Stereogenys, Andrews, 1901
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publication ID |
https://doi.org/ 10.1206/350.1 |
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persistent identifier |
https://treatment.plazi.org/id/C95DDC2B-FF2E-5ECD-FE9B-A4F59ABBD63B |
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treatment provided by |
Felipe |
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scientific name |
Stereogenys |
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Stereogenys Figures 75–82 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig
PREFRONTAL
Preservation: In DPC 4120 both prefrontals are complete except for parts of the ventral processes. BMNH R.3007, BMNH R.3190, and AMNH 14736 have the prefrontal area preserved but no visible sutures and no internal information. BMNH R.3189 has both prefrontals preserved, pitted on the dorsal surface but some sutures visible. BMNH R.3191 preserves none of the prefrontal.
Contacts: In Stereogenys the prefrontal contacts the other prefrontal on the midline, the maxilla anteroventrolaterally, the frontal posteriorly, and the palatine ventrally, as in the other subtribe Stereogenyina .
Structures: The dorsal plate in Stereogenys is flat with an anteriorly protruding dorsal margin of the apertura narium externa that, based on the relatively complete nose of AMNH 14736, does not overlap the premaxillae in dorsal view. The prefrontal forms the anterodorsal margin of the orbital rim, as in the other subtribe Stereogenyina .
FRONTAL
Preservation: In DPC 4120 the right frontal is complete, but the left one lacks its lateral margin. BMNH R.3007 and AMNH 14736 have the frontal area preserved, slightly crushed, and no visible sutures, no internal information. BMNH R.3189 has both frontals preserved, pitted and crushed on the dorsal surface, but some sutures are visible. BMNH R.3190 has both frontals preserved, pitted and crushed on the dorsal surface, with no sutures visible. BMNH R.3191 preserves none of the actual frontal bone, but probably has some of the posteroventral imprint of the frontal sulcus olfactorius.
Contacts and structures: The frontal in Stereogenys contacts the other frontal on the midline, the prefrontal anteriorly, the postorbital posterolaterally, and the parietal posteriorly. On the ventral surface the posteroventral process of the frontal contacts the dorsal process of the palatine. On the ventral surface, the frontal forms the sulcus olfactorius, which can be seen in both endocasts (figs. 79, 80, 82).
PARIETAL
Preservation: In DPC 4120 both parietals lack the dorsal plate over the fossa temporalis and preserve only the sections over the cavum cranii. The ventral portions are complete. BMNH R.3007 has the parietal area preserved, slightly crushed, and no visible sutures. The right parietal is complete; the left is broken laterally. AMNH 14736 has the parietal area partially preserved but badly eroded. BMNH R.3189 has both parietals present anteromedially, pitted on the dorsal surface, but some sutures are visible; they lack the dorsal plate over the fossa temporalis and preserve only the sections over the cavum cranii. BMNH R.3190 has portions of both parietals preserved, pitted and crushed on the dorsal surface, with no sutures visible. The right one lacks the dorsal plate over the fossa temporalis, but the left one has most of the skull roof, slightly crushed, with no sutures. BMNH R.3191 preserves only a small part of the posterolateral vertical wall of the parietal on the left side. It also preserves the endocast of the cavum cranii, largely formed by the parietal.
Contacts of dorsal plate: Based on the well-preserved DPC 4120, the contacts can be seen, but DPC 4120 does not have the skull roofing elements preserved posteriorly, and the other specimens that are complete do not show sutures. As preserved, the parietal in Stereogenys contacts the other parietal on the midline, the postorbital anterolaterally, and the frontal anteriorly. There is probably a quadratojugal contact posterolaterally, and BMNH R.3190 shows a crack in the right place, but the material really is too poorly preserved to be sure. What can be determined from the material is that Stereogenys had a fully roofed skull with only a slight emargination exposing the posterior edge of the otic chamber in dorsal view, much as in Bairdemys venezuelensis .
