PERSPECTIVE doi:10.1111/evo.12773 On the serial homology of the pectoral and pelvic girdles of tetrapods Karen E. Sears, 1,2,3 Terence D. Capellini, 4 and Rui Diogo 5 1 School of Integrative Biology, University of Illinois, Urbana, Illinois 61801 2 Institute for Genomic Biology, University of Illinois, Urbana, Illinois 61801 3 E-mail: kesears@life.illinois.edu 4 Human Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 5 Howard University College of Medicine, Washington, District of Columbia 20059 Received May 28, 2014 Accepted September 9, 2015 While fore- and hindlimbs are commonly assumed to be serially homologous, the serial homology of the pectoral and pelvic girdles is more ambiguous. We investigate the degree to which a common history, developmental program, and gene network are shared between the girdles relative to the rest of the appendicular skeleton. Paleontological data indicate that pectoral appendages arose millions of years before pelvic appendages. Recent embryological and genetic data suggest that the anatomical similarity between the fore- and hindlimbs arose through the sequential, derived deployment of similar developmental programs and gene networks, and is therefore not due to ancestral serial homology. Much less developmental work has however been published about the girdles. Here, we provide the first detailed review of the developmental programs and gene networks of the pectoral and pelvic girdles. Our review shows that, with respect to these programs and networks, there are fewer similarities between pelvic and pectoral girdles than there are between the limbs. The available data therefore support recent hypotheses that the anatomical similarities between the fore- and hindlimbs arose during the fin-to-limb transition through the derived co-option of similar developmental mechanisms, while the phylogenetically older pectoral and pelvic girdles have remained more distinct since their evolutionary origin. KEY WORDS: Development, embryology, forelimb, gene network, hindlimb, paleontology, pelvis, scapula. The tetrapod appendicular skeleton is comprised of the fore- and hindlimbs, and their respective pectoral and pelvic girdles that connect the limbs to the body (Fig. 1). Fossil evidence suggests that the origin of the pectoral girdle predates that of the pelvic girdle by 20 million years. Definitive pectoral girdle and fin structures are first observed in fossil osteostracans (jawless vertebrates closely related to gnathostomes, which are vertebrates with jaws), dating to the Early Silurian (430 million years ago) (Coates 2003; Janvier 1996; Johanson and Trinajstic 2014; Trinajstic et al. 2015). In contrast, pelvic girdle and fin structures do not appear until the rise of fossil gnathostomes in the early Devonian (413 mya) (Zhu et al. 2012, 2013) (Fig. 2). Interest- ingly, the first pelvic appendages (girdles + fins) were anatomi- cally markedly different from the first pectoral appendages, which were in general strongly anchored to dermal bones of the head or cephalic region (Diogo et al. 2013; Zhu et al. 2012, 2013; Diogo and Ziermann 2015). The fin-to-limb transition that occurred 40 million years later (e.g., Coates and Ruta 2007; Coates et al. 2008) is characterized by a similar pattern in the sense that the evolution of a more tetrapod-like pectoral girdle preceded the evo- lution of a more tetrapod-like pelvic girdle (Boisvert 2005; Clack 2009, 2012). The independence of pectoral and pelvic girdle evolution suggests that pectoral and pelvic girdle morphogenesis is regulated, to some degree, by distinct developmental controls. However, it is possible that a similar (or single) developmental module was independently recruited to generate the pectoral and pelvic girdles at different times in tetrapod history, as is com- monly assumed for the more distal elements of the appendicular skeleton (Ouimette et al. 2010; Rabinowitz and Vokes, 2012). Through the morphological diversification of their appendic- ular skeleton, tetrapods were able to infiltrate almost every habi- tat in the world, and develop a wide range of feeding and social 1 C  2015 The Author(s). Evolution