Albertwoodemys testudinum, Gaffney & Meylan & Wood & Simons & De Almeida Campos, 2011

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 : 35-37

Albertwoodemys testudinum , n. gen. et sp.

TYPE SPECIMEN: AMNH 5088 (figs. 83, 84), plastron with articulated left lateral and posterior peripherals.

TYPE LOCALITY: One mile west of AMNH quarries in the Lower Fossil Wood zone ( Simons, 1968: 21), Fayum Depression, Egypt.

HORIZON: Jebel Qatrani Fm., early Oligocene ( Seiffert, 2006).

DIAGNOSIS: As for genus.

ETYMOLOGY: In reference to the high-domed, tortoiselike nature of the shell.

REFERRED MATERIAL: None, but a shell, KNM-WK17139, from the Miocene of Kalodirr, Kenya, is very similar and may be the same species or genus.

PREVIOUS WORK: Described in Wood (1971).

DISCUSSION: It may seem the epitome of self-indulgent hypocrisy to name another new shell taxon, particularly after thoroughly criticizing those who have done so ( Gaffney et al., 2006). However, we have a good reason (doesn’t everyone?) for breaking our own rules. Albertwoodemys testudinum is both diagnosable and subject to phylogenetic analysis, just not right now. The key to the relationships of Albertwoodemys lies in the skull of UCMP 42008 (currently under study by Bramble, Hutchison, and Gaffney), which has a shell with a number of synapomorphies in common with Albertwoodemys . The skull, however, is not yet named and described, although we have enough information to place it in the cladogram as the sister taxon to Dacquemys (fig. 98). Albertwoodemys is probably the sister taxon to UCMP 42008 or it could be the shell of Dacquemys itself (although Dacquemys is Eocene, it is very late Eocene and Albertwoodemys is very early Oligocene), in any case, it is clearly closely related to UCMP 42008 and Dacquemys . So we name the plastron with the presumption that description of the skull of UCMP 42008 will clarify its relationships.

DESCRIPTION: There are several unique features on the plastron (figs. 83, 84). The humeral and pectoral scales are either fused or one or the other of these pairs of scales is totally missing. The anal scales are so small and posteriorly placed that they make no contact at all along the midline. On the inner surface of the posterior lobe of the plastron parallel to and just inside the lateral borders are bulbous ridges of unknown significance. All of these features are unique in pleurodires.

Excellent preservation characterizes the only known specimen of Albertwoodemys . The anterior lobe of the plastron is relatively short and rounded; the posterior lobe is somewhere between 1.5 and 2 X as long as the anterior one. The bridge is unusually long, being nearly twice as long as the posterior lobe. The lateral borders of the posterior lobe converge only slightly toward the midline as they extend posteriorly. The tips of the xiphiplastra are sharply pointed and separated from each other by a deeply incised U-shaped notch. Based on UCMP 42008 and specimens associated with it, a caudal buckler or osteoderm apparently fits here. The ventral surface of the plastron is a convex bulge curving gently from anterior to posterior. On either side of the midline in the area of the hypo-xiphiplastral sutures are localized oval depressions reversing this trend. A slight transverse concavity is present between the bridges at right angles from the midline. The plastron measures 43.1 cm in length.

In anteroposterior cross section (fig. 84B, C) the epiplastra are wedge shaped, so that the greatest thickness occurs posteriorly at the junction between the epiplastra and the entoplastron. This thickening, however, is not uniform, and the resultant structure is not truly comparable to the epiplastral thickening and excavation present in nearly all Testudinidae . Instead, between the laterally situated ridges there is a concavity on the midline suture anteriorly (figs. 83, 84). The entoplastron affords a striking example of how the shape of a bone may vary between the exterior and interior of a chelonian shell. Ventrally it is roughly diamond shaped, while dorsally it is a trapezoid whose leading edges are so shallow that its shape might almost be described as that of an equilateral triangle (fig. 83B).

Superimposed on the dorsal surface of the entoplastron is a Y-shaped ridge whose open end faces anteriorly. The distal ends of the bifurcated arms lead into the transverse ridges just described at the posterior edges of the epiplastra. Along the lateral border of the inner surface of the posterior lobe of the plastron are paired bony protuberances. These structures extend over an area covering the posterior portion of the hypoplastron and an adjacent region on the anterior end of the xiphiplastron (fig. 84D).

Prominent pelvic scars are also present on the inner surface of the plastron. The posterolateral ends of the ischial scars are continuous with the U-shaped anal notch and then curve forward until the proximal ends are nearly parallel to each other and the midline. This entire paired structure is slightly raised on a pedestal above the inner plastral surface. The pubic scars are spaced in the form of obtuse-angled triangles whose apices are directed posterolaterally. A very small portion of the right pubis has been preserved, which shows that the basal thickness quickly diminishes to produce a bladelike leading edge. Noting else is known of the pelvis.

The bone sutures on the plastron of Albertwoodemys are consistent with a typical podocnemidid. Roughly trapezoidal epiplastra form the anteriormost portion of the plastron. Immediately behind these paired elements is the more or less diamond-shaped entoplastron, whose posterior extension lies behind the level of the axillary notches of the bridge. This same condition can be seen in Neochelys . Sutures between the hyoplastra and hypoplastra bisect the middle of the bridge and terminate laterally in a junction with the mesoplastral sutures. The mesoplastra themselves are roughly hexagonal elements. Sutures separating the hypoplastra from the xiphiplastra diverge laterally and posteriorly from the midline at an angle of approximately 75 °.