Contacts and structures of processus inferi- or parietalis: The processus can be seen in DPC 4120 and the endocast made from it. It shows that the contacts in Stereogenys are with the palatine anteroventrally, the pterygoid ventrally, the prootic posteroventrally, and the supraoccipital posteriorly. The processus inferior parietalis forms the medial wall of the sulcus paltinopterygoideus. In the lateral wall, the parietal contacts the postorbital anteriorly, the palatine anteroventrally, and the pterygoid posteroventrally (fig. 78). The foramen nervi trigemini is formed by the parietal anterodorsally, prootic dorsolaterally, and pterygoid ventrally.
JUGAL
Preservation: In DPC 4120 none of the left jugal remains. The right jugal has its medial process and the anterior portion of the lateral plate. BMNH R.3007 has the jugal area preserved on both sides, slightly crushed, with possible sutures, but sutures appear the same as cracks. Both jugals are broken posteriorly. AMNH 14736 has the jugal area partially preserved but badly eroded. BMNH R.3189 has both jugals present medially, pitted and poorly preserved, with only the medial process present. BMNH R.3190 has portions of both jugals preserved, pitted and crushed on the dorsal surface, with no sutures visible. BMNH R.3191 preserves none of the jugal.
Contacts and structures of lateral plate: The posterior extent of the jugal is not known in Stereogenys , but its known contacts are with the maxilla anteroventrally and the postorbital dorsally. The extent of the cheek emargination in Stereogenys is seen in BMNH 3190, and it is nearly absent—less than in Bairdemys venezuelensis .
Contacts of medial process: In the anterior surface of the septum orbitotemporale in
Stereogenys , the jugal contacts the maxilla anteriorly and laterally and the palatine medially. In the posterior surface of the septum orbitotemporale, the jugal contacts the postorbital dorsomedially, the palatine ventromedially, and the maxilla ventrally. The usual pterygoid contact is prevented by a large dorsal process of the palatine that reaches the postorbital. This condition is also in Shweboemys but not in any other subtribe Stereogenyina .
Structures of medial process: The jugal in Stereogenys participates in the formation of the septum orbitotemporale and its enclosed space, the sulcus palatinopterygoideus (fig. 80). The septum orbitotemporale of the subtribe Stereogenyina , as seen in Stereogenys , especially the endocasts (figs. 79, 80, 82), is a relatively thick wall, thicker and longer than the processus inferior parietalis, the wall that actually encloses the cavum cranii proper. Presumably the thick wall and large sulcus are associated with the secondary palate.
QUADRATOJUGAL
Preservation: The quadratojugal in Stereogenys is not preserved in DPC 4120. BMNH R.3007 has the quadratojugal area preserved on the right side, slightly crushed, and no apparent sutures. AMNH 14736 has the quadratojugal area partially preserved on the right side. BMNH R.3189 lacks both quadratojugals. Some of both quadratojugals are probably present in BMNH R.3190, but the areas are poorly preserved and lack sutures. BMNH R.3191 preserves none of the quadratojugal.
Contacts and structures: The few specimens that preserve the area of the quadratojugal have no sutures. A cheek emargination is absent.
SQUAMOSAL
Preservation: The squamosal in Stereogenys is not preserved in DPC 4120. BMNH R.3007 has the squamosal area preserved on both sides, slightly crushed, without sutures. The right side is more complete than the left. AMNH 14736 has both squamosals preserved but eroded. BMNH R.3189 lacks both squamosals. In BMNH R.3190 the right squamosal is probably complete, although poorly preserved, with no sutures, and a portion of the left is probably present. BMNH R.3191 preserves none of the squamosal.
Contacts and structures: None of the specimens of Stereogenys with the squamosal preserved have sutures. The shape of the squamosal is similar to that in Latentemys , there is no vertical flange as seen in Bairdemys .
POSTORBITAL
Preservation: The left postorbital in Stereogenys is missing in DPC 4120, but the right one preserves its medial process and most of the lateral plate. BMNH R.3007 has the postorbital area preserved on both sides, slightly crushed, and lacking sutures. AMNH 14736 has the postorbital area preserved but eroded. BMNH R.3189 probably has portions of both postorbitals in the orbital rims, but sutures are not visible. On the left side of BMNH R.3190 the postorbital area is preserved, but there are no sutures. BMNH R.3191 preserves none of the postorbital.
