Ontogeny of the Shell Bones of Embryos of Podocnemis unifilis (Troschel, 1848) (Testudines, Podocnemididae)

Abstract

The aim of the present study was to investigate the sequence of shell bone formation in the embryos of the Pleurodira, Podocnemis unifilis. Their bones and cartilage were collected and cleared before staining. The shell was also examined by obtaining a series of histological slices. All the bony elements of the plastron have independent ossification centers, which subsequently join together and retain two fontanelles until the period of hatching. This turtle has a mesoplastra, which is characteristic of the Podocnemididae. The carapace begins to form concurrently with the ossification of the ribs at the beginning of stage 20. All the plates, except the suprapygal, initiate ossification during the embryonic period. The main purpose of the histological investigation was to highlight the relationship between the formation of the carapace and ribs with that of the neural plates. The costal and neural plates were found not to independent ossification centers, but to be closely related to components of the endoskeleton, originating as expansions of the perichondral collar of the ribs

Bone shell microstructure of Condorchelys antiqua Sterli, 2008, a stem turtle from the Jurassic of Patagonia

“…Moreover, the presence of structural fibers occupying partially (peripherals) or entirely (neurals) the bone tissue in the internal cortex suggests that the visceral portion of these plates were partially or entirely imbedded within the dermis. Interestingly, this position coincides with that observed in the early stages of development of several extant taxa, including Podocnemis unifilis, Emydura subglobosa and Pelomedusa subfrula (Lima et al, 2011;, but differs from Pelodiscus sinensis, in which neural plates develop entirely outside of the dermis (Hirasawa et al, 2013).…”

“…In some costal plates, an enlarged, roughly circular cavity is present in the inner core of the elements. This central cavity has been previously described in costal plates of fossil and extant turtles (Lima et al, 2011;Lyson et al, 2013;Scheyer et al, 2014a). Scheyer et al (2014a) interpreted this tubular structure as representing the original location of the embryonic rod-like cartilaginous rib early in costal growth.…”

“…Scheyer et al (2014a) interpreted this tubular structure as representing the original location of the embryonic rod-like cartilaginous rib early in costal growth. The internal diameter of this structure (1,45 mm in MPEF-PV 10849) is, however, relatively larger than the diameter of the costal growth center of turtle embryos (Hirasawa et al, 2013;Lima et al, 2011;Scheyer et al, , 2014a. As pointed out by Scheyer et al (2014a), the size of the primordial ribs varies among embryos of extant turtles, which appears to be linked to the individual size of the specimen sampled, as well as the final adult size of the respective species.…”

“…Recent studies (Gilbert et al, 2001;Hirasawa et al, 2013;Lima et al, 2011; have provided support for the second hypothesis. In this sense, ontogenetic data of extant turtles reveal that the costal and neural plates start their development as initial outgrowths of the periosteum of the ribs and vertebral arches, respectively, before the mode of mineralization switches to metaplastic ossification (i.e., transformation of pre-existing, non-osseous tissue into bone) of the dermis (Lima et al, 2011;. Metaplastic tissue has been inferred by the I.A.…”

“…The costals and neurals have been interpreted either as separate dermal elements that secondarily fuse with the underlying endoskeletal elements (e.g., ribs and vertebrae/neural arches) or as solely derived from these endoskeletal elements (see review in Rieppel, 2013). Recent studies (Gilbert et al, 2001;Hirasawa et al, 2013;Lima et al, 2011; have provided support for the second hypothesis. In this sense, ontogenetic data of extant turtles reveal that the costal and neural plates start their development as initial outgrowths of the periosteum of the ribs and vertebral arches, respectively, before the mode of mineralization switches to metaplastic ossification (i.e., transformation of pre-existing, non-osseous tissue into bone) of the dermis (Lima et al, 2011;.…”