Coates, M.I.: Fins to Limbs: What the fossils say

The fin to limb transition is one of the pivotal morphological transformations in vertebrate history. Fossil outgroups of tetrapods bear fins with a simple iterative pattern oriented proximo-distally; several features of these fins are also present in extant lungfish. Certain examples of fossil fins long considered close to tetrapod limbs, have now been removed to more remote phylogenetic positions. Their apparent similarities thus appear convergent. The most derived ‘fish’ (i.e. pre-limbed) members of the tetrapod stem-lineage (panderichthyids) show modest enlargement of the fin-girdles. This coincides with reduction of endoskeletal outgrowth, the presence of an expanded plate adjacent to the probable homologue of the intermedium, and changes in the proportions of proximal fin radials to more closely resemble the humerus, radius and ulna of primitive, digited, limbs. The earliest limbs date from the uppermost Devonian (360 mya). Of these, the forelimb of Acanthostega is arguably the most primitive. Like other Devonian examples it is polydactylous, but, uniquely, the wrist includes only a single, cylindrical, bone, quite unlike cross-articulating wrist and ankle structures of more recent limbs. All known primitive hindlimbs include a well ossified ankle, but in the earliest examples this includes fewer and larger elements than in more recent, derived, limbs. Functionally, these simple ankles are consistent with a likely paddle-like function. The most recent phylogenetic analyses indicate that pentadactyly evolved once, at a node within the tetrapod stem-lineage preceding the divergence of amniotes and lissamphibians. By this point in tetrapod phylogeny, it is also noticeable that markers of muscle insertion areas have moved proximally, corresponding to greater development of the endoskeletal girdles.