Contacts of lateral plate: The posterior part of the postorbital is missing in DPC 4120. The contacts as preserved in Stereogenys are with the frontal anteromedially, the jugal ventrally, and the parietal posteromedially. The possible quadratojugal contact is not determinable.
Structures of lateral plate: As preserved, the postorbital of Stereogenys agrees with Bairdemys in the formation of the orbital rim and the absence of a temporal emargination.
Contacts of medial process: In the anterior surface of the septum orbitotemporale in Stereogenys the postorbital contacts the frontal dorsomedially, the palatine ventrally, and the jugal ventrolaterally. In the posterior wall of the septum orbitotemporale, the postorbital contacts the parietal dorsomedially, the jugal ventrolaterally, and the palatine ventrally. The usual pterygoid contact is prevent- ed by the large palatine from reaching the parietal and postorbital, as in Shweboemys .
Structures of medial process: The postorbital in Stereogenys is not actually exposed within the sulcus palatinopterygoideus, although it forms part of the septum orbitotemporale on the outside of the structure. This is also seen in Latentemys , but the material of other subtribe Stereogenyina is not well enough preserved or prepared to allow further comparisons.
PREMAXILLA
Preservation: The premaxilla in Stereogenys is not preserved in DPC 4120. BMNH R.3007 lacks the premaxilla; the apparent presence of them in Andrews (1906: fig. 95) shows only the underside of the skull roof, which appears to be premaxillae. AMNH 14736 has both premaxillae preserved but eroded. BMNH R.3189 has both premaxillae present; they are not well preserved and are crushed dorsomedially. BMNH R.3190 has both premaxillae, complete but partially deformed. BMNH R.3191 preserves none of the premaxilla.
Contacts and structures: DPC 4120 has no premaxilla but shows a sutural margin along the anteromedial edge of the right maxilla. BMNH R.3190 has the best-preserved premaxillae, but they show no definite sutures. There is a relatively thick labial ridge in Stereogenys that forms the apex of an anterior projection of the skull, in contrast to Bairdemys venezuelensis . The triturating surface of the premaxilla has a low accessory ridge that parallels the labial ridge.
MAXILLA
Preservation: In DPC 4120 the left maxilla is missing. The right one is nearly complete but lacks its anteromedial edge and a small portion of its anterior edge. BMNH R.3007 has the maxilla preserved on both sides, slightly crushed, and showing sutures posteriorly ( Andrews, 1906: pl. 25, fig. 2); dorsal and ventral surfaces are visible. AMNH 14736 has both maxillae preserved but eroded. BMNH R.3189 has both maxillae present; they are not well preserved and are crushed dorsomedially, although the left is better preserved and retains the labial ridge. BMNH R.3190 has both maxillae present, and relatively complete, except for posterior cracking and breakage. BMNH R.3191 preserves none of the maxilla.
Contacts and structures of vertical plate: The maxilla in Stereogenys contacts the premaxilla anteromedially, the jugal posterodorsally, and the prefrontal anterodorsally, as in the other subtribe Stereogenyina . There is either no cheek emargination or a very slight one, based on BMNH R.3190.
Contacts of horizontal plate: The contacts are with the premaxilla anteromedially, the palatine posteromedially, and the jugal posterolaterally.
Structures of horizontal plate: The maxilla forms the anterior third of the secondary palate characteristic of the subtribe Stereogenyina . It is much wider than in Bairdemys and lacks the convex swellings seen in B. venezuelensis . The labial ridge in Stereogenys is relatively low and thick, as in Shweboemys and Brontochelys , in contrast to the higher, narrower one in Bairdemys . The choanal passages can be seen in DPC 4120 and the endocast made from it. The passages are mostly in the palatine, but they begin anteriorly in the fossa nasalis, which is formed by the maxilla and premaxilla. The maxilla forms the lateral part of the fossa orbitalis; the orbital rim has a slight raised edge, lower than that seen in Bairdemys venezuelensis .
