Phosphatochelys tedfordi, GAFFNEY, EUGENE S. & TONG, HAIYAN, 2003

GAFFNEY, EUGENE S. & TONG, HAIYAN, 2003, Chapter 26 Phosphatochelys, a New Side-Necked Turtle (Pelomedusoides: Bothremydidae) from the Paleocene of Morocco, Bulletin of the American Museum of Natural History 279 (279), pp. 644-659 : 646-657

Phosphatochelys tedfordi , new species

TYPE SPECIMEN: AMNH 30008, complete skull without lower jaws. Gift from François Escuillié who purchased the specimen in France.

TYPE LOCALITY: Oued Zem, Ouled Abdoun Basin, Morocco.

HORIZON: Thanetian phosphates, late Paleocene. The skull has been dated by shark teeth included in the surrounding matrix which indicate Thanetian age (Cappetta, personal commun.).

DIAGNOSIS: Same as for genus.

ETYMOLOGY: For Dr. Richard H. Tedford, in recognition of his lifelong contributions to vertebrate paleontology in general and to the American Museum of Natural History in particular.

DESCRIPTION

The type skull of Phosphatochelys tedfordi, AMNH 30008 ( figs. 26.1, 26.2 ), has a premaxilla­condyle median length of 68.0 mm, a maximum width of 69.0 mm, and a height from condylus mandibularis of the quadrate to the top of the skull roof of 47.5 mm.

PREFRONTAL

Both prefrontals in AMNH 30008 are present and nearly complete with clear sutures. A small amount of the ventral process seems to be broken on both sides. The left prefrontal is completely clear of matrix but the right has some matrix on its ventral surface.

The prefrontal in Phosphatochelys is a relatively large element, larger even than in Azabbaremys and much larger than in pelomedusids and Araripemys . The prefrontal forms a distinct, anterior process on its ventral surface.

The contacts of the prefrontal in Phosphatochelys are with the maxilla anterolaterally, the frontal posteromedially, the parietal posterolaterally, and with the other prefrontal anteromedially. The parietal­prefrontal contact of Phosphatochelys is unusual and, in fact, unique among pleurodires and cryptodires. It is not even approached by any other bothremydid, which generally have large prefrontals. Unlike most pelomedusids in which the prefrontals meet on the midline for their entire length, Phosphatochelys has a midline length much shorter than its maximum length. The suture with the frontal is strongly convex anteriorly, quite different from the straight suture in Azabbaremys and other bothremydids. The median prefrontal contact length is less than the total prefrontal length, as in chelids, Araripemys , and FR 4922. This may be a primitive condition; however, its absence in Azabbaremys and other close relatives of Phosphatochelys makes this unlikely. The prefrontal in Azabbaremys has a distinct, anterior projection slightly subdividing the apertura narium externus.

Because of its large size and thickness, the prefrontal in Azabbaremys forms much of the roof of the fossa nasalis and fossa orbitalis. On the ventral surface, the prefrontal contacts the parietal. The frontal is not exposed on the ventral surface. The ventral process of the prefrontal contacts the dorsal process of the maxilla as in other pelomedusoides. This contact area is quite narrow in Phosphatochelys in contrast to the broad contact in Azabbaremys . The entire snout area of Phosphatochelys is telescoped in comparison to Azabbaremys .

FRONTAL

Both frontals are present and complete with clear sutures. The frontal in Phosphatochelys is very unusual for pleurodires. It is small in size, widely separated from the orbital margin, and not exposed on the ventral surface in the cavum cranii. The frontal contacts the prefrontal anterolaterally, the parietal posterolaterally, and the other frontal medially.

PARIETAL

Both parietals are nearly complete, but some of the ventral surface is damaged or covered by matrix.

