Sokatra antitra, Gaffney & Krause, 2011

Gaffney, Eugene S. & Krause, David W., 2011, Sokatra, a New Side-Necked Turtle (Late Cretaceous, Madagascar) and the Diversification of the Main Groups of Pelomedusoides, American Museum Novitates 2011 (3728), pp. 1-28 : 4-13

Sokatra antitra , new species

TYPE SPECIMEN: UA 9769 (field number MAD 07433), a partial skull, lacking the premaxillae, basioccipital, and with damaged basisphenoid, quadrates, prootics, and opisthotics. The holotype skull of Sokatra antitra was discovered in the austral winter of 2007 as part of the joint Stony Brook University / Université d’Antananarivo Mahajanga Basin Project , which was initiated in 1993. This discovery enabled the identification and referral of most of the other, previously discovered specimens described in this report .

TYPE LOCALITY: MAD 93-35, Berivotra Study Area , northwestern Madagascar .

HORIZON: Anembalemba and Masorobe members, Maevarano Formation, Maastrichtian, Upper Cretaceous. All of the specimens described in this report were recovered from localities in the Anembalemba and Masorobe Members (Maastrichtian, Upper Cretaceous) of the Maevarano Formation in the Berivotra and Masiakakoho study areas, Mahajanga Basin, northwestern Madagascar (see description and map in Gaffney et al., 2009: fig. 1). Geographic coordinates for these localities are on file at the Field Museum (Chicago), Stony Brook University, and the Université d’Antananarivo and are available to qualified investigators.

DIAGNOSIS: As for genus.

ETYMOLOGY: Antitra (pronounced AHN-tah-trah), Malagasy for “old.”

REFERRED SPECIMENS: FMNH PR 2649 (field number MAD 07673), a partial skull with articulated or associated quadrates (both right and left), opisthotics (both right and left), left exoccipital, right prootic, left pterygoid, and right palatine; UA 9864 (field number MAD 07637), a partial right quadrate; and UA 9865 (field number MAD 07862), a partial left maxilla. These three specimens are referred to Sokatra antitra based on direct comparisons and morphological agreement with the holotype. Five other incomplete specimens are more tentatively referred: FMNH PR 2647 (field number MAD 07581), supraoccipital; FMNH PR 2648 (field number MAD 07583), left frontal; UA 9770 (field number 96144), right parietal; UA 9771 (field number MAD 05564), right dentary; UA 9866 (field number MAD 07895), left dentary .

In addition to the holotype specimen, FMNH nos. PR 2647– PR 2649 and UA nos. 9864 and 9865 were recovered from a small (2 m 2) quarried surface in locality MAD 93-35 (Berivotra Study Area). UA 9770 was collected from locality MAD 93-34 (Berivotra Study Area) and UA 9771 was collected from locality MAD 05-61 ( Masiakakoho Study Area ). UA 9866 was collected from locality MAD 07-66 ( Berivotra Study Area ), which lies stratigraphically in the Masorobe Member of the Maevarano Formation ; it is the only specimen not recovered from the Anembalemba Member .

Shell material was not found associated with any of the Sokatra skull elements, but the relatively common presence of pleurodire shell material in these units suggests that at some time it may be possible to associate shell morphology with Sokatra .

DESCRIPTION

PREFRONTAL (figs. 1, 2, 5)

PRESERVATION: Both prefrontals are present in UA 9769.

CONTACTS: The prefrontal in Sokatra meets the prefrontal on the midline, the maxilla anteroventrolaterally, and the frontal posteriorly, as in other Pelomedusoides. The frontal sends a process anteriorly on the ventral surface along the midline, preventing the prefrontal from roofing the sulcus olfactorius.

STRUCTURES: The dorsal plate of the prefrontal forms the dorsal margin of apertura narium externa, which is slightly protruding on the midline, similar to the condition in Podocnemis . The prefrontal also forms the anterodorsal margin of the orbit. On the ventral surface, the prefrontal forms the roof of the fossa nasalis, which is a shallow concavity just lateral to the midline. As in pelomedusids and euraxemydids, the prefrontal in Sokatra has a ventral process projecting anteroventrolaterally that forms the edge of the foramen orbito-nasale.

FRONTAL (figs. 1, 2, 5)

PRESERVATION: Both frontals are present in UA 9769 but they do not have clear posterior sutures. An isolated left frontal, FMNH PR 2648, is well preserved and, although incomplete, helps with the identification of the frontal sutures and the ventral morphology. This disarticulated frontal is a pleurodire as indicated by the presence of a sulcus palatinopterygoideus; it is also possible that it belongs to the bothremydid Kinkonychelys ( Gaffney et al., 2009) , but we consider this unlikely because Kinkonychelys has a deeper ridge for the sulcus palatinopterygoideus compared to Sokatra . Nonetheless, it might belong to a taxon not yet identified from the Maevarano Formation. Neither the diagnosis nor any of the phylogenetic characters rely on FMNH PR 2648 belonging to Sokatra .