The maxillary (along with the palatine) postmortem crushing in some of the BMNH specimens led Andrews (1901: 442; 1906: 297) to suggest that Stereogenys may have had a completely closed palate, sutured on the midline. He did state that this was ambiguous, some specimens had a midline cleft and some seemed to be in contact. This was not clarified until Swinton (1939: 551; and later, Wood, 1970) recognized that both the better preserved Shweboemys as well as Stereogenys had a cleft that separated palatines and maxillae on the midline. All three authors suggested that the cleft, open or not, was covered either by a membrane (Swinton, ibid.) or ‘‘broad, horny plates’’ (Andrews, ibid.), producing a secondary palate of the sort seen in mammals (and the cheloniids Osteopygis, Erquellinesia , and Rhetechelys ). Despite the passage of years, it is still not clear whether the palate of the Stereogenyina had a functionally open slit down its midline or whether it was covered with something. The retention of the cleft in this group for a long time period over a fairly large area of marine landscape suggests that it was successful in doing whatever it did. Presumably, if the cleft was open in life, there could have been communication with the choanal passages, allowing some sort of air movement, either for breathing or sensing. Secondary palates have come and gone, evolutionarily speaking, but a complete secondary palate with a median fissure is something unique in tetrapods.
PALATINE
Preservation: In DPC 4120 the right palatine is complete; the left one lacks its anterolateral portion. BMNH R.3007 has the ventral and lateral surfaces of the palatine preserved on both sides, slightly crushed; only the anterior sutures are visible. AMNH 14736 has both palatines preserved but eroded. BMNH R.3189 has both palatines present; they are not well preserved and are crushed dorsomedially, coming in contact on the midline. This was the specimen that led Andrews (1901, ibid.; 1906: ibid.) to suggest that the palate in Stereogenys may have been completely closed. Both palatines of BMNH R.3190 are present but cracked and poorly preserved. BMNH R.3191 preserves the posteromedial parts of both palatines.
Contacts: The contacts in ventral view in Stereogenys are with the maxilla anterolaterally, the pterygoid posteriorly, and the basisphenoid posteromedially. The basisphenoid contact occurs only in Stereogenys and Shweboemys among the subtribe Stereogenyina , and is due to the unusually long basisphenoid. It completely separates the pterygoids on the midline. On the dorsal surface the palatine contacts the processus inferior parietalis. In the floor of the fossa orbitalis, i.e., the anterior surface of the septum orbitotemporale, the palatine contacts the maxilla anterolaterally, the jugal laterally, the postorbital posterolaterally, and the frontal dorsomedially. In the posterior surface of the septum orbitotemporale, the palatine contacts the jugal anteroventrally, the postorbital anterodorsally, the parietal posterodorsally, and the pterygoid posteroventrally. The large dorsal process of the palatine that reaches the parietal and postorbital is found only in Stereogenys and Shweboemys among the subtribe Stereogenyina .
Structures on dorsal surface: In common with the other subtribe Stereogenyina members, Stereogenys has an unusually deep posterior pocket or concavity in the posterior wall of the fossa orbitalis (the septum orbitotemporale). The palatine forms the anterior part of the sulcus paltinopterygoideus.
The dorsal process of the palatine in Stereogenys is completely preserved and visible internally as well as externally. The process is particularly large and massive (possibly related to the extensive secondary palate, a major part of which directly underlies the process). The sulcus palatinopterygoideus, which is lateral to the palatine process is consequently reduced, particularly anteriorly where, instead of a large opening visible in anterior view (as in living pelomedusoides), it is reduced to a small opening far posterior to the postorbital wall itself. The sulcus palatinopterygoideus can be seen clearly in DPC 4120. When seen along its axis from a posteroventrolateral position, the sulcus palatinopterygoideus narrows significantly at its anterior end to become funnel shaped.
Structures on ventral surface: The palatine forms the lateral wall of the choanal passage or groove (fig. 82), and the long secondary palate, characteristic of the subtribe Stereogenyina . Stereogenys and Shweboemys have particularly long secondary palates, even among the subtribe Stereogenyina , and these are mostly formed by posterior extensions of the palatine. The medial edges of the palatine in Stereogenys and Shweboemys and Lemurchelys are parallel, rather than curved as in all the other subtribe Stereogenyina .