Scale sulci on the plastron of Albertwoodemys are very clearly incised. Consequently, the absence of a humero-pectoral sulcus cannot be attributed to inadequate preservation. Much as in most living and extinct Testudinidae , the scale furrows are not simply grooves on the bony surface of the plastron, but are instead enclosed within slightly raised parallel ridges. At the front of the plastron, the pentagonal intergular scale is extremely large and broad. Small triangular gulars are wedged in on either side of the anterior end of the intergular. The pectoral-abdominal sulcus traverses the belly of the plastron anterior to the mesoplastra. The position of the abdominal-femoral sulcus is in no way remarkable. But at the posterior end of the plastron, the anal scales are so reduced that they are prevented from meeting at the midline by the xiphiplastral notch.

At both the anterior and posterior ends of the plastron, the scales extended far onto the dorsal surface of the plastron (fig. 83B). Anteriorly, most of the visceral surface of the epiplastra was covered by scales. The trapezoidal prolongation of the intergular was flanked on either side by rhombic gulars. In contrast to the situation for these scales on the external surface of the plastron, the combined areal extent of the two gulars exceeded that of the intergular. Triangular wedges of the pectoral scales stretched from the lateral borders of the gular scales back to the axillary notch. On the posterior lobe of the plastron, the femoral scales extend dorsally to cover the outer surface of the lateral bony ridges. Apparently the anal scales completely encompassed the tips of the xiphiplastra, encircling them in glovelike fashion.

Part or all of six peripheral bones have been preserved on the left side. Assuming that Albertwoodemys had the usual chelonian complement of 11 peripherals, it is probable that numbers five through 10 are represent- ed. A lateral carina is lacking, and the vertical orientation of the peripherals shows that the carapace was high vaulted and rounded (as in UCMP 42008 and the Kalodirr shell) rather than being low and flat as in typical aquatic forms. Unlike any tortoise, however, the distal ends of the pleural scales extend well down onto the peripheral bones in agreement with the pattern encountered in pleurodires, emydids, and baenids. The sulcus separating what is probably the sixth marginal scale from the abdominal scale crosses the distal portion of the mesoplastron in a manner similar to that seen in the type of ‘‘ Stereogenys ’’ libyca ( Andrews, 1906: 303, fig. 97). An unusual feature on the inner surface of the posterior peripherals is a thick ridge (fig. 84E), which continues forward and around the circumference of the inguinal notch as a bladelike crest that ultimately connects to the anterior extension of the thick ridge on the inner surface of the posterior lobe of the plastron. Such a structure is often found in testudinids but never in pleurodires.

The depositional environment of the Jebel Qatrani Fm. has been described as ‘‘fluviomarine,’’ but Simons (1968: 15, 16) and Bown and Kraus (1988) indicate a more terrestrial depositional environment, with channel sands and conglomerates together with clayey flood plains deposits reinforcing this interpretation. The tortoiselike structure of the shell of Albertwoodemys , suggests that it (and UCMP 42008) was a terrestrial form.

UCMP 42008, unnamed new genus and species

DIAGNOSIS: Podocnemidid known from skull and shell; differing from all other pleurodires on the basis of these characters: triangular-shaped and fully roofed skull, cheek and temporal emargination completely absent, due to unique possession of unusually large squamosal with ventral flange on occipital surface and extensive parietal contact, laterally and anteriorly expanded supraoccipital with extensive squamosal and parietal contacts (longer than in Dacquemys , the only form to approach it), large quadratojugal with extensive squamosal and jugal contacts; anteriorly extensive prefrontal with very long maxilla contact and ventral flange forming snout region; apertura narium externa small and partially subdivided on midline; other distinguishing characters: orbits facing anterolaterally; interorbital groove absent; medial expansion of triturating surface present, median maxillary ridge present, accessory ridge present; complete cavum pterygoidei present; processus trochlearis pterygoidei present and at right angles to midline; carapace high domed with deep nuchal embayment; caudal buckler or cloacal cover formed by fused osteoderms.

SPECIMEN: UCMP 42008, skull lacking major portions of palate and braincase, partial shell with carapace and plastron elements.

LOCALITY: UCMP locality V4898, Moruorot Hill (often alternately spelled Muruarot Hill), 20 miles NE of Lodwar, northcentral Kenya.

HORIZON: Early Miocene Lothidok Formation ( Boschetto et al., 1992).

REFERRED MATERIAL: A caudal buckler, UCMP 41918, and a peripheral, UCMP 42141, both from UCMP locality V48100, are identified with this taxon. It should be mentioned that National Museum of Kenya WK-17139, a shell from the Miocene of Kalodirr, Kenya, is very similar to the shell of UCMP 42008 and may be the same species or genus.

DISCUSSION: UCMP 42008 is currently being studied by J.H. Hutchison, D. Bramble, and E.S. Gaffney. Enough work has been done to include it in the phylogenetic analysis. The unusual shell morphology of this specimen links it with Albertwoodemys , based on the high-domed carapace, the deeply incised anterior margin of carapace, thickened plastral edges, and the fusion (or loss) of the humeral and pectoral scales. See discussions under Dacquemys and Albertwoodemys .