CONTACTS: The frontal in UA 9769 (best preserved on the right side) contacts the frontal on the midline, the prefrontal anteriorly, the postorbital posterolaterally, and the parietal posteriorly. The latter contact is poorly preserved and not clearly visible in UA 9769; it is also not visible in FMNH PR 2648, in which the posterior edge is broken. Nonetheless, considering these specimens and the isolated parietal, UA 9770, it can be deduced that the frontal has a posterolateral contact with the postorbital similar to that in pelomedusids except that it is relatively short. Similarly, as restored, the postorbital contact with the frontal is longer in Sokatra than in pelomedusids.

STRUCTURES: The frontal forms the posteromedial portion of the orbital margin. Sokatra has a temporal emargination comparable to that seen in pelomedusids and Araripemys , in contrast to the more completely roofed skulls of euraxemydids, bothremydids, and podocnemidids. On the ventral surface, the frontal forms most of the sulcus olfactorius. Lateral to the sulcus it forms a low ridge paralleling the orbital margin that separates the orbit from the sulcus palatinopterygoideus. The frontal forms the anterior part of the sulcus palatinopterygoideus, as in other pleurodires ( Gaffney et al., 2006: fig. 25).

PARIETAL (figs. 1, 2, 5)

PRESERVATION: Both parietals are present but poorly preserved in UA 9769, with the sutures distinguishable only with difficulty. A disarticulated parietal, UA 9770, is identified as Sokatra on the basis of the extremely close agreement with the parietal in UA 9769.

CONTACTS OF DORSAL PLATE: As in pelomedusids, euraxemydids, and other Pelomedusoides, the contacts of the parietal in Sokatra are with the parietal on the midline, the frontal anteriorly, and the postorbital anterolaterally. The postorbital contact is not well preserved in any specimen but, based on both UA 9769 and UA 9770, it is relatively narrow, due to the extent of the temporal emargination in Sokatra .

Pelomedusids, unlike euraxemydids, have no parietal-quadratojugal contact. Although this area is poorly preserved in UA 9769, and the referral of UA 9770 is not certain, UA 9769 is well enough preserved to show that a parietal-quadratojugal contact is very unlikely.

STRUCTURES OF DORSAL PLATE: The temporal emargination in Sokatra extends anteriorly to expose the foramen stapedio-temporalis and all the otic chamber in dorsal view, a condition typical of pelomedusids and Araripemys , but not found in Podocnemidera (including Euraxemydidae ). UA 9769 has a temporal emargination that is narrower posteriorly than in Kinkonychelys and podocnemidids. On the ventral surface, the parietal forms the posterior part of the shallow sulcus palatinopterygoideus.

CONTACTS OF PROCESSUS INFERIOR PARIETALIS: The processus inferior parietalis is very poorly preserved in UA 9769, and lacking a ventral edge in UA 9770. Although the ventral contacts are not determinable, those of the pterygoid, prootic, and supraoccipital are so universal in turtles that their presence is highly likely. Based on the anterior extent of the processus inferior parietalis in UA 9769, it is also very likely that a palatine-parietal contact was present, as in pelomedusids, euraxemydids, and many other Pelomedusoides. On the ventral surface of the right parietal in UA 9769, it can be seen that there is no parietal-pterygoid contact above the processus trochlearis pterygoidei.

STRUCTURES OF PROCESSUS INFERIOR PARIETALIS: As preserved in UA 9769, the processus inferior parietalis of Sokatra is similar to that in pelomedusids, euraxemydids, and most other Pelomedusoides, as this structure does not vary much in this group. The foramen nervi trigemini is identifiable on the left side of UA 9769 and seems to be formed by the usual elements: the parietal anterodorsally, the prootic dorsolaterally, and the pterygoid ventrally.

JUGAL (figs. 1, 2, 5)

PRESERVATION: The lateral plate of the jugal is present on both sides of UA 9769; the sutures are relatively clear except that the maxilla is broken on both sides to a varying extent so the exact position of the cheek emargination is not definite. The medial process is not as well preserved, but most of its sutures can be seen on the left side.

CONTACTS OF LATERAL PLATE: The jugal contacts the maxilla anteroventrally, the postorbital dorsally, and the quadratojugal posterodorsally, as in pelomedusids. The posterior contact with the quadratojugal is V-shaped with the apex pointing posteriorly. As this unusually shaped contact is preserved only on one side of one damaged specimen, it might be considered suspect, but the shape of this suture can be seen on the internal surface as well.