QUADRATE
Preservation: The right quadrate is present in DPC 4120 but lacks its anterior cheek plate, and there is some breakage along its posterolateral edge. BMNH R.3007 has the quadrate preserved on both sides; the right is crushed with no cavum tympani visible, the left has a cavum tympani. AMNH 14736 has both quadrates partially preserved but erod- ed. BMNH R.3189 has both quadrates missing, although on the left side the margin of the otic chamber lies on the canalis stapedio-temporale and some parts of quadrate may remain, but no sutures are visible. Most of the right quadrate in BMNH R.3190 is present showing the cavum tympani, but it is poorly preserved. Most or all of the left quadrate in BMNH R.3190 is missing. BMNH R.3191 preserves none of the quadrate.
Contacts on lateral surface: As preserved, none of the lateral surface contacts are preserved in Stereogenys , although the squamosal contact is presumed to be posterodorsal.
Structures on lateral surface: The cheek is not emarginated, although sutures are not visible in the specimens that show this. Only the posterior half of the cavum tympani is decently preserved in Stereogenys (in DPC 4120) and it shows a relatively deep cavum, similar to that in Bairdemys . The incisura columellae auris is completely enclosed along with the eustachian tube, as in the other subtribe Stereogenyina . The antrum postoticum in Stereogenys is very small, much as in Lemurchelys , Bairdemys venezuelensis , and B. hartsteini . The eustachian tube is not separated from the fenestra postotica in Stereogenys as it is in Bairdemys .
Contacts on dorsal and anterior surface: The quadrate in Stereogenys contacts the prootic anteromedially, the opisthotic posteromedially, presumably, the squamosal posteriorly and posterolaterally.
Structures on dorsal and anterior surface: The foramen stapedio-temporale is preserved in DPC 4120 and formed between the prootic and quadrate as in most turtles.
Contacts on ventral surface: On the ventral surface of DPC 4120 the contacts are with the pterygoid anteromedially and the basioccipital posteromedially. There is a very narrow contact with the basisphenoid that is virtually pinched out by the expanded pterygoid.
Structures on ventral surface: The quadrate forms the posterolateral part of the cavum pterygoidei, which appears to be about the same depth in Stereogenys as in Lemurchelys and Bairdemys , but not as deep as in Mogharemys . The cavum pterygoidei in Stereogenys has the cavum cranii opening in its roof via the foramen cavernosum, formed by the pterygoid laterally and the quadrate medially.
Contacts on posterior surface: The quadrate of Stereogenys contacts the squamosal dorsolaterally, the opisthotic dorsomedially, and the basioccipital ventromedially.
Structures on posterior surface: The fenestra postotica in Stereogenys is not separated by bone from the sulcus eustachii as it is Latentemys and Bairdemys . The incisura columellae auris includes the eustachian tube as in the other subtribe Stereogenyina . The foramen chorda tympani inferius is not visible, but may be small and one of a number of small foramina in a damaged area.
PTERYGOID
Preservation: Both pterygoids are present in DPC 4120. The right one is complete except for the pterygoid flange and some breakage around the cavum pterygoidei. The left one is also nearly complete, but lacking the pterygoid flange and with some breakage posteriorly. BMNH R.3007 has the pterygoid preserved on both sides, slightly crushed, lacking sutures, with no internal surfaces visible. AMNH 14736 has both pterygoids preserved but eroded. BMNH R.3189 has portions of both pterygoids present, but they are not well preserved. The right one exposes the cavum pterygoidei; the left one retains the processus trochlearis pterygoidei. Parts of both pterygoids are present in BMNH R.3190 but poorly preserved with no sutures. The left pterygoid retains the processus trochlearis pterygoidei and both have some parts of the pterygoid flange base. BMNH R.3191 preserves the posteromedial parts of both pterygoids.
Contacts on ventral surface: The pterygoid in Stereogenys contacts the palatine anteriorly, the basisphenoid posteromedially, and the quadrate posterolaterally. Due to the anteriorly extensive basisphenoid, which reaches the palatines, the pterygoids do not meet on the midline. This also occurs in Shweboemys but not in any other subtribe Stereogenyina member.
