Shweboemys (Jain, 1986)

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 : 164-171

Shweboemys Figures 71–74

This taxon is based on two skulls, GSI 17255 (holotype) and BMNH R.8432. Both skulls are similar in state of preservation: they both lack the otic chambers, most of the occiput, and skull roofing elements covering the fossa temporalis. As we have been able to see only BMNH R.8432, nearly all of the following is based on it. Observations on GSI 17255 are based on the literature ( Swinton, 1939: figs. 1, 2; Wood, 1970: pl. 1, figs. A, B). The internal part of the skull in BMNH R.8432 is filled with matrix and unavailable at present for examination. Of the two skulls, BMNH R.8432 is the more complete.

PREFRONTAL

Preservation: Both prefrontals are preserved and nearly complete in GSI 17255, and BMNH R.8432. The anterior margins in both specimens are broken, although BMNH R.8432, is more complete. The dorsal margin of the apertura narium externa in BMNH R.8432 appears to have only a small amount of bone missing or perhaps nothing is missing at all.

Contacts: The prefrontal in Shweboemys contacts the other prefrontal on the midline, the maxilla anteroventrolaterally, and the frontal posteriorly, as in the other subtribe Stereogenyina .

Structures: As preserved, the dorsal margin of the apertura narium externa in BMNH R.8432 has no anterior protrusion seen in some other subtribe Stereogenyina , e.g., Bairdemys . It is very similar in extent to that seen in Lemurchelys .

FRONTAL

Preservation: Both frontals are present and complete in GSI 17255 and BMNH R.8432. The ventral surfaces are covered by matrix in the latter.

Contacts: The frontal in Shweboemys contacts the other frontal on the midline, the prefrontal anteriorly, the postorbital posterolaterally, the parietal posteriorly, and the palatine ventrally, as in all other subtribe Stereogenyina .

Structures: The frontal forms a section of the dorsal orbital rim, with less exposure than in Brontochelys but more than in Lemurchelys .

PARIETAL

Preservation: The parietals in GSI 17255 and BMNH R.8432 are present only anteromedially; the posterior and lateral portions forming the fossa temporalis covering are broken away.

Contacts and structures of dorsal plate: The parietal in Shweboemys contacts the other parietal on the midline, the frontal anteriorly, and the postorbital laterally. Other cheek contacts are not preserved. The degree of emargination is unknown. In the septum orbitotemporale the parietal contacts the postorbital anteriorly, the palatine anteroventrally, and the pterygoid ventrally (fig. 74F).

Contacts and structures of processus inferi- or parietalis: Only the most posterior part of the processus inferior parietalis is visible in BMNH R.8432. It contacts the pterygoid ventrally and forms the anterodorsal rim of the foramen nervi trigemini.

JUGAL

Preservation: Both jugals are present in BMNH R.8432, but the thin cheek plate covering the fossa temporalis is broken away on both sides, so the presence and extent of the cheek emargination is unknown. The left jugal has visible sutures, but the right one is covered by matrix. The presence and extent of the jugal in GSI 17255 is not known to us.

Contacts of lateral plate: The preserved portion of the jugal in BMNH R.8432 contacts the maxilla anteroventrally and the postorbital dorsally, but the posterior contacts of the lateral plate are unknown.

Structures of lateral plate: The jugal in Shweboemys forms most of the posterior orbital rim, as in Bairdemys but in contrast to the small portion in Cordichelys . The possibility of a cheek emargination is unknown.

Contacts and structures of medial process: In the septum orbitotemporale in posterior view in BMNH R.8432, the jugal contacts the postorbital dorsomedially, the palatine posteromedially, and the maxilla

[M. Rightmyer, del.]

ventrally, as in Stereogenys , but in contrast to all other subtribe Stereogenyina . The wide dorsal process of the palatine prevents a jugal-pterygoid contact. There is no jugal contribution to the triturating surface.

QUADRATOJUGAL

The quadratojugal is not preserved in GSI 17255 and BMNH R.8432.

SQUAMOSAL

The squamosal is not preserved in GSI 17255 and BMNH R.8432.

POSTORBITAL

Preservation: The anterior and medial portions of both postorbitals are mostly preserved in BMNH R.8432. The thin, posterior sheets covering the fossa temporalis are gone on both postorbitals.

Contacts of lateral plate: The preserved contacts of the postorbital are with the frontal anteromedially, the jugal ventrally, and the parietal posteromedially.

Structures of lateral plate: The postorbital makes up the posterodorsal part of the orbital rim. The degree of exposure in Shweboemys is about the same as in Latentemys and Stereogenys .

Contacts and structures of medial process: In the posterior surface of the septum orbitotemporale, facing the fossa temporalis, the postorbital contacts the parietal dorsomedially, the palatine ventromedially, and the jugal ventrally. As in the other subtribe Stereogenyina , the postorbital makes up part of the roof and lateral wall of the sulcus palatinopterygoideus and the dorsal portion of the septum orbitotemporale. The presence of a fossa orbitalis posterior enlargement is not determinable.

