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173 Fossil Turtles from the Northern Neotropics: The Urumaco Sequence Fauna and Finds from Other Localities in Venezuela and Colombia Marcelo R. Sánchez-Villagra and Torsten M. Scheyer Turtles probably constitute the most readily recognizable group of all vertebrates, with their characteristic shell consisting of two parts, a dorsal carapace and a ventral plastron. Both parts are connected through a bridge, which completes the encasing of organs and even the shoulder girdle, the latter a unique feature among living vertebrates (present only in at least some placodonts among extinct clades, Rieppel 2002). This basic “grundplan” has existed in the group since at least the Late Triassic (ca. 210 Ma), the time from which the oldest stem-turtles are known (Rougier, de la Fuente, and Arcucci 1995). This general conservatism does not imply lack of variation in turtles. In fact, the Urumaco turtle fauna alone, the main subject of this chapter, is a good example of the variation in size, shape, and form that characterizes the turtle shell. Cranial variation also exists in turtles and is relevant at all taxonomic levels. This is evident in the Urumaco turtles discovered so far for a particular genus, for which several species have been recently described. There are two major clades of turtles existing today, distinguished by two distinct neck-retraction mechanisms. Cryptodires (“hidden necked” turtles) retract the head by bending the neck in a vertical plane, whereas pleurodires (“side necked” turtles) bend the neck in a horizontal plane. Living cryptodires are taxonomically and ecologically the more diverse group of the two, and have a worldwide distribution. Living pleurodires are restricted to freshwater environment in the Southern Hemisphere. However, ancient pleurodires were much more diverse ecologically and taxonomically than they are today (Gaffney, Tong, and Meylan 2006), as illustrated by the turtle fauna from the Urumaco sequence. Cryptodires are less abundant in Urumaco, but some occurrences are of singular biogeographic and ecological significance. A total of about eight turtle species are known from the Urumaco fauna. Since the Harvard expeditions of the early 1970s (Sánchez-Villagra this volume), the abundance of fossil turtles in Urumaco has been apparent. Subsequent collections have confirmed the abundance and singularity of this fauna. Other localities 9 Sánchez-Villagra and Scheyer 174 that have yielded an abundance of fossil turtles, such as the Miocene Castillo and La Venta formations of Colombia, as well as the Acre vertebrate fauna of Brazil, will be compared with the Urumaco sequence (Quiróz and Jaramillo this volume). Pelomedusoides: The Stupendous Stupendemys (Fig. 9.1) Because of its size, the podocnemidid Stupendemys geographicus (Wood 1976a) is perhaps the most remarkable turtle from Urumaco. A specimen with a carapace length of 330 cm and a carapace width of 220 cm was first reported by Aguilera (2004), making it the world’s largest. Wood (1976a) described a well-preserved shell (MCZ [P] 4376) of 218 cm midline length. Casts of this specimen are on display at the Museum of Comparative Zoology at Harvard University, the American Museum of Natural History in New York, the Aquarium in Osaka, and probably in other places unknown to us as well. This species has been subsequently reported from Miocene deposits in Acre, Brazil (Lapparent de Broin, Bocquentin, and Negri 1993; Gaffney, Campbell, and Wood 1998). As stressed by Wood (1976a) in the original description and by Gaffney , Campbell, and Wood (1998), the most distinctive element in the carapace of Stupendemys besides its gigantic size is the nuchal in the anterior part of the carapace. This bone has a thickened and dorsally curved margin that forms a large flange or collar at a right angle to the main plate. The humerus is a very massive bone, with distal and proximal ends both markedly expanded, with a triangular shaft in cross-section. The cervicals of Stupendemys are also diagnostic: the centra are particularly massive, and the central articular facets are close to the main body of the centrum (Gaffney, Campbell, and Wood 1998). It would be interesting to study whether these features are to be expected in a turtle of this size, given allometric relationships for cervical features in Pelomedusoides, or whether there is a deviation associated with particular adaptations and constraints. As pointed out by Gaffney, Campbell, and Wood (1998, 8), another singular feature of Stupendemys is the scapula, which “is unique among pelomedusids in having a dorsal scapular process that is strongly bowed with a flattened flange projecting laterally from the main axis.” Wood (1976a, fig. 6...

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