Structures on ventral surface: The processus trochlearis pterygoidei in Stereogenys lies at a right angle to the midline as in the other subtribe Stereogenyina , and has a concavity on its ventral surface at the base. The cavum pterygoidei in Stereogenys has the cavum cranii opening in its roof via the foramen cavernosum, formed by the pterygoid laterally and the quadrate medially, as in Latentemys . The foramen nervi facialis is just anterior to the foramen cavernosum and is largely formed by the prootic. Probably some prootic enters the margin of the foramen cavernosum but this is unclear. The pterygoid forms the lateral rim of the anterior opening of the cavum pterygoidei into the cavum cranii.
The base of the pterygoid flange is preserved on many specimens of Stereogenys , but best seen in DPC 4120. It is a posterior extension of the medial limits of the concavity on the base of the processus trochlearis pterygoidei, parasagittal in position, and reaching the ventral opening of the cavum pterygoidei, as in other subtribe Stereogenyina that are well enough preserved to show these structures.
Contacts on dorsal surface: The posterior surface of the septum orbitotemporale shows these contacts with the pterygoid: the palatine anteroventrally and the parietal dorsally. The postorbital and jugal contacts seen in other subtribe Stereogenyina are prevented by the large dorsal process of the palatine in Stereogenys and Shweboemys . The crista pterygoidea contacts the palatine anteriorly and the parietal dorsally.
Structures on dorsal surface: The processus trochlearis pterygoidei in Stereogenys is similar to that in the other subtribe Stereogenyina . The floor of the sulcus palatinopterygoideus is formed anteriorly by the palatine and posteriorly by the pterygoid. It is more of an enclosed tunnel in the subtribe Stereogenyina than in other podocnemidids because of the thick septum orbitotemporale in this group. The foramen nervi trigemini is formed by the parietal anterodorsally, the prootic posterodorsally, and the pterygoid ventrally, as in other podocnemidids.
SUPRAOCCIPITAL
Preservation: Only the anteroventral part of the supraoccipital from the foramen magnum forward, remains in DPC 4120.
BMNH R.3007, BMNH R.3189, and AMNH 14736 have the anterior part of the supraoccipital preserved, but the crista supraoccipitalis is missing. BMNH R.3190 has the anterior portion as well as some of the crista supraoccipitalis. BMNH R.3191 preserves a small part of supraoccipital just anterior to the foramen magnum on the left side.
Contacts and structures: The supraoccipital contacts the parietal dorsally and anteriorly, the prootic anterolaterally, the opisthotic posterolaterally, and the exoccipital posteroventrally, as in other podocnemidids. The crista supraoccipitalis is missing in the available specimens, but AMNH 14735 (cast of the type) shows an indication of the horizontal shelf on the ventral edge of the crista in the subtribe Stereogenyina . The supraoccipital forms the dorsal edge of the foramen magnum. There is apparently no exposure of the supraoccipital on the skull roof in Stereogenys .
EXOCCIPITAL
Preservation: In DPC 4120 the right exoccipital is complete; the left one lacks its lateral process. BMNH R.3007, BMNH R.3189, and AMNH 14736 have the exoccipital area preserved on both sides, somewhat crushed, the condylus occipitalis is missing, and they lack sutures. BMNH R.3190 has both exoccipitals and part of the condylus occipitalis surface preserved. BMNH R.3191 preserves most of both exoccipitals.
Contacts: The exoccipital in Stereogenys contacts the supraoccipital dorsally, the opisthotic laterally, and the basioccipital ventrally.
Structures: As far as can be determined, the condylus occipitalis is formed equally by the basioccipital and the two exoccipitals. The foramen nervi hypoglossi are two openings medially that join in a recessed foramen to open posteriorly as one foramen, as in other members of the subtribe Stereogenyina . The foramen jugulare posterius is completely closed by bone, but in contrast to forms such as Latentemys , the exoccipitals do not meet to form the bar; the opisthotic is between the two lateral processes of the exoccipital and this closes the opening. Such variation is commonly seen in many recent turtles as well. The exoccipital does not enter the fenestra postotica.