The dorsal plate of the parietal is large as in Azabbaremys , but it is nearly rectangular rather than an irregular quadrangle. It contacts the frontal anteromedially and the prefrontal anterolaterally. As mentioned (see Prefrontal), the broad prefrontal contact is unique among pleurodires and cryptodires. Also unique among pleurodires is the exposure of the parietal in the margin of the orbit. This condition is not approached by any oth­ er pleurodire or cryptodire. Laterally the parietal contacts the postorbital anteriorly and the quadratojugal posteriorly. The quadratojugal­parietal contact only occurs in Phosphatochelys and Taphrosphys among the Bothremydidae . Although it also occurs in FR 4922 and Podocnemididae , (including Hamadachelys ), it seems to be an independent acquisition within the Taphrosphys Group of the Bothremydidae .

The posterior margin of the parietal along with the quadratojugal forms the posterior limits of the posterior temporal emargination. The temporal emargination in Phosphatochelys is not different in extent from Azabbaremys , but the margin in Phosphatochelys is transverse, while in Azabbaremys it is also straight but trends anterolaterally from the midline. This transverse edge is unique in Bothremydidae but a number of taxa ( Rosasia , Arenila , Zolhafah ) are incompletely known in the skull roof.

The processus inferior parietalis in Phosphatochelys , exposed only on the left side, is very thin as in Azabbaremys and enters the foramen nervi trigemini also as in Azabbaremys . The processus inferior parietalis contacts the pterygoid ventrally from the foramen interorbitale anteriorly to the foramen nervi trigemini posteriorly. The parietal also sends a process ventrally on the lateral side of the sulcus pterygopalatinus ( Antunes and de Broin, 1988), which contacts the palatine anteriorly and the pterygoid posteriorly. This process as well as an enclosed sulcus pterygopalatinus is absent in Azabbaremys . As in Azabbaremys , the foramen interorbitale of Phosphatochelys is relatively small compared with other pelomedusoids. Posteriorly the parietal contacts the supraoccipital in a nearly vertical suture above the prootic contact.

JUGAL

Neither jugal is complete, but the left one is missing only part of its posterior edge.

The jugal of Phosphatochelys is widely exposed in the posteroventral margin of the fossa orbitalis, contacting the postorbital dorsally and the maxilla ventrally, all as in Azabbaremys . In Phosphatochelys , however, there appears to be a free posterior edge along the margin of the left jugal, indicating a cheek emargination. The right jugal is damaged posteriorly in this area. On the right side, the quadrate and maxilla meet. This may be due to postmortem distortion, and the rest of the skull also supports this interpretation. It seems likely then, that there was a narrow cheek emargination, as shown in the restored lateral view ( fig. 26.3 ) with part of the jugal exposed on the margin of the emargination. Nonetheless, the dorsal placement of the quadratojugal and the close approximation of quadrate and maxilla is similar to Azabbaremys .

The medial process of the jugal is best preserved on the right side. It is barely visible in ventral view and does not extend onto the triturating surface. The posterior wall of the orbit is absent as in Azabbaremys , so the jugal has only a ventromedial process that reaches the maxilla and palatine in the orbital floor.

QUADRATOJUGAL

The quadratojugal in AMNH 30008 is present only on the right side. The quadratojugal is complete except along its anterior margin where some of its edge has been eroded.

In most turtles the quadratojugal is a large C­shaped element lying along the anterior margin of the quadrate on the cheek. This is the case in bothremydids like Foxemys , but in Azabbaremys and Phosphatochelys , the quadratojugal lies well above the main body of the quadrate and there is a jugal­quadrate contact. Azabbaremys and Phosphatochelys differ, however, in the shape and position of the postorbital. In Phosphatochelys the postorbital is small and lies anterior to the quadratojugal, while in Azabbaremys it is large and lies above the quadratojugal.

The quadratojugal in Phosphatochelys contacts the parietal medially, the postorbital anteriorly, the quadrate ventrolaterally, and the squamosal posteroventrally. The area of the postorbital contact is damaged, but the fact of the contact is not doubtful. A quadratojugal­parietal contact also occurs in the podocnemidids Erymnochelys and Peltocephalus , but among bothremydids it is definitely known only in Taphrosphys . In Taphrosphys , the quadratojugal is more extensive ventrally, but whether or not a jugal­quadrate

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contact is present is unknown so far. It is the pattern of a dorsally placed quadratojugal seen in such bothremydids as Taphrosphys , Phosphatochelys , Azabbaremys , and some undescribed species that argues for the identification of this bone as a quadratojugal.

