Stereogenys libyca, Andrews, 1903

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 : 65-67

‘‘Stereogenys’ ’ libyca Andrews, 1903

TYPE SPECIMEN: BMNH R.3039, a carapace and plastron lacking part of the left side. This specimen was returned to the CGM in the 1960s, but the authors have been unable to locate it or determine its CGM number, so we use the original BMNH number.

TYPE LOCALITY: North of Lake Qarun, Fayum Depression, Egypt ( Andrews, 1903).

HORIZON: Jebel el-Qatrani Fm., Early Oligocene ( Andrews, 1903).

DIAGNOSIS: Known material supposedly differs from shells attributed to ‘‘ Stereogenys ’’ cromeri by having a broader and shorter nuchal bone, a U-shaped anal notch, a pentagonal intergular, and triangular humeral scales, but see discussion below.

REFERRED MATERIAL: BMNH R.3100, anterior plastral lobe; BMNH R.3120, anterior plastral lobe; AMNH 5089, weathered shell.

PREVIOUS WORK: Reinach (1903b), Dacqué (1912), Williams (1954b), Lapparent de Broin (2000a).

DISCUSSION: The type specimen, as based on figures in Andrews (1903) and his description, is a relatively well-preserved and complete shell, allowing a reasonable description and reconstruction (fig. 85, based on Andrews, 1903: pl. 7). Although this shell may be lost, we summarize its morphology below because it remains as one of the betterknown shell taxa from the Fayum region. In any case, there are no skull associations with this material, so it is not identifiable as Stereogenys , and there is no character set that allows a shell-based generic identification. The supposed differences between the relatively well-preserved shell material of ‘‘Stereogenys’ ’ libyca and the poorly preserved shell attributed to ‘‘ S. ’’ cromeri are minor and could easily be individual variation or preservational (R.C.W.).

DESCRIPTION: The carapace is low arched and oval, slightly expanded posteriorly. A strong lateral carina crosses the bridge peripherals, and the shell bulges outward in the area covered by the last vertebral scale. The trapezoidal nuchal bone is slightly indented anteriorly, similar to Cordichelys antiqua , and is about one and a half times wider than long. The first pair of costals meet on the midline behind the nuchal bone separating it from the neurals. There are seven neurals, the anterior ones longer than wide, the posterior ones wider than long. The first neural is pentagonal, the rest are hexagonal. The midline meeting of the eighth costals separates the seventh neural from the suprapygal. The suprapygal is a broadly rounded diamond with the long axis perpendicular to the midline. There is a small notch at the posterior end of the rectangular pygal bone, marking the sulcus separating the last two marginal scales. Eleven pairs of peripherals border the eight costal bones. Peripherals four to seven form the bridge, the fifth and sixth contact the mesoplastra. The posterior peripherals are more expanded than the anterior. According to Andrews (1906: 304) ‘‘both buttresses are weaker and less developed than in most pleurodires, the inguinal being the stronger of the two.’’ The first of the five vertebral scales is relatively small and pentagonal. In contrast to the commoner condition in pleurodires, the first vertebral is narrower than the nuchal bone underlying it. The remaining vertebrals are hexagonal and much larger than the first. Marginal scales 1, 5, 7, 9, and 11 are pentagonal; the others are all quadrilateral.

The posterior lobe of the plastron is longer than the anterior lobe. The front margin of the anterior lobe is squared off and slightly concave. Andrews noted (1906: 304) that ‘‘The upper surface in this region is also gently concave from side to side, the concavity being bounded by thickened ridges borne on the epiplastrals and anterior part of the hyoplastrals.’’ Differences in the shape of the entoplastron were cited by Andrews (1903: 119) as a means of distinguishing between cromeri and libyca . Although the entoplastron of libyca is smaller than that of cromeri it is the same shape. In view of the variability in the shape of this bone in some pleurodires (e.g., fajumensis Dacque´, 1912: fig. 7), the taxonomic significance of the entoplastron shape is questionable. The posterior extremity of the entoplastron extends past the level of the axillary notch. Small, hexagonal mesoplastra, longer than broad, are wedged between the distal extremities of the hyo- and hypoplastra. The anal notch is shallow and U-shaped. Parallel to the outer border of the posterior lobe, there is a ridge on the inner surface, which fades out midway along the xiphiplastron ( Reinach, 1903b: pl. 13, fig. 9). The lateral surface of this ridge slopes gently outward, tapering to a thin edge. The scales of the plastron overlap onto the dorsal plastral surface to a greater extent than in the living podocnemidids. Wide separation of the small gular and humeral scales is related to the large, pentagonal intergular, extending posteriorly to the middle of the entoplastron. A sulcus separating the sixth marginal from the abdominal scale slants across the mesoplastron, but the pectoral scale does not reach the mesoplastron.

DESCRIPTION OF SHELL ATTRIBUTED BY ANDREWS (1906) TO STEREOGENYS CROMERI : Although we do not feel that there is any evidence to associate this shell with the skull of Stereogenys cromeri , or anything else, it is worth discussing the interpretation that this shell is the same taxon as the shell-based ‘‘S.’’ libyca .

The shell material of this presumed taxon is poorly preserved, and there is also ambiguity in the various Andrews descriptions as to which specimen he is describing. The carapace is oval, its greatest lateral expansion slightly anterior to the inguinal notch. There may have been an indentation along the anterior carapace margin. The nuchal illustrated ( Andrews, 1906: fig. 96) is slightly broader than long. The total number of neurals cannot be determined, but seven is most likely, based on the reconstruction and the close similarity to the shells of ‘‘S.’’ libyca . Instead of contacting the nuchal bone, the first neural is separated by the midline contact of the first costal bones. The first neural is pentagonal, the only other preserved one, probably the fifth, is hexagonal, as in the better preserved ‘‘S.’’ libyca . The pygal is trapezoidal, and there are the usual eight pairs of costals, but the number of peripherals is indeterminate.

The plastron of ‘‘ Stereogenys ’’ cromeri has a bridge longer than the posterior lobe that is longer than the anterior lobe. The anterior lobe is more angular than rounded and its anterior edge is slightly concave. The epiplastra are broader laterally than at the midline giving the anterior lobe a squared-off shape. ‘‘In some specimens the upper surface of the epiplastrals is raised into a sort of boss near their posterior angle, and from this a slight ridge is continued backwards to the axillary buttress’’ ( Andrews, 1906: 300). The external outline of the entoplastron, although poorly preserved, is variable in the available specimens. Being rhomboidal to escutcheon shaped in specimens described by Andrews (ibid.). Mesoplastron sutures are absent. ‘‘The posterior border of the plastron seems to have been notched in the middle line as in St. libyca , but in no specimen is this region well preserved’’ ( Andrews, 1906: 300). And yet the anal notch shape was used to distinguish cromeri from libyca . Later work by one of us (R.C.W.) suggests that a specimen containing a left xiphiplastron (BMNH R.3200) shows that the anal notch is small and broadly V-shaped in contrast to ‘‘ S. ’’ cromeri . Little of the plastral scale pattern is preserved in the attributed material. There is a large, apparently, seven-sided intergular scale, although the actual difference between this and the large five-sided intergular of ‘‘ S. ’’ libyca is a matter of interpretation. The intergular scale extends posteriorly to the middle of the entoplastron and completely separates the humerals and the gulars. The gulars are small, triangular scales and the humerals are slightly larger and trapezoidal. Both are entirely on the epiplastra. No other scale markings are apparent in the available material.