Pectoral fins of rhizodontids and the evolution of pectoral appendages in the tetrapod stem-group

Abstract

The Rhizodontida are an extinct order of large, predatory, lobe-finned fishes, found world-wide in Devonian and Carboniferous freshwater deposits. They are thought to be basal members of the tetrapod lineage. In this paper the pectoral fin skeleton of Rhizodus hibberti, a derived member of the group, is described in detail for the first time. It shows that muscular processes of the humerus (the deltoid and supinator processes) may have appeared later in tetrapod evolution than previously thought. The rhizodontids share an unusual fin-ray morphology, with highly elongated basal hemisegments, overlapping extensively with the endoskeleton. This morphology raises the possibility that segmentation and endoskeletal overlap share some common upstream genetic control during lepidotrichial development. The relative size of the lepidotrichia and the complexity of the endoskeleton does not fit with a 'clockface' model of limb developmental evolution.