SQUAMOSAL

Both squamosals are present, the right one is complete, the left one lacks some of its anterior process. The squamosal in AMNH 30008 is the usual cone­shaped element lying on the posterolateral corner of the quadrate. It contacts the opisthotic medially on the dorsal, medial, and ventral surfaces. A short process of the squamosal contacts the quadratojugal along the lateral edge of the temporal embayment as in Azabbaremys . In Phosphatochelys , there is a ventral, parasagittal flange or process extending from the posteroventral part of the bone. This is absent in Azabbaremys , but occurs in Taphrosphys .

POSTORBITAL

The postorbital is present on both sides of AMNH 30008. The left one is nearly complete, only some of its posterior edge is damaged, but the right postorbital is damaged with its posterior margin missing. The internal surface is visible on the left side but only partially on the right.

The postorbital of Phosphatochelys is a roughly square element, contacting the parietal medially, the quadratojugal posteriorly, the quadrate posteroventrally, and the jugal anteroventrally. The absence of a frontal contact, due to the prefrontal­parietal contact, is unusual and seems to be unique to Phosphatochelys . The short postorbital of Phosphatochelys is completely separated from the posterior temporal emargination by the parietal­quadratojugal contact, quite different from the long postorbital of Azabbaremys that reaches the temporal emargination. It is possible that a very narrow postorbital­prefrontal contact was present, but as preserved, the right side is eroded and the left side is broken at this point. Thus we have restored the postorbital with no prefrontal contact and a narrow orbital exposure of the parietal. We consider this to be equivocal, however.

In most bothremydids, the postorbital has a medial wall that contacts the jugal and palatine to form a strong posterior wall for the fossa orbitalis. In Phosphatochelys , Azabbaremys , and Taphrosphys , however, this wall is absent. In Phosphatochelys , the medial surface of the postorbital has a vertical ridge continuous with one from the jugal that represents the fossa orbitalis margin. A much lower ridge is present in Azabbaremys .

PREMAXILLA

Both premaxillae are present in AMNH 30008, but the right one lacks the labial ridge.

The premaxilla in Phosphatochelys forms a deep, acute labial ridge with a median notch in contrast to the hook seen in Azabbaremys . In Azabbaremys and many pelomedusoids, the ventral rim of the apertura narium externa protrudes well anterior to the labial ridge. But in Phosphatochelys , the lower rim of the apertura is distinctly recessed, unlike any other bothremydid, so that the labial ridge is the anteriormost part of the skull. In Azabbaremys , there is a median ridge with low troughs on either side communicating with the apertura narium externa. In Phosphatochelys , there is also a median ridge but it is acute, not blunt as in Azabbaremys , and there are no troughs. Phosphatochelys does have a low concavity on the premaxilla that produces the recessed shape of the lower rim of the apertura. Below the concavity on the anterior face in Phosphatochelys , there is a distinct pattern of the nutrient foramina presumably for the horny beak. These occur in a band along the ventral edge of the labial ridge, premaxilla, and maxilla that is slightly raised above the more dorsal parts of these bones. In Azabbaremys , Taphrosphys , and other bothremydids, the nutrient foramina are not so prominent on the outer surface of premaxilla and maxilla. The horizontal plate of the premaxilla forms the floor of the fossa nasalis, and in Phosphatochelys , these form an acute dorsal ridge on the midline absent in Azabbaremys .

On the ventral surface, the premaxilla in Phosphatochelys forms the anterior part of the very high and acute labial ridge, higher than in Azabbaremys or any other bothremydid. Also in contrast to Azabbaremys and other bothremydids, the horizontal part of the triturating surface in Phosphatochelys is very narrow, particularly on the premaxilla, where it is hardly developed at all. The lingual ridge might be identified as the very low, rounded edge separating the triturating surface proper from the median concavity that leads into the apertura narium interna. This concavity is large in Phosphatochelys , wider and more open posteriorly in comparison to Azabbaremys .