PREMAXILLA

Preservation: The premaxilla is present in BMNH R.8432, but both are broken anteriorly. In GSI 17255 the Swinton figure (1939: fig. 2) shows no premaxillae, but the photo in Wood (1970: pl. 1) has what could be interpreted as premaxillae. In any case, the latter specimen is less complete than BMNH R.8432.

Contacts: The premaxilla contacts the maxilla posterolaterally and the other premaxilla medially on the midline.

Structures on dorsal surface: The ventral margin of the apertura narium externa is not preserved and the fossa nasalis is filled with matrix.

Structures on ventral surface: The shape of the premaxilla in BMNH R.8432 seems to show a slight pinching, but the figures of GSI 17255 show no pinching. There is a slight midline concavity not defined posteriorly, but sloping dorsally into the apertura narium interna, similar to that seen in Bairdemys venezuelensis . There are no accessory ridges. The foramen praepalatinum is present on the left side. Only a small part of the labial ridge is present in either skull. It shows a low, blunt edge as is present on the maxillary labial ridge.

MAXILLA Preservation: Both maxillae are present in GSI 17255 and BMNH R.8432. In BMNH

R.8432 the labial ridges are slightly damaged and the posteroventral margin is broken. In GSI 17255 the breakage and missing areas are more extensive.

Contacts of vertical plate: The maxilla as preserved in Shweboemys contacts the premaxilla anteromedially, the jugal posterodorsally, and the prefrontal anterodorsally.

Structures of vertical plate: The ventral margin of the orbital rim is formed by the maxilla and is a relatively sharp ridge, as in all other subtribe Stereogenyina except Lemurchelys . There is no remnant of possible cheek emargination.

Contacts of horizontal plate: The horizontal plate of the maxilla contacts the premaxilla anteromedially, the palatine posteromedially, and the jugal posterolaterally.

Structures of horizontal plate: The triturating surface in Shweboemys , as in Stereogenys , has parallel medial margins forming the cleft-shaped apertura narium interna. However, the length of the secondary palate is shorter in Lemurchelys than in the other two genera. There is a slight convexity on the maxilla in Shweboemys , similar to that in Lemurchelys . The maxilla in Shweboemys is relatively thick and swollen as in Lemurchelys , in contrast to the thin and narrow maxilla in Latentemys and Cordichelys .

VOMER

It can be determined that there is no vomer in Shweboemys .

PALATINE

Preservation: Both palatines are present in GSI 17255 and BMNH R.8432. In both skulls the bones are damaged posteriorly but intact anteriorly.

Contacts: The palatine contacts the maxilla anterolaterally and the pterygoid posteriorly. There is a very slight midline contact with the basisphenoid in BMNH R.8432, separating the pterygoids on the midline, as in Stereogenys but no other subtribe Stereogenyina .

Structures on ventral surface: The palatine is longer than the maxilla in Shweboemys , Brontochelys , and Stereogenys . The palatine surface is slightly concave, as in Latentemys and Cordichelys , but not as deep as in Bairdemys venezuelensis .

QUADRATE

Preservation: Only the medialmost contact area of the left quadrate remains in BMNH R.8432. None of the cheek or skull roof contacts remain. None of the quadrate is apparent in GSI 17255.

Contacts: The only remaining quadrate contacts are with the pterygoid anteromedially, the basisphenoid medially, the basioccipital posteromedially, and what seems to be the prootic medially.

Structures: The medial part of the incisura columellae auris is present on the left side of BMNH R.8432. At this position it is an oval tunnel formed mostly by the quadrate and probably the prootic, although sutures are unclear.

PTERYGOID Preservation: The anterior and medial portions of both pterygoids are present in BMNH R.8432. Only anterior portions seem to be present in GSI 17255.

Contacts on ventral surface: The pterygoid contacts the palatine anteriorly, the basisphenoid posteromedially, and the quadrate posterolaterally. In Shweboemys there is no midline contact with the other pterygoid.

Structures on ventral surface: The processus trochlearis pterygoidei is broken away on both Shweboemys specimens. The posterior portion of the cavum pterygoidei is visible on the left side of BMNH R.8432, and is floored by the pterygoid. There is no sign of a foramen palatinum posterius. See Quadrate for cavum pterygoidei description.

Contacts on dorsal surface: The contacts of the pterygoid in BMNH R.8432 at the base of the processus trochlearis pterygoidei are with the palatine anteriorly and the parietal dorsally. The contacts of the crista pterygoidea are unclear due to matrix and the only contact visible is with the parietal anterodorsally.

Structures on dorsal surface: The sulcus palatinopterygoideus and its associated septum orbitotemporale are formed in BMNH R.8432 by the postorbital, parietal, pterygoid, and the unusually large palatine. As in the other subtribe Stereogenyina , the foramen nervi trigemini in BMNH R.8432 is formed by the parietal anterodorsally, the prootic dorsolaterally, and the pterygoid ventrally.

