Cranial or postcranial-Dual origin of the pectoral appendage of vertebrates combining the fin-fold and gill-arch theories?

Abstract

Two main theories have been used to explain the origin of pectoral and pelvic appendages. The "fin-fold theory" proposes that they evolved from a trunk bilateral fin fold, while Gegenbaur's theory assumes they derived from the head branchial arches. However, none of these theories has been fully supported. The "fin-fold" theory is mainly often accepted due to some existing developmental data, but recent developmental studies have revived Gegenbaur's theory by revealing common mechanisms underlying the patterning of branchial arches and paired appendages. Here I review developmental data and many others lines of evidence, which lead to a crucial question: might the apparent contradictions between the two theories be explained by a dual origin of the pectoral appendage, that is, the pectoral girdle and fin/limb being mainly related to the head and trunk, respectively? If this is so then (a) the pectoral and pelvic girdles would not be serial homologues; (b) the term "developmental serial homologues" could only potentially be applied to the pectoral and pelvic fins/limbs. Fascinatingly, in a way this would be similar to what Owen had already suggested, more than 170 years ago: that the pectoral and pelvic girdles are mainly related to the head and trunk, respectively.