Meeting Abstract
Paleontological studies of the fin-to-limb transition have focused almost exclusively on endoskeletal evolution. However, comparative analyses of dermal fin rays can also inform the behavior, ecology, and developmental evolution of tetrapodomorph fishes. Here we use computed tomography to describe the dermal rays of the pectoral fins of three tetrapodomorph species: Sauripterus taylori, Eusthenopteron foordi, and Tiktaalik roseae. We find that in the lineage leading to crown group tetrapods, fin rays were simplified (segmentation and branching was lost), the fin web was reduced in size, and asymmetry evolved between dorsal and ventral hemitrichia. In tetrapodomorph pectoral fins, dorsal hemitrichia generally cover the endoskeleton to a greater degree than the ventral hemitrichia, indicative of dorsoventral asymmetries in the distribution of fin musculature. Notably, in Tiktaalik, dorsal hemitrichia cover the third and fourth mesomeres, while ventral hemitrichia are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip, analogous to a fleshy palm. Additionally, in tetrapodomorph pectoral fins, dorsal and ventral hemitrichia differ in cross sectional area. Eusthenopteron dorsal hemitrichia are slightly larger than ventral hemitrichia, and the magnitude of difference is consistent between individuals of different sizes; while Tiktaalik dorsal hemitrichia are several times larger than ventral hemitrichia, and magnitude of asymmetry is greater in larger individuals. This indicates a transition from isometric to allometric scaling between the dorsal and ventral hemitrichia in elpistostegids. We argue that dermal fin ray evolution in tetrapodomorphs shows convergence with benthic actinopterygians and adaptation to substrate-based loading and prior to the origin of digits.
