Morphology of the anterior vertebral region in elasmobranchs: special focus, Squatiniformes

Fishes are known to have a great deal of variability in their vertebrae especially near the occipital region. Despite this observed variability, there is new and compelling evidence for developmental conservatism in the region for actinopterygians (Bemis & Forey 2001; Britz & Johnson 2010; Johnson & Britz 2010). Authors have even hypothesized that the occiput could be homologous among chondrichthyans (Britz & Johnson 2010). This study is a first attempt at characterizing the ontogeny of the occipito-vertebral region, specifically in the derived chondrichthyans, Squatiniformes. Squatiniformes is a group of dorsoventrally-flattened cartilaginous fishes with shark and batoid-like morphology. These fishes were shuffled among many branches of elasmobranchs; sometimes aligned closely with batoids, sometimes squaloids, and sometimes both depending on the study (Seret 1986; Shirai 1992a, 1992b, 1996 and references therein). Most recently two well supported but drastically different competing hypotheses based on morphology and molecules posit batoids as derived sharks closely related to angel sharks and sawsharks (Shirai 1992a, 1992b, 1996; Carvalho 1996), or as the sister group to a monophyletic group of sharks with no relationship to angel sharks respectively (Douady et al. 2003; Winchell et al. 2004; Heinicke et al. 2009, V lez-Zuazo & Agnarsson 2011) (Fig. 1). The superficially similar morphology of angel sharks to batoids prompted our new investigation of both extinct and extant squatiniform-taxa, with particular emphasis placed on the anterior portion of the axial skeleton. This region of the axial skeleton is of particular interest because of its possible functional and systema-