MAXILLA

Both maxillae are present in AMNH 30008 and are slightly damaged. The left maxilla is nearly complete but has a horizontal break running through the main body; not much bone seems to be missing, however. The right maxilla is missing its anteriormost edge and has a broken area below the orbit. Both have the internal portion preserved.

In lateral view, the maxilla of Phosphatochelys contacts the premaxilla anteriorly, the prefrontal anterodorsally, and the jugal posterodorsally. It is more curved, convex anterolaterally, than the very flat maxilla of Azabbaremys , and has a relatively larger fossa orbitalis and apertura narium externa than in Azabbaremys . As preserved, the maxilla contacts the quadrate on the right side. Based on the apparent free edges of the maxilla and jugal on the left side and some distortion on the right side, we have restored Phosphatochelys with a narrow cheek emargination. The prefrontal­maxilla contact is best preserved on the left side. It is much narrower than in Azabbaremys .

The medial parts of the maxilla help form the fossa nasalis anteriorly and the fossa orbitalis posteriorly. The fossa nasalis is relatively large for a bothremydid and does not have the posterolateral pocket seen in Azabbaremys . Along the ventrolateral margin of the fossa nasalis is a deep groove beginning as a shallow groove on the premaxilla and running posterolaterally to the fossa orbitalis. This groove has two large foramina in it that open ventrolaterally into the main body of the maxilla. In Podocnemis and Pelusios, Albrecht (1976) described the canalis infraorbitalis and canalis alveolaris superior system that connects a series of foramina and contains the supramaxillary and superior alveolar arteries. The groove and foramina in Phosphatochelys seem to be part of this system. Although we have not seen it developed to this extent in any other pelomedusoids, other taxa do have foramina in the same place in the fossa nasalis that communicate with the alveolar canals.

In the floor of the fossa orbitalis, the horizontal plate of the maxilla contacts the palatine posteromedially and the jugal posterolaterally. The lower border of the orbital opening is relatively high above the orbital floor, resulting in a deep pocket, deeper than seen in Azabbaremys and much deeper than seen in Bothremys , Foxemys , Podocnemis , and pelomedusids.

The ventral portion of the maxilla bears the triturating surface and forms part of the palate. The labial ridge of Phosphatochelys is much deeper than in Azabbaremys or other bothremydids. The snout is unusually foreshortened and the labial ridge is very deep, resulting in a deep, horseshoe­shaped space. The flat portion of the triturating surface width is very small and not distinctly separated from the labial ridge or the lingual edge. The lingual ridge barely exists; it is just the rounded margin for the apertura narium interna. The triturating surface has the usual nutrient foramina but it is smooth, not corrugated as in Azabbaremys .

In ventral view, the maxilla contacts the premaxilla anteromedially, the jugal posteriorly, and the palatine posteromedially. The area around the premaxilla­maxilla suture is broken on both sides as the suture approaches the apertura narium interna. The vomer, presumably present, is missing and it is possible that the maxilla contacted the vomer; if not, it did come close to it. The jugal does not extend onto the triturating surface in Phosphatochelys . The palatine contact is also farther from the triturating surface than it is in Azabbaremys .

VOMER

There is no vomer present, possibly due to postmortem loss.

PALATINE

Both palatines are present but are missing some of the anterior margin, although the right seems to be nearly complete. Most of the dorsal surfaces are visible except posteromedially.

