Skeletal morphology and development of phrynops hilarii (Pleurodira: Chelidae)

BONA, P. & ALCALDE, L.

Neuquen

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados.; 2008

Asociación Paleontológica de Argentina

Morphology is a central topic in practically all aspects of evolutionary biology. Although the anatomical and morphological variability of turtles has been studied by several authors, only few works contain comprehensive descriptions of the development of the entire skeletal system (Rieppel 1993; Sheil 2003, 2005). This paucity of information is particularly evident for the cranial and postcranial ontogeny of pleurodiran turtles because proposals of homology and cranial and postcranial patterns of “Testudines” are based largely on cryptodiran anatomy (Gaffney 1979; Rieppel 1994, and works cited therein). Pleurodira is one of the major monophyletic assemblages of extant turtles and comprises two major clades (Joyce, 2007), both distributed in the Southern Hemisphere: the pelomedusoids, restricted to Africa, Madagascar and South America (de Broin 1988) and the Chelidae, distributed in Australasia and South America. Although the monophyly of both Pleurodira and Chelidae is widely accepted, the phylogenetic relationships within each clade remain controversial (Gaffney 1977; Georges et al. 1998; Joyce, 2007). Furthermore, the species studied here, Phrynops hilarii, belongs to Phrynops (sensu lato), a group of chelids endemic to South America, and recorded probably since the Mio-Pliocene in Argentina (e.g. Ph. geoffroanus complex, Cione et al. 2000), which includes taxa (e.g., Phrynops, Mesoclemmys, and Batrachemys) with unsettled interrelationships (Georges et al. 1998; McCord et al. 2001).The present work is the first comprehensive study of osteological development for a pleurodiran chelid turtle. We studied nineteen Phrynops hilarii embryos (stages 19, 21, 22, 23, 25 and 26) and one subadult (juvenile, lacking external sexual dimorphism), double-stained and cleared according to the technique of Taylor and Van Dyke (1985). The dry skeletons of two adults were also studied. All specimens are housed at the herpetological collections of Museo de La Plata (La Plata, Buenos Aires, Argentina - MLP R.) and Fundación Miguel Lillo (Tucumán, Argentina - FML). The most remarkable features found are: 1) absence of ascending process on pterygoquadrate cartilage; 2) presence of ossification centers for the epiotics; 3) contact between ilium and carapace occurs later in ontogenetic development; 4) suture between ischia, pubes and plastron occurs in posthatching specimens; 5) contrary to previous interpretations, the phalangeal formula of the pes of P. hilarii is 2:3:3:3:5; 6) the hooked bone represents the fifth metatarsal. These results are relevant to increase our understanding of morphological variation in turtles, discuss primary homologies of several cranial and postcranial elements, and propose structural and developmental patterns for different elements of the axial and appendicular skeleton of this species. Broin, F. de. 1988. Les Tortues et le Gondwana. 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