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.

Not deconstructing serial homology, but instead, the a priori assumption that it generally involves ancestral anatomical similarity: An answer to Kuznetsov's paper

“…This is because the proximal muscles of the upper limb and the proximal muscles of the lower limb have evolved independently: fishes do not have them, so to assume a priori that they have to be similar, makes no sense at all. They were never similar-as shown in our detailed recent muscle reconstructions of early tetrapod fossils (Molnar, Diogo, Hutchinson, & Pierce, 2018, 2020, and continue to not be similar in living tetrapods, as shown by a huge number of comparisons between the upper and lower limbs of numerous tetrapods, including detailed developmental works and using other state-of-the-art techniques such as anatomical network analysis (e.g., Diogo & Molnar, 2014 Sears, Capellini, & Diogo, 2015;Siomava, Shkil, Voronezhskaya, & Diogo, 2018;Ziermann, Freitas, & Diogo, 2017;Ziermann, Miyashita, & Diogo, 2014). Only the muscles related to the distal region, particularly the autopods, are similar, because of the secondary convergence-due to "gene piracy", as we explain-between the evolution of the tetrapod hand and foot.…”

“…Moreover, paleontological data shows that the pectoral appendage arose in time before the pelvic one, not the other way round. In fact, in a recent paper written about at the same time in which we wrote ours, and at the same time that wrote a paper specifically about the upper and lower limbs (Diogo, 2020), Sleight and Gillis (2020) showed how developmental data does support instead the idea that the pectoral girdle comes very likely from the head, exactly as I defend in that upperlower limb paper. A similar idea had been also defended in a paper by Nagashima et al, 2016, which had provided very strong developmental data to support it, which my colleagues and me had put forward three years before (Diogo et al, 2013).…”

“…This is very different from a historical definition of serial homology, and Kuznetsov does not only fails to explain why he used a developmental definition but also to understand—seemingly—that we are not following such a definition. As we explain in our paper, using a historical definition of homology, there is no serial homology of the pectoral and pelvic appendages as a whole, and thus also of the upper and lower limbs of tetrapods as a whole, as shown by the empirical data compiled in a recent paper (Diogo, 2020; see below).…”

Evolution, Homology, and Development of Tetrapod Limb Muscles

“…Recently, Diogo [87] reviewed the question of the origin of the paired appendages in face of the huge amount of gross anatomical and developmental data about the fore and hindlimb muscles accumulated over the past quarter century. Two main theories have been used to explain the origin of pectoral and pelvic appendages: the "fin-fold" theory and the "gill-arch" theory.…”

Imaging With the Past: Revealing the Complexity of Chimaeroid Pelvic Musculature Anatomy and Development

“…In recent years, comparative anatomy, specifically examinations of the musculature, has been used, by a vocal minority, to challenge the long held view that pectoral and pelvic fins are serial homologs (Diogo and Molnar, 2014;Diogo and Ziermann, 2015;Ziermann et al, 2017;Diogo, 2020;Siomava et al, 2020). Among these studies, only a single paper (Ziermann et al, 2017) has looked at fin muscle development despite its essential role in addressing questions such as the validity of serial homology (Ziermann et al, 2017).…”