In terrestrial vertebrates, the shape of the pelvic girdle is a reliable predictor of both phylogeny and locomotor mode (Gambarian, 2002; Lovejoy et al., 2009; Romer and Parsons, 1977; Snyder, 1954). However, in marine vertebrates, gravitational forces typically exerted on terrestrial pelvic girdles are essentially released. This is in part due to teleost fishes being positively buoyant and to the majority of these bony fishes and elasmobranch fishes (sharks, skates, and rays) performing swimming as a primary form of locomotion (Moyle and Cech, 1999). Rather than providing support for the posterior half of the body, as in terrestrial vertebrates, the pelvic girdle of fish is unattached from the vertebrae, and serves as a site of muscle attachment and support for fins in most fishes (Fig. 1c). Harris (1938) concluded that rigid pelvic fins of sharks (Elasmobranchii) are not involved in the primary force production during locomotion, but rather, have extremely limited function and can be removed without affecting overall locomotion of the shark (Harris, 1938). The pelvic fins of more derived teleost fishes stabilize the body and assist in maneuverability, and would likely transmit very low forces to the pelvic girdle (Standen, 2008; Harris, 1938; Fig. 1c). However, some teleost fish (e.g., batfish, flying gurnards, frogfish, and lungfish; (Helfman et al., 1997; Pough et al., 2004; Renous et al., 2000; Ward, 2002) and elasmobranch fish (epaulette and bamboo sharks, horn sharks, and batoids (skates and rays); (Compagno, 1999; Goto et al., 1999; Koester and Spirito, 2003; Lucifora and Vassallo, 2002; Macesic and Kajiura, 2010; Pridmore, 1995; Wilga and Lauder, 2001) perform bottom walking with their pelvic fins (Figs. 1a,b). Within the mostly benthic batoids, the pelvic fins are used to walk (each fin alternately; (Lucifora and Vassallo, 2002) and punt (both fins synchronously; (Koester and Spirito, 2003; Macesic and Kajiura, 2010) on the substrate. While this pelvic fin locomotion would require considerably less support of the body by the pelvic girdle than in terrestrial locomotion, small ground reaction forces would exist. In addition to true punting, performed with just the pelvic fins, some batoids perform augmented punting, in which the pectoral fins are also employed and are thought to generate supplemental thrust (Macesic and Kajiura, 2010). Despite this augmented feature, kinematics such as distance and speed per punt are not greater than in the true punters. The pectorals are likely generating thrust, thus reducing the amount of force experienced by the pelvic fins alone during a punt (Macesic et al., 2013). Moreover, several pelagic rays likely do not perform any form of punting. Therefore, the goal of this current study was to determine if the shape of the batoid pelvic girdles vary with punting ability.
Figure 1. Morphological variation of pelvic girdles from a batoid (a, b), illustrating the articulation between the girdle and the propterygia (a) and the musculature associated with the pelvic skeletal elements (b, adapted from Macesic and Kajiura, 2010: Ventral musculature: proximal fin depressor—turquoise, distal fin depressor—purple, distal propterygium depressor—green, proximal propterygium depressor—red; Dorsal musculature: proximal fin levator—turquoise, distal fin levator—purple, proximal propterygium levator—green, distal propterygium levator—red), a rainbow trout [(c) adapted from Standen (2008)], and Acanthostega, a primitive sprawled-gait tetrapod [(d) adapted from Coates (1996)]. ct = connective tissue.
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We hypothesize that true punters will exhibit traits similar to those of a sprawled-gait terrestrial vertebrates, including a broad, anteroposteriorly expanded pelvic girdle, with exaggerated lateral processes (Boisvert, 2005; Clack and Coates, 1995; Coates et al., 2008, 1980, 1996; Figs. 1a,b,d). Similarly, we also hypothesize that, like the laterally facing acetabula of sprawled-gait terrestrial vertebrates (Fig. 1d), the angle at which the propterygium (the stylopodial element that is primarily used in punting) articulates with the pelvic girdle will vary such that the true punters have the most laterally facing articulations. We employ geometric and linear morphometrics to examine the overall shape of the pelvic girdle and to measure the angle of articulation between the pelvic girdle and propterygium among batoids that are diverse with respect to phylogeny, swimming (Rosenberger, 2001; Webb, 1984), and punting modes. Using the data from extant specimens, we also examine fossil specimens (Fig. 2, deCarvalho et al., 2004) and place them with respect to phylogeny and locomotor mode based on our results from the extant species in this study.