SUPRAOCCIPITAL

Preservation: Only a small portion of the base of the supraoccipital is preserved in BMNH R.8432, the crista supraoccipitalis is broken away. The supraoccipital seems to be missing in GSI 17255.

Contacts: As in other subtribe Stereogenyina the supraoccipital contacts the parietals dorsally and anteriorly, the prootic anterolaterally, the opisthotic posterolaterally, and the exoccipitals posteroventrally.

Structures: Although most of the structure is broken away, the base of the crista in BMNH R.8432 is not deep as in Lemurchelys .

EXOCCIPITAL

Preservation: Most of the left exoccipital is present in BMNH R.8432. The break is just lateral to the condylus occipitalis and that structure is missing. The exoccipital is missing in GSI 17255.

Contacts: The exoccipital in Shweboemys contacts the supraoccipital dorsally, the opisthotic laterally, and the basioccipital ventrally.

Structures: The foramina nervi hypoglossi are combined into a single recessed opening, as seen in Lemurchelys . The foramen jugulare posterius is closed by bone, which seems to include the opisthotic and basioccipital.

BASIOCCIPITAL

Preservation: The left part of the basioccipital in BMNH R.8432 is mostly present, but it is broken just lateral to the midline, missing the condylus occipitalis. The basioccipital is missing in GSI 17255.

Contacts: The basioccipital in Shweboemys contacts the basisphenoid anteriorly, the quadrate laterally, and the exoccipitals posterodorsally.

Structures: Compared with forms like Latentemys and Cordichelys, BMNH R. 8432 has a tuberculum basioccipitale, similar to the one in those genera.

PROOTIC

Preservation: Most of the left prootic in BMNH R.8432 is present; its lateral margins are broken edges, however. The prootic seems to be missing in GSI 17255.

Contacts: The prootic in Shweboemys contacts the parietal dorsomedially, the quadrate laterally, the supraoccipital posterodorsally, the pterygoid ventrally, the basisphenoid ventrally, and the opisthotic posteriorly, as in other podocnemidids.

Structures: The prootic in Shweboemys is very similar to that bone in the other subtribe Stereogenyina . The medial edge of the foramen stapedio-temporale is preserved. The prootic forms part of the foramen cavernosum in the roof of the cavum pterygoidei, and this can be seen in section on the left side of BMNH R.8432. The canalis cavernosus is visible dorsal to the incisura columellae auris.

OPISTHOTIC

Preservation: The medial part of the left opisthotic is preserved in BMNH R.8432. The opisthotic is missing in GSI 17255.

Contacts: Enough of the contacts can be made out in BMNH R.8432 to see that they are as in other podocnemidids: the supraoccipital anteromedially, the prootic anteriorly, the quadrate anterolaterally, and the exoccipital posteromedially. The squamosal contact is not preserved.

Structures: The opisthotic forms the lateral wall of the foramen jugulare posterius and, probably, the medial wall of the fenestra postotica, although the latter opening is not preserved.

BASISPHENOID

Preservation: The basisphenoid in BMNH R.8432 is cracked down its midline, with the left portion complete and the right portion lacking its distal end. The basisphenoid seems to be missing in GSI 17255, although its anteriormost margin may be present. We don’t know.

Contacts on ventral surface: The basisphenoid contacts the pterygoids anterolaterally, the basioccipital posteriorly, the prootic dorsolaterally (within the cavum pterygoidei), and the quadrate laterally. The basisphenoid in BMNH R.8432 barely contacts the palatines to separate the pterygoids on the midline. The degree of contact varies from side to side; on the left side it is longer than on the right side. This contact differs from that in Stereogenys , which has a broad contact with palatines and widely separates the pterygoids. Cordichelys has a basisphenoid that approaches the palatines and separates the pterygoids for most of their length.

Structures on ventral surface: The basisphenoid shape in Shweboemys is roughly as wide as long, similar to that in Bairdemys venezuelensis and Cordichelys . This differs from the very short basisphenoid seen in Brontochelys and Lemurchelys . In BMNH R.8432 the foramen anterius canalis carotici interni can be seen in the basisphenoid on the left side where the cavum pterygoidei is broken away laterally exposing the medial wall.

Figures

Fig. 71. Shweboemys pilgrimi Swinton, 1939. Partially restored skull based on BMNH R.8432. A, dorsal; B, ventral; C, lateral. [F. Ippolito, del.]

Fig. 72. Shweboemys pilgrimi Swinton, 1939. Partially restored ventral view based on BMNH R.8432.

Fig. 73. Shweboemys pilgrimi Swinton, 1939. BMNH R.8432. A, dorsal; B, ventral; C, right lateral; D, anterior; E, left lateral; F, posterior. [F. Ippolito, del.]

Fig. 74. Shweboemys pilgrimi Swinton, 1939. BMNH R.8432. A, dorsal; B, ventral; C, right lateral; D, anterior; E, left lateral; F, posterior. [F. Ippolito, del.]