BASIOCCIPITAL
Preservation: In DPC 4120 the basioccipital is complete except for the condylus occipitalis. BMNH R.3007, BMNH R.3189, and AMNH 14736 have the basioccipital area preserved, slightly crushed, and lacking sutures, with the condylus occipitalis missing. BMNH R.3190 has the basioccipital area with no sutures but with the condylus occipitalis preserved. The basioccipital is present but damaged in BMNH R.3191.
Contacts and structures: The basioccipital contacts the basisphenoid anteriorly, the quadrate laterally, the exoccipitals posterodorsally, and the opisthotic dorsolaterally. The basioccipital is relatively short in Stereogenys as in Shweboemys and in contrast to other subtribe Stereogenyina .
PROOTIC
Preservation: In DPC 4120 the right prootic is complete, and the left one lacks its lateral part. BMNH R.3007 and AMNH 14736 have the prootic area preserved on both sides, slightly crushed, with no sutures and no indication of the foramen stapedio-temporale. In BMNH R.3189 the prootic is exposed on the left side showing the canalis stapedio-temporalis and the foramen stapedio-temporale. In BMNH R.3190 the prootic appears to be missing, badly damaged, or covered with plaster on both sides. The left prootic in BMNH R.3191 is present and nearly complete, but the right one is poorly preserved.
Contacts: The prootic in Stereogenys contacts the parietal dorsomedially, the quadrate laterally, the pterygoid ventrally, and the opisthotic posteriorly, as in other podocnemidids. Within the cavum pterygoidei the prootic also contacts the basisphenoid ventrally, the pterygoid laterally, and the quadrate medially.
Structures: The prootic in Stereogenys participates in the formation of the foramen nervi trigemini and the foramen stapedio-temporale, which are formed by the same additional elements as in Bairdemys and Latentemys . Within the cavum pterygoidei the prootic forms the foramen nervi facialis in the roof of the cavum, and probably enters the foramen cavernosum. Within the cavum cranii the prootic forms the fossa acustico-facialis, as in all turtles. The fossa acustico-facialis is unique in Stereogenys among turtles in being so far recessed that it appears to be a canal when seen from inside the cavum cranii. The two small foramina nervi acustici lie dorsally in the canal and the facial nerve is a larger canal extending ventrolaterally.
OPISTHOTIC
Preservation: In DPC 4120 the left opisthotic is missing, but the right one is complete. BMNH R.3007 and AMNH 14736 have the opisthotic area preserved on both sides, with no sutures. In BMNH R.3189 portions of both opisthotics are present, the left one is clearer, but sutures are lacking. BMNH R.3190 has most of both opisthotics, but the areas are poorly preserved. The medial portions of both opisthotics are present in BMNH R.3191.
Contacts: The opisthotic in Stereogenys contacts the supraoccipital anteromedially, the prootic anteriorly, the quadrate ventro- and anterolaterally, probably the squamosal posterolaterally, and the exoccipital posteromedially.
Structures: The opisthotic in Stereogenys enters the lateral margin of the foramen jugulare posterius and separates it from the fenestra postotica.
BASISPHENOID
Preservation: In DPC 4120 the basisphenoid is complete and visible internally as well as externally. BMNH R.3007 and AMNH 14736 and BMNH R.3189 have the basisphenoid area preserved, but no sutures are visible. The area of the basisphenoid in BMNH R.3190 is preserved, but there are no sutures. The basisphenoid is preserved in BMNH R.3191.
Contacts on ventral surface: The basisphenoid in Stereogenys contacts the palatines anteriorly, the pterygoids laterally, the basioccipital posteriorly, and the quadrate posterolaterally in a very narrow suture.
Structures on ventral surface: The large basisphenoid of Stereogenys and Shweboemys roofs much of the posterior part of the skull and forms the roof of the posteriorly displaced apertura narium interna. The basisphenoid forms the medial and anteroventral walls of the cavum pterygoidei, and the foramen posterius canalis carotici interni.
Contacts on dorsal surface: The basisphenoid contacts the pterygoid anterolaterally, the prootic laterally, the exoccipital posterolaterally, and the basioccipital posteriorly.