In ventral view ( fig. 26.4 ), the palatine in Phosphatochelys is smaller than the unusually large palatine of Azabbaremys . The palatine­pterygoid suture is roughly transverse in Phosphatochelys , not concave anteriorly as in Azabbaremys . The edges of the apertura narium interna in Phosphatochelys are not completely preserved, but they seem to be more circular rather than triangular as in Azabbaremys . As in Azabbaremys , only a small part of the palatine enters onto the triturating surface. The palatine forms the roof of the choanal opening into the mouth, the apertura narium interna, and in Phosphatochelys , Azabbaremys , and Nigeremys , this roof is highly arched dorsally in contrast to the flatter surface of other bothremydids. The foramen palatinum posterius in Phosphatochelys is formed almost entirely by the palatine, but is very close to the pterygoid suture, and a small spur of the pterygoid may enter the foramen. The foramen palatinum posterius of Phosphatochelys is in a comparable position to that seen in Azabbaremys , but it is much more medial and closer to the apertura narium interna in Phosphatochelys than in Azabbaremys or other bothremydids.

The dorsal surface of the palatine is complex, as it is involved in the fossa orbitalis, the sulcus pterygopalatinus, and the postorbital wall. In Phosphatochelys as in Azabbaremys and Taphrosphys , the postorbital wall, so prominent in other bothremydids, is largely absent. In dorsal view, the palatine of Phosphatochelys is similar to Azabbaremys in forming most of the floor of the orbit and in having a broad anterolateral maxilla contact and a broad lateral jugal contact. The surface of the orbital floor is deeply concave, even more than in Azabbaremys . In contrast to Azabbaremys , Phosphatochelys has a completely enclosed sulcus pterygopalatinus. In Azabbaremys , the lateral wall of the sulcus is gone; in Phosphatochelys it is narrow but complete. Medially the palatines in Phosphatochelys contact each other in a relatively short suture, which may have been longer before postmortem damage.

QUADRATE

Both quadrates are nearly complete. The left one lacks its anterior edge and the right one has some breakage along the jugal­maxilla contact.

The cavum tympani in Phosphatochelys has a completely enclosed incisura columellae auris as in Azabbaremys and most bothremydids. Also as in Azabbaremys , the cavum is hemispherical and lacks a precolumellar fossa. However, Phosphatochelys has a well­developed antrum postoticum, larger than in Taphrosphys sulcatus , which is in strong contrast to Azabbaremys , which completely lacks an antrum postoticum. At the posterior margin of the cavum tympani in Phosphatochelys is a groove for the eustachian tube as in Azabbaremys . In Azabbaremys , this groove is open, but in Phosphatochelys , there is a well­developed ventral process or overhang that partially encloses the eustachian tube. As in Azabbaremys , there is a shelf along the ventral margin of the cavum tympani. It is very similar in size and shape in both Phosphatochelys and Azabbaremys .

In lateral view the quadrate of Phosphatochelys contacts the jugal anteriorly, the postorbital anterodorsally, the quadratojugal dorsally, and the squamosal posterodorsally. As preserved, there is a maxilla contact anteroventrally and this may have been the case in life although we have interpreted the presence of a narrow cheek emargination (see Maxilla).

In posterior view, the quadrate contacts the squamosal dorsolaterally, the opisthotic dorsally, the exoccipital medially, and the basioccipital ventromedially (not quite visible in occipital view). The quadrate and opisthotic combine to form a fully enclosed fenestra postotica, very similar to that in Azabbaremys . In the shelf leading into the fenestra, both dorsally and ventrally, are low spurs suggesting a division of stapedial artery and lateral head vein, but not distinct ridges as seen in Taphrosphys . As in Azabbaremys , the fenestra postotica of Phosphatochelys is widely separated from the foramen jugulare posterius by a well­developed opisthoticquadrate contact. In Phosphatochelys and Azabbaremys , the quadrate contacts the exoccipital and basioccipital ventral to the foramen jugulare posterius. On the posterior surface of Phosphatochelys is a continuation of the groove, apparently for the eustachian tube, from the cavum tympani. This groove is roughly horizontal and is well defined dorsally by a shelf that has a slight ventral ridge resulting in an overhang partially enclosing the groove. Medially the groove flattens out and disappears.

