UCM 53971 was subjected to high-resolution X-ray micro-computed tomography using a North Star Imaging scanner at the University of Texas High-Resolution X-ray Computed Tomography Facility, Austin, Texas, U.S.A. The custom-built scanner employs a gantry configuration based on the North Star Imaging X500 scanner and the detector is a 2048 × 2048 Perkin-Elmer flat-panel. Scanning was performed with 3000 projections over 360°, a voltage of 160 kV, a current of 200µA, and the use of an aluminum filter. 1896 coronal slices were obtained with a voxel size of 28.1 µm. 3D models were generated using the software Amira 2019.2 (https://www.fei.com/software) and reconstructions were obtained through interpolated slice-by-slice segmentation. The 3D models were exported as .ply-files and the software Blender 2.79b (https://www.blender.org) was used to create the images used in the figures. The original set of coronal slices were deposited at UCM and will be made available to qualified researchers. The 3D models generated as part of this study are available at MorphoBank (http://morphobank.org/permalink/?P3919).
The phylogenetic relationships of Uluops uluops were investigated by modifying the third paracryptodiran matrix employed by Joyce and Rollot (2020), which is based on Lyson and Joyce (2011). As our matrix only samples paracryptodires (and two basal testudinatans), our analyses are only suited to address paracryptodiran in-group relationships and provide no formal test of alternative positions for potential paracryptodiran taxa (such as Kallokibotion bajazidi and helochelydrids) with regard to their global position outside or inside of Paracryptodira. The matrix was expanded to include Kallokibotion bajazidi from the Maastrichtian of Romania, as described by Gaffney and Meylan (1992), Perez-Garcia and Codrea (2018), and Martín-Jiménez et al. (2021); the helochelydrid Aragochersis lignitesta Perez-Garcia et al., 2020 from the lower Albian of Spain, as described by Perez-Garcia et al. (2020); the helochelydrid Helochelydra nopcsai Lapparent de Broin and Murelaga, 1999 from Barremian of England, as described by Joyce et al. (2011) and personal observations of the type material housed at the Natural History Museum, London UK; and the helochelydrid Naomichelys speciosa from the Aptian–Albian of Texas, U.S.A., as described by Joyce et al. (2014). 11 new characters were added to the analysis (characters 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, and 107 in our matrix), 6 were modified (16, 19, 27, 28, 80, and 88 in our matrix), and 7 were deleted (characters 24, 31, 38, 69, 75, 84, and 98 from the original matrix of Joyce & Rollot, 2020; see Additional file 2 for justifications). The analyses were subjected to a traditional parsimony analysis using TNT (Goloboff et al., 2008b). 21 characters that form morphoclines were run ordered (characters 6, 14, 16, 18, 27, 28, 31, 34, 39, 40, 41, 46, 48, 60, 63, 80, 88, 95, 97, 98, and 101). The initial analysis was carried out under equal weight and, following the recommendations of Goloboff et al., (2008a, 2018), implied weighting factors of K = 3, K = 6, K = 9, and K = 12 were implemented for a second set of analyses. 1000 random addition sequences were followed by a round of tree bisection reconnection. Trees suboptimal by 10 steps and with a relative fit difference of 0.1 were retained as part of the first search. A tree collapsing rule was implemented with a minimum length of 0. Proganochelys quenstedti was selected as the outgroup.
Nomenclature
We use phylogenetic nomenclature as recently codified by the PhyloCode (Cantino and de Queiroz, 2020). All clade names used herein are highlighted as such through the use of italics.
Our anatomical nomenclature follows that of Gaffney (1972b) with modifications in regard to the terminology of the carotid and facial nerve system as recently summarized by Rollot et al. (2021). Following the rationale outlined by Rollot et al. (2021), however, we introduce two additional terms, which are modifications of previous terms introduced by Rabi et al. (2013b).
