The Evolution of Paired Fins

Abstract

The Archipterygium is Gegenbaur’s most lasting contribution to the study of vertebrate limb evolution. This transformational hypothesis of gill arches to limb girdles, rays to fins, and proposal of a vertebrate fin-limb groundplan, is generally treated as a flawed alternative to the more widely accepted lateral fin-fold hypothesis of vertebrate limb evolution. When compared to the phylogenetic distribution and diversity of fins and limbs, both hypotheses fail. Dermal skeletal lateral folds, spines and keels originate repeatedly in vertebrate evolution, but paired fins with girdles originate at pectoral level and are anteroposteriorly restricted. Pelvic fins emerge later in phylogeny; pectoral and pelvic appendages primitively differ. Endoskeletal girdles never exhibit characteristics of gill arches. The emergent sequence of paired fin evolution depends upon phylogenetic hypotheses within which extant agnathan interrelationships are uncertain; positions of jawless fossil fish along the gnathostome stem are insecure; the fossil data set is patchy. However, certain features of the data set are robust. This has prompted a reconsideration of Gegenbaur’s hypothesized arch-girdle relationship, and an iterative homology between scapulocoracoid and extrabranchial cartilages is suggested. No transformation of arch to girdle is necessarily implied, but some signal of developmental relatedness is predicted.