In ventral view, the quadrate of Phosphatochelys contacts the pterygoid anteromedially, the basisphenoid medially, and the basioccipital posteromedially, as in Azabbaremys and other bothremydids. The quadrate and the basioccipital form the tuberculum basioccipital, which is higher and more prominent than in Azabbaremys . As in Azabbaremys , there is a narrow contact with the basisphenoid between the broader basioccipital and pterygoid contacts. The quadrate contacts the quadrate ramus of the pterygoid in a suture extending from the basisphenoid along the processus articularis of the quadrate as in Azabbaremys and other pleurodires.

The foramen posterius canalis carotici interni is formed in the pterygoid­quadrate suture, but more is formed by the quadrate than the pterygoid. In contrast to Azabbaremys , there is no contribution from the basisphenoid. There is a distinct groove on the quadrate leading anteriorly into the foramen posterius canalis carotici interni.

The processus articularis of the quadrate in Phosphatochelys is longer than in Azabbaremys so that the condylus mandibularis is much farther from the plane of the palate in Phosphatochelys than it is in Azabbaremys . This is presumably related to the very deep labial ridge in Phosphatochelys , although a lower jaw will be needed to demonstrate this.

The anterior face of the otic chamber can be seen on both sides, but it is best exposed on the left side. The quadrate enters into the margin of the foramen nervi trigemini along its posteroventral edge between the prootic and pterygoid. In Azabbaremys the quadrate does not enter the foramen nervi trigemini as preserved, but this region is damaged on both sides, and originally the quadrate may have been present. The foramen stapedio­temporalis in Phosphatochelys is very close to the foramen nervi trigemini, being separated from it only by narrow process of quadrate below and prootic above. The foramen stapedio­temporalis, formed ventrally by the quadrate and dorsally by the prootic, in Phosphatochelys is completely on the anterior surface of the otic chamber and not visible in dorsal view. This area is somewhat eroded in Azabbaremys , but to the extent it is preserved, it is very similar to Phosphatochelys . There is a strong ridge that begins dorsally along the anterior margin of the foramen nervi trigemini and extends posteroventrally straight to the medial margin of the condylus mandibularis. This ridge is absent in Azabbaremys . On the anterior surface of the quadrate in Phosphatochelys is a low ridge, lateral to the foramen stapedio­temporalis, that runs roughly dorsoventrally and is presumably related to muscle attachments.

In dorsal view, the quadrate in Phosphatochelys contacts the prootic medially, the opisthotic posteromedially, and the squamosal posterolaterally. As in Azabbaremys and Taphrosphys , there is no quadrate­supraoccipital contact that is seen in the Bothremys Group.

PTERYGOID

Both pterygoids are present and nearly complete. The processus trochlearis pterygoidei is displaced on the right side, but intact on the left. In ventral view, the pterygoid contacts are as in other bothremydids: the palatine anteriorly, the quadrate posterolaterally, the basisphenoid posteromedially, and the other pterygoid medially. The midline pterygoid contact is slightly longer than in Azabbaremys .

The foramen posterius canalis carotici interni (see Quadrate) lies in the pterygoid­quadrate suture midway between the basisphenoid and lateral edge of the quadrate. Nonetheless, the foramen is in a similar position in Azabbaremys despite the fact that the basisphenoid enters the foramen in Azabbaremys . The basisphenoid is much wider in Azabbaremys than in Phosphatochelys and that may be a factor. The pterygoideus muscle scar is weakly developed in Azabbaremys , but it is absent in Phosphatochelys . There is no indication at all of its presence. However, there is a dorsally directed trough formed by pterygoid and quadrate in Phosphatochelys that may have held an insertion for the M. pterygoideus. This trough has a sharp ridge on the quadrate as its lateral margin and the crista pterygoidea and processus inferior parietalis as its medial limits. It trends posteroventrally from the parietal down to the condylus mandibularis and is open anteriorly. As it reaches the ventral edge of the pterygoid, its posterior wall ends and only the lateral ridge reaches the condylus mandibularis. Thus, in ventral view, the end of the trough lies behind the processus trochlearis pterygoidei just lateral to the thin web of bone behind the processus. This structure developed to the extent seen in Phosphatochelys is not seen in other pleurodires. It is possible that this trough contains a division of the M. pterygoideus ( Schumacher, 1973), probably either the pars ventrolateralis, or possibly the pars ventroposterior (seen in Podocnemis ).