Structures on dorsal surface: The rostrum basisphenoidale in DPC 4120 is unclear as the anterior edge of the basisphenoid here is broken. The sulcus cavernosus extends from the anterior opening of the cavum pterygoidei forward, as in other subtribe Stereogenyina members. The dorsum sellae is unusual in being almost completely flat, with no clear step separating it from the sella turcica. Just medial to the anterior opening of the cavum pterygoidei lies the foramen anterius canalis carotici interni, connected by a short canalis caroticus internus to the foramen posterius canalis carotici interni, which is in the cavum pterygoidei. Between the anterior opening of the cavum pterygoidei and the foramen anterius canalis carotici interni is a short processus clinoideus. A foramen nervi abducentis was not seen but could be present.
The dorsal surface of the basisphenoid is visible and well preserved in Stereogenys (DPC 4120) and Latentemys (BMNH R.11998). In both the anteriormost limits are missing and in DPC 4120 there may be some erosion of bone surfaces. In Latentemys the rostrum basisphenoidale is fused in front of the sella turcica to form a rodlike projection. In Stereogenys there is no fusion and the trabeculae are very short, barely extending past the foramen anterior canalis carotici interni. The sella turcica in Latentemys is a distinctly formed, oval concavity, similar to Podocnemis , but in Stereogenys the sella turcica appears to be very shallow. The dorsum sellae is relatively high and overhanging slightly in Latentemys , but in Stereogenys it is low and flat, with no midline indentation or concavity at the level of the foramen anterior canalis carotici interni. These paired foramina are relatively close together and hidden in dorsal view in Latentemys . In Stereogenys the foramina canalis carotici interni are visible dorsally and wider apart than in Latentemys . In both specimens the paired processus clinoideus is broken off on each side, but the processus base is preserved and seems to be much smaller in Stereogenys . Unfortunately, there is insufficient material of other taxa to make useful systematic use of these features at present.
The cavum pterygoidei is broken open on the left side of DPC 4120 revealing its internal morphology. Basically its structure in Stereogenys is comparable to that in Podocnemis with the important exception of the lateral head vein. In Podocnemis the lateral head vein is separated by bone from the cavum pterygoidei, and there is a well-defined canalis cavernosus. In Stereogenys (and Latentemys , Erymnochelys , and Peltocephalus ) the lateral head vein enters the cavum pterygoidei in the dorsal roof of the latter just posterior to the foramen for VII and immediately ventral to the aditus stapedio temporalis.
Both the facial nerve and the lateral head vein exit the aditus together in a short canal that should be the canalis cavernosus. This canal, however, is very different in length to the more normal canalis cavernosus seen in Podocnemis . This condition of the lateral head vein entering the cavum pterygoidei also occurs in Latentemys , Erymnochelys , and Peltocephalus (see discussion in character list). The facial nerve and the lateral head vein enter the cavum pterygoidei by means of a joint depression in the roof of the chamber. On the left side of DPC 4120 the break goes through the middle of this depression, between facial nerve and lateral head vein, so that the entry foramen of both is missing. The facial nerve shortly exits (actually it’s entering) while the lateral head vein goes anteromedially to enter the cavum cranii at the extreme anterior end of the cavum pterygoidei.
The bones making up the cavum pterygoidei in Stereogenys are complex but comparable to Podocnemis despite the absence of the lateral head vein in the chamber of Podocnemis . The floor is formed by the pterygoid, which has ridges that attach to the more dorsal elements on each side. Laterally the quadrate contacts the pterygoid posteriorly, and the prootic contacts it more anteriorly. This suture rises anteriorly to reach the roof of the chamber anteromedially, and it enters the edge of the foramen for the entry of the facial nerve and lateral head vein.
On the medial wall of the cavum pterygoidei, the pterygoid is lower and contacts the quadrate posteriorly and the basisphenoid anteriorly. The foramen posterius canalis carotici interni is formed by the basisphenoid ventrally and the prootic at least anterodorsally. The bone making up the posterior rim is not clearly identifiable but seems to be basisphenoid. The prootic-basisphenoid suture is unclear. The anterior opening of the cavum pterygoidei is formed by pterygoid ventrally, basisphenoid ventromedially, and prootic dorsally.
SHELL MORPHOLOGY OF SOUTH AMERICAN LATE CRETACEOUS AND EARLY TERTIARY PODOCNEMIDIDS
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