Podocnemis Wagler, 1830

Gaffney, Eugene S., Meylan, Peter A., Wood, Roger C., Simons, Elwyn & De Almeida Campos, Diogenes, 2011, Evolution Of The Side-Necked Turtles: The Family Podocnemididae, Bulletin of the American Museum of Natural History 2011 (350), pp. 1-237 : 27-29

Podocnemis Wagler, 1830

TYPE SPECIES: Emys expansa Schweigger

TABLE 2 Comparison of Peirópolis Skulls (measurements in mm)

INCLUDED SPECIES: Podocnemis expansa , P. vogli , P. unifilis , P. erythrocephala , P. lewyana , P. sextuberculata , P. bassleri .

DISTRIBUTION: Northern South America.

DIAGNOSIS: A podocnemidid with postorbital small allowing jugal-parietal contact; jugal unusually large; interorbital groove present, uniquely among Podocnemididae ; skull relatively high and narrow in contrast to Bauruemys ; orbits facing dorsolaterally; temporal emargination slight in contrast to Bauruemys but deeper than in Peltocephalus ; parietal-quadratojugal contact long; cheek emargination reaches level of orbit; medial expansion of triturating surface, median maxillary ridge, present; accessory ridge or ridges present in triturating surface; vomer absent (except in P. vogli ); fossa precolumellaris varying from shallow to moderate but not as deep as in Bauruemys ; foramen jugulare posterius closed; interparietal scale equilateral triangle or elongate triangle; cavum pterygoidei with small to moderate anterior opening for foramen cavernosum; horizontal occipital shelf present as in Peiropemys ; chorda tympani not enclosed in processus retroarticularis.

Postcrania with cervical centra saddle shaped; nuchal bone width greater than length; seven neurals extending to costal eight; axillary musk duct absent from buttress (except in P. erythrocephala ), axillary buttress reaching second peripheral (except in P. vogli ), pectoral scales do not contact mesoplastra, but do contact entoplastron and epiplastra.

PREVIOUS WORK: Baur (1890) gave a comparative diagnosis for Podocnemis , Peltocephalus , and Erymnochelys (repeated un- der Erymnochelys discussion), when he authored Erymnochelys . Boulenger (1889) gives a diagnosis for Podocnemis sensu lato, synonymizing Peltocephalus and Erymnochelys , for the Recent species of the family as currently constituted. The same is true for Siebenrock (1902), Williams (1954a), and Wermuth and Mertens (1961, 1977). The Pritchard and Trebbau (1984) diagnosis was published after the separation of the three genera was widely accepted. All of these emphasize characters visible in the preserved whole animal with limited if any osteologic characters. Although there is no systematic information, Groombridge (1982) has references for biology of the various living species. Fritz and Havas (2007) and Bickham et al. (2007) are the most recent to produce species lists and synonymies of variable usefulness.

DISCUSSION: We follow Iverson (1992) who, in turn, followed Williams (1954a) and Pritchard and Trebbau (1984), in the recognition of six living species of Podocnemis sensu stricto. See all three references for an introduction to the literature on this genus. As our present work is not primarily concerned with these species, we do not review this literature. The specimens listed below as Referred Material are only a few of the ones we have actually seen and do not reflect the material available in collections or referred to in the literature.

Although there have been few suggestions of relationships among the living Podocnemis species, Mittermeier and Wilson (1974) argued that erythrocephala , unifilis , lewyana , vogli , form a monophyletic group within Podocnemis . They called this the ‘‘vomerine group’’ (ibid.: 157), based on the possession of at least two parallel ridges on the triturating surface (three in vogli , which they interpreted as primitive) and elongate heads. Frair et al. (1978) produced a resolution of: ( P. expansa , P. vogli , P. lewyana , P. unifilis , P. erythrocephala ( P. sextuberculata )), based on a serological analysis. Cadena et al. (2010) did not resolve these species.

Our resolution of these species is: ( P. vogli ( P. lewyana ( P. unifilis ( P. erythrocephala ( P. sextuberculata , P. expansa ))))). There is virtually nothing in common with this resolution and the molecular study of Vargas-Ramírez et al. (2008). Most of the characters we have used that actually resolve these species are frequently subject to homoplasy in other turtle taxa, and are reversed within our own study, so we do not consider this part of the analysis to be strongly supported as the resolution collapses into a multichotomy at one step more than the shortest resolution (fig. 98). Osteologically, these Recent species are very similar to each other and have relatively conservative morphologies even within a group that has changed little since the late Cretaceous.

In our resolution of the character list presented here, the following characters are unique synapomorphies for Podocnemis : interorbital groove, jugal-parietal contact, postorbital smaller than orbit (except in P. unifilis ), and foramen nervi abducentis large (also in Pricemys , and not determinable in many fossil skulls).

For the living species of Podocnemis , our analysis produces the following characters (see also Character List under Conclusions) supporting the resolution in fig. 98:

( P. expansa , P. sextuberculata ) – character 6, parietal and pterygoid contact in septum orbitotemporale present and narrow; character 30, fossa precolumellaris present but shallow.

(( P. expansa , P. sextuberculata ) P. erythrocephala ) – character 36, anterior opening of cavum pterygoidei moderate in size.

((( P. expansa , P. sextuberculata ) P. erythrocephala ) P. unifilis ) – character 70, axillary musk duct with three openings.

(((( P. expansa , P. sextuberculata ) P. erythrocephala ) P. unifilis ) P. lewyana ) – character 8, interparietal scale elongate triangle; character 22, vomer absent; character 68, axillary buttress reaches peripheral two.

((((( P. expansa , P. sextuberculata ) P. erythrocephala ) P. unifilis ) P. lewyana ) P. vogli ) – characters for genus Podocnemis listed above.