The processus trochlearis pterygoidei of Phosphatochelys is relatively well preserved on both sides; in many pleurodire fossils it is damaged due to the thin bone. In Azabbaremys , only part of one is preserved, but it is similar to that in Phosphatochelys . The processus in both genera extends posteriorly at an angle of roughly 30 ° from the midline. There is a low ridge along the ventrolateral margin of the processus in Phosphatochelys also as in Azabbaremys , but absent in pelomedusids. The base of the processus trochlearis pterygoidei contacts the parietal anterodorsally and the palatine anteroventrally. The foramen palatinum posterius (see Palatine) is formed almost entirely by the palatine, with a very narrow contribution from the pterygoid.

The crista pterygoidea in Phosphatochelys is completely covered with matrix on the right side and only partially exposed on the left. But the left side does show the anterior margin and the foramen nervi trigemini clearly. The crista pterygoidea in Phosphatochelys is narrow in comparison to Bothremys Group taxa, but it is not as narrow as in Azabbaremys . In Azabbaremys , both the processus inferior parietalis and the crista pterygoidea are roughly half the width of these walls in Phosphatochelys . The posterior edge of the crista pterygoidea forms the anteroventral margin of the foramen nervi trigemini in Phosphatochelys , as in Azabbaremys . In Phosphatochelys , in contrast to Azabbaremys , there is a high, thin ridge along the anterior margin of the foramen nervi trigemini, separating it from the more anterior part of the crista pterygoidea.

SUPRAOCCIPITAL

The supraoccipital in AMNH 30008 is nearly complete and well preserved, with all sides visible. The supraoccipital of Phosphatochelys underlies the two parietals on the midline but not to the extent seen in Azabbaremys , presumably related to the shorter parietals in Phosphatochelys , because the supraoccipitals in both Phosphatochelys and Azabbaremys are quite similar. The crista supraoccipitalis is complete in Phosphatochelys and short as in Azabbaremys , extending only slightly past the level of the condylus occipitalis. Phosphatochelys has a slightly shorter crista than in Azabbaremys . The blade of the crista is deeper in Azabbaremys than Phosphatochelys , but this seems to be related to the larger foramen magnum of Phosphatochelys , in turn presumably related to its smaller size.

The laterally projecting otic portion of the supraoccipital in Phosphatochelys contacts the prootic anterolaterally, the opisthotic laterally, and the exoccipital posterolaterally.

EXOCCIPITAL

Both exoccipitals are preserved in AMNH 30008, complete, free of matrix, and with clear sutures.

The exoccipital in Phosphatochelys forms all of the condylus mandibularis; the basioccipital enters the neck of the condyle and almost reaches the articulation surface, in contrast to the condyle in Azabbaremys where the basioccipital is more posterior. The exoccipitals are slightly eroded on the midline, or perhaps were not fully ossified, giving the occipital condyle a bilobate appearance as in BMNH R 16370, the type specimen of Azabbaremys moragjonesi . The foramen jugulare posterius is formed mostly by the exoccipital, with between a third and a half being formed by the opisthotic, in contrast to Azabbaremys , in which only a very narrow process of the opisthotic enters the foramen margin. The foramen jugulare posterius is entirely enclosed by bone, as in Taphrosphys , Arenila , and Bothremys , but in contrast to the open condition of Foxemys and Polysternon .

Between the foramen jugulare posterius and the condylus occipitalis are the two foramina nervi hypoglossi entirely formed by the exoccipital. Their positions differ significantly in Phosphatochelys and Azabbaremys . In Phosphatochelys the more medial foramen lies on the roughly flat posterior surface of the exoccipital and opens posterolaterally and is clearly visible in posterior view. The more lateral foramen lies within the entry to the foramen jugulare posterius and is so far within the margin that it is completely concealed in posterior view on the right side and only barely visible on the left. The more medial foramen nervi hypoglossi in Phosphatochelys is larger in diameter than the more lateral one. In Azabbaremys , both foramina are very close to one another, are the same size, and neither are close to the foramen jugulare posterius.

