Stereogenys Andrews, 1901

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 : 56-57

Stereogenys Andrews, 1901

TYPE SPECIES: Stereogenys cromeri Andrews, 1901 .

INCLUDED SPECIES: At the present time, we are including only the skull-based Stereogenys cromeri in the genus. We realize that a number of other taxa have been named as species of Stereogenys that are shell based. These species are listed below, but we do not know what generic allocation would be likely for them.

DIAGNOSIS: A podocnemidid of the Tribe Stereogenyini known from the skull; uniquely possessing a secondary palate longer than in any other Tribe Stereogenyini , having a triturating surface of premaxilla with accessory ridge medial to labial ridge, and a basisphenoid that widely contacts the palatines and widely separates the pterygoids; pinched snout in contrast to Infratribe Bairdemydita ; medial edges of palatal cleft parallel in contrast to the Infratribe Bairdemydita ; basioccipital shortest of all Tribe Stereogenyini ; palatine with large dorsal process in septum orbitotemporale that contacts parietal as in Shweboemys in contrast to all other Tribe Stereogenyini ; triturating surface of premaxilla with accessory ridge medial to labial ridge in contrast to all other Stereogenyini .

DISTRIBUTION: Late Eocene, Egypt.

DISCUSSION: Andrews (1906: 298) assigned shells to Stereogenys by the following reasoning:

Although in no case have the carapace and plastron been found associated with the skull in such a manner as to leave no doubt that they belong to the same individual, nevertheless the shell now to be described may be regarded with reasonable certainty as belonging to the present species. In the first place, this form of shell, like the skull, is the commonest occurring in the Qasr el-Sagha beds, and in the second place it differs widely from the shell of Podocnemis , the only other Pleurodiran genus found in this horizon.

Commonality of occurrence can be used to associate skulls and shells in turtles only when there are a large number of specimens. There are rarely enough specimens (and that is certainly the case here) to make a statistical argument, and there are too many exceptions known. The ‘‘ Podocnemis ’’ referred to above was P. antiqua , here considered as Cordichelys antiqua , another tribe Stereogenyini member, closely related to Stereogenys , so that part of the argument is also problematic. To date there have been no skull-shell associations for Stereogenys . The shell-based species ‘‘ Stereogenys ’’ libyca Andrews, 1903 , and ‘‘Stereogenys’ ’ podocnemoides Reinach, 1903b , are not assignable to genus on the basis of the shell, and they do not have skull material associated with them.

Nonetheless, the shells claimed by Andrews (1903: 119) to belong to the skull-based taxon Stereogenys cromeri , do have a distinctive set of characters: carapace oval and depressed, seven neural bones, the first one pentagonal and not meeting the nuchal bone but separated from it by midline contact of first pair of costal bones, buttresses relatively small, intergular scale large, separating both gulars and humerals, gular and humeral scales entirely on epiplastra. These shells could in fact belong to Stereogenys cromeri , as this shell type has not yet been associated with any skull. Although we describe this shell taxon below, there is no evidence linking it with Stereogenys .