STRUCTURES OF LATERAL PLATE: The jugal of Sokatra is of about the same relative size as in pelomedusids and euraxemydids, but not as large as in Podocnemis . The jugal forms the posteroventral part of the orbital rim. It probably contributes to the anterior part of the cheek emargination, although this is not preserved in any specimen. On the left side of UA 9769 there is only a small part of the jugal missing, and it is likely that this part entered the cheek emargination rather than the quadratojugal sending a process ventrally to meet the maxilla. As seems likely based on this area of UA 9769, the cheek emargination in Sokatra was not as extensive dorsally as in Podocnemis , but very similar to that in pelomedusids and euraxemydids.

CONTACTS OF MEDIAL PROCESS: In the floor of the orbit (in dorsal view), the jugal contacts the maxilla anteriorly and laterally. It probably contacts the palatine medially on the left side of UA 9769, but this is obscured by breakage. In the septum orbitotemporale the jugal in UA 9769 has the usual pelomedusid and euraxemydid contacts with the postorbital dorsomedially, the palatine ventromedially, the pterygoid posteriorly, and the maxilla ventrally.

STRUCTURES OF MEDIAL PROCESS: The jugal in UA 9769 forms part of the fossa orbitalis floor, as in pelomedusids and euraxemydids, and the lateral part of the septum orbitotemporale, as in other Pelomedusoides. The jugal does not extend onto the triturating surface.

QUADRATOJUGAL (figs. 1, 2, 5)

PRESERVATION: The quadratojugal is present only in UA 9769 on the left side and it is incomplete. The dorsal margin is a broken edge, although the edge is very thin and probably did not extend much farther. The ventral margin is mostly broken but a small section of finished margin shows the extent of the cheek emargination. The posterior contact with the quadrate is broken, but the position of most of the suture is determinable.

CONTACTS: The quadratojugal in UA 9769 contacts the quadrate posteriorly, the jugal anteriorly, and, probably, the postorbital anterodorsally. The latter contact is not actually visible but is likely. There is no squamosal-quadratojugal contact in Sokatra , but both Pelusios and Pelomedusa have narrow ones. The absence of a quadratojugal-parietal contact, which appears to be the case in Sokatra , is characteristic of pelomedusids and Araripemys , but it is present in euraxemydids, podocnemidids, and many bothremydids.

STRUCTURES: The quadratojugal in UA 9769 is a flat plate entering the cheek emargination ventrally and the temporal emargination dorsally.

SQUAMOSAL (figs. 1, 2, 5)

PRESERVATION: The squamosal in Sokatra is present on both sides of UA 9769; both are largely complete but damaged with some crushed areas.

CONTACTS: The squamosal in Sokatra contacts the quadrate anteriorly and anteromedially, and the opisthotic medially, on the dorsal, posterior, and ventral surfaces. Due to the temporal emargination there is no contact with the quadratojugal. Sokatra differs from pelomedusids, euraxemydids, and Araripemys in having no contact between the squamosal and quadratojugal above the cavum tympani of the quadrate. In Araripemys and many pelomedusids the contact between these bones is very narrow, but in Sokatra they are widely separated, exposing a substantial length of quadrate in the temporal margin.

STRUCTURES: The squamosal in Sokatra has the usual conical shape, with a large antrum postoticum internally.

POSTORBITAL (figs. 1, 2, 5)

PRESERVATION: The postorbital in Sokatra is present on both sides of UA 9769, but both are incomplete. The left one has a broken posterior margin and the dorsal contact is in a broken area, particularly posteriorly, while the right one is more complete but also has a broken posterior margin.

CONTACTS OF LATERAL PLATE: The postorbital definitely contacts the frontal anteromedially, the jugal ventrally, and the parietal posteromedially. It probably contacts the quadratojugal posteroventrally, but this is not actually preserved.

STRUCTURES OF LATERAL PLATE: The postorbital in Sokatra forms part of the dorsal margin of the orbit. It probably also formed part of the temporal margin posteriorly, but this is broken on both sides. Nonetheless, a small part of the edge of the right postorbital is very thin and seems to be close to or on the temporal margin, showing that there is no quadratojugal-parietal contact and that the postorbital does form part of the temporal emargination. All of these features of the cheek are very similar in Sokatra , euraxemydids, and pelomedusids.

CONTACTS OF MEDIAL PROCESS: In the septum orbitotemporale, facing the fossa orbitalis, the postorbital contacts the frontal dorsomedially and the jugal ventrolaterally. Neither of these is well preserved. Facing the fossa temporalis, the postorbital contacts the parietal dorsomedially, the jugal ventrolaterally, and, probably, the pterygoid ventromedially.

STRUCTURES OF MEDIAL PROCESS: The postorbital forms most of the roof of the sulcus palatinopterygoideus, which is relatively thin as in pelomedusids. It is thicker in euraxemydids. The ventral surface has a shallow concavity forming the roof of the sulcus palatinopterygoideus.