The exoccipital in Phosphatochelys contacts the supraoccipital dorsally, the opisthotic laterally, the quadrate ventrolaterally, and the basioccipital ventrally, as in Azabbaremys and other bothremydids.

BASIOCCIPITAL

The basioccipital in AMNH 30008 is complete and clearly defined. The basioccipital in Phosphatochelys contacts the basisphenoid anteriorly, the quadrates laterally, and the exoccipitals posteriorly, as in Azabbaremys . Phosphatochelys has a broadly curved anterior margin rather than the straight suture seen in Azabbaremys . The basisphenoid contact is smaller and the quadrate contact more extensive in Phosphatochelys . The basioccipital in Phosphatochelys makes up the medial half of the very low tuberculum basioccipitale. Between the paired tubercula is a median concavity that is deeper and more clearly defined than in Azabbaremys . The concavity in Phosphatochelys is almost exactly coincident with the basioccipital.

PROOTIC

Both prootics are present and complete; the left one is free of matrix, but the right one is covered anteromedially. Both have clear sutures.

The prootic in Phosphatochelys contacts the supraoccipital posteromedially, the parietal medially, the pterygoid ventrally (internal to the foramen nervi trigemini), the quadrate laterally, and the opisthotic posterolaterally. The prootic forms the dorsomedial margin of the foramen nervi trigemini, the parietal forms its anterodorsal margin, the pterygoid forms its anteroventral margin, and the quadrate its posteroventral margin. The foramen nervi trigemini is visible and complete on the left side of AMNH 30008 as well as the foramen stapedio­temporale. The two foramina are very close together in Phosphatochelys and only separated by contact of the prootic and quadrate. The prootic forms the dorsal half of the foramen stapedio­temporale, which lies entirely on the anterior face of the otic chamber and is not visible in dorsal view. This is the same condition in Azabbaremys , Taphrosphys , Bothremys , Rosasia , and Foxemys . The quadrate forms the lower half of the foramen stapedio­temporale.

OPISTHOTIC

Both opisthotics are complete and well preserved with clear sutures in AMNH 30008. The opisthotic in Phosphatochelys has the usual bothremydid contacts: supraoccipital dorsomedially, prootic anteromedially, quadrate anterolaterally, squamosal posterolaterally, quadrate (again) ventrolaterally, and exoccipital posteromedially.

The opisthotic forms the roof of the fenestra postotica, the ventral and greater portion being formed by the quadrate. The fenestra in Phosphatochelys is oblong with small ridges distally presumably indicating the positions of the stapedial artery and lateral head vein (see Quadrate). The processus interfenestralis of the opisthotic forms the wall between the fenestra postotica and foramen jugulare posterius, contacting the quadrate ventrally. This wall is quite thick in Phosphatochelys , as it is in Azabbaremys and also Bothremys .

BASISPHENOID

The basisphenoid is complete and clearly defined in AMNH 30008, but its dorsal surface is covered with matrix. The basisphenoid of Phosphatochelys is roughly triangular but nearly pentagonal, similar to Taphrosphys and distinct from Azabbaremys . It has a straight posterior contact with the basioccipital, posterolateral contacts with the quadrates, and anterolateral contacts with the pterygoids. The basisphenoid in Phosphatochelys is about as wide as long in contrast to the very short and wide basioccipital in Azabbaremys .

Figures

Fig. 26.1. Phosphatochelys tedfordi, n. gen. and n. sp., AMNH 30008, Phosphates of Oued Zem, Ouled Abdoun Basin, Thanetian, late Paleocene, Morocco. Skull: A, ventral view; B, dorsal view; C, right lateral view; D, occipital view; E, left lateral view; F, anterior view.