record. As a result of changes in skeletal fin structures and the lack of homologous elements throughout the group, three morphological modules are used as the unit of comparison rather than individual elements of the fin. Ordination plots from thin plate spline analyses, including fossil and extant taxa, show correspondence with the actinopterygian phylogeny as trends become apparent through the group. Basal actinopterygians and basal teleosts tend to have a narrow, elongate fin shape. Following a shift in morphospace, derived teleosts have a wider fin base, with an elongate anterior margin of the fin relative to the posterior trailing edge. Linear and squared-change parsimony reconstructions of continuous data allow the phylogeny to be traced through morphospace to approximate the path of pectoral fin evolution. The shift in morphospace occurs at Acanthomorpha, and is correlated with the evolution of a physoclistous swim bladder and a change in the position and orientation of pectoral fins. As a result of these changes, new swimming modes are made available, and the expansion of locomotory modes of Acanthomorpha is here hypothesized to be associated with a sharp increase in pectoral fin disparity