Chondrogenesis of the limbs and mesopodial ossification of Podocnemis expansa Schweigger, 1812 (Testudines: Podocnemidae)

Abstract

We address the chondrogenic formation of the limbs and the mesopodial ossification pattern of the Pleurodira Podocnemis expansa, to resolve the homology of these elements as well as the pattern of connection of the autopodial elements and the origin of the digital arch. Embryos and juveniles of P. expansa were cleared and stained for cartilage and bone. The fore- and hind-limbs were also studied histologically. We describe the development of the stylopodium and zeugopodium originating from a Y-shaped cartilaginous condensation, and the differentiation of the primary axis and the digital arch in the initial stages of limb development. The most pronounced changes were observed in the chondrogenic pattern and ossification of the mesopodium, although development of the digits is similar and we found no ontogenetic reduction such as that described for other Testudines. In this study, as in previous research involving several groups of reptilian sauropsids, we found an inconsistent pattern between the chondrogenic formation and mesopodial ossification of the limbs, indicating that these developmental events are dissociated. In summary, the chondrogenic and ossification sequences of these elements do not follow the same pattern. In addition, the differences found between P. expansa and other species to which it was compared clearly indicate that these events follow more than one pattern in Testudines.

The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology

“…2E, F). This corresponds to the observation of Fabrezi et al (2009) for Podocnemis unifilis, and of Vieira et al (2011) for Podocnemis expansa. In addition we also discovered a distinct perichondral ossification along the distal half of the hooked element in histological sections of an early specimen of E. subglobosa (Fig.…”

“…Araeoscelis casei; Reisz et al 1984). Second, we are able to confirm the observations of Fabrezi et al (2009 and Vieira et al (2011) that the hooked element of pleurodires exhibits endochondral ossification, and we are able to extend this observation for a number of cryptodires. The endochondral ossifications seen in pleurodires are relatively large compared with the full size of the hooked element, and endochondral ossification persists throughout ontogeny.…”

“…unifilis ossifies endochondrally like a carpal or tarsal element (Fabrezi et al 2009). The endochondral nature of the hooked element was confirmed for other pleurodires by and Vieira et al (2011), respectively, and serves as the best line of evidence in favour of a distal tarsal identity of the hooked element.…”

“…The large body of recent literature pertaining to the development of extant turtles placed much effort into densely sampling ontogenetic stages to clarify sequences of chondrification and ossification (Rieppel, 1993a;Sheil, 2003aSheil, ,b, 2005Sheil & Greenbaum, 2005;S anchez-Villagra et al 2007aS anchez-Villagra et al ,b, 2008Sheil & Portik, 2008;Bona & Alcalde, 2009;Vieira et al 2011). Although much variation is apparent, all available developmental data from recent turtles agree in that the hooked element ossifies after metatarsals I-IV, but prior to all tarsals.…”

“…This new hypothesis has gone uncontested to date (e.g. de la Fuente & Fern andez, 2011;Vieira et al 2011), although some authors prefer retaining traditional nomenclature by continuing to refer to this element as the hooked fifth metatarsal (e.g. Delfino et al 2010).…”