Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning

Significance The fossil fish Rhizodus hibberti , a member of the tetrapod stem group, shows a unique skeletal pattern in the pelvic fin. Rather than the highly conserved one-to-two pattern of a femur, tibia, and fibula (seen in all known tetrapods, including the extinct, fishlike members of the group), the fin of Rhizodus comprises a femur articulating distally with three bones, each with a distinct morphology. This reveals an early stage in the evolution of limb development, in which the processes patterning the proximal parts of the embryonic fin/limb (the stylopod and zeugopod) were not constrained in the way seen in living tetrapods and could produce more varied skeletal patterns in the adult.