Foramen posterius canalis carotici basisphenoidalis—the posterior foramen to the canalis caroticus basisphenoidalis (foramen posterius canalis carotici cerebralis of Rabi et al., 2013b), which serves as an entry for the cerebral artery into the skull. The foramen posterius canalis carotici basisphenoidalis is typically located in the lateral margin of the parabasisphenoid and concealed when the carotid split is not exposed. The foramen posterius canalis carotici basisphenoidalis is exposed on the ventral side of the skull when the branching off of the internal carotid artery into its primary branches is not ventrally covered by bone. We favor the term foramen posterius canalis carotici basisphenoidalis over foramen posterius canalis carotici cerebralis, as the artery traversing the canal may supply blood to more tissues than just the brain and because it restores a connection with a significant body of literature (Rollot et al., 2021).
Foramen posterius canalis carotici lateralis—the posterior foramen to the canalis caroticus lateralis (foramen posterius canalis caroticus palatinum of Rabi et al., 2013b), which serves as an entry for the palatine artery into the skull. The foramen posterius canalis carotici lateralis is located along the pterygoid–parabasisphenoid suture lateral to the foramen posterius canalis carotici basisphenoidalis, and typically exposed on the ventral side of the skull when the branching off of the internal carotid artery into its primary branches is not ventrally covered by bone. We favor the term foramen posterius canalis carotici lateralis over foramen posterius canalis carotici palatinum, as the canal may contain arteries other than palatine artery and because it restores a connection with a significant body of literature (Rollot et al., 2021).
Systematic paleontology
Testudinata Klein, 1760 [Joyce et al., 2020].
Paracryptodira Gaffney, 1975 [Joyce et al., 2021].
Pleurosternidae Cope, 1868 [Joyce et al., 2021].
Uluops uluops Carpenter & Bakker, 1990
Type specimen
UCM 53971 (holotype), a cranium (Carpenter & Bakker, 1990: Fig. 4).
Type locality
Main Breakfast Bench Quarry, Albany County, Wyoming, U.S.A. (Carpenter & Bakker, 1990); Morrison Formation, Tithonian, Late Jurassic.
Referred material and range
None.
Emended diagnosis
Uluops uluops can be diagnosed as a pleurosternid by the presence of a jugal that does not extend deeply ventrally, a plate-like supraoccipital exposure on the skull roof between the parietals, an anteriorly convex nasal–frontal suture, anterior tubercula basioccipitale on the parabasisphenoid, and the exclusion of the exoccipitals from the articular surface of the occipital condyle. Uluops uluops is distinguished from all other pleurosternids by the following combination of features: a reduced lingual ridge anteriorly, a foramen palatinum posterius entirely formed by the palatine, a midline contact of the pterygoids of about 10–40% of their length, a length between orbit and cheek emargination equal to the diameter of the orbit, a maximum combined width of the parietals greater than their length, the presence of a canalis caroticus lateralis, a raised ridge on the dorsal surface of the paroccipital process, and anterior abducens nerve foramina that are entirely formed by the pterygoid.
Description
Skull
The skull of UCM 53971 is well preserved, as most of the preserved bones remain in articulation and show only minor signs of crushing (Figs. 1, 2, 3). However, a substantial part of the right side of the skull is missing, in particular the right maxilla, palatine, postorbital, jugal, quadratojugal, and both premaxillae (Fig. 2B). The left otic capsule is heavily damaged externally and internally by a multitude of cracks. The vomer is slightly displaced towards the left, but its sutural contacts with the surrounding bones are preserved (Fig. 1B). The good preservation of at least one of each paired bone with the exception of the premaxillae allows for detailed observation of every bone and all internal structures.
The skull of UCM 53971 resembles that of baenodds by being relatively short, broad, and highly domed (Figs. 1A and 2A). The orbits are laterally oriented as in Compsemys victa (Lyson & Joyce, 2011). The skull is nearly as long as wide. The length reaches 44.4 mm from the foramen magnum to the anterior tip of the nasals while the width is 39.6 mm between the squamosals dorsal to the cavum tympani, which is the maximum width of the cranium. The lateral surfaces of the skull are vertically oriented (Fig. 2C), as was mentioned by Carpenter and Bakker (1990). The maxilla, jugal, quadratojugal, and quadrate jointly form a distinct cheek emargination that crosses an imaginary line between the lower margin of the orbit and the incisura columella auris (Fig. 2A). The cheek emargination is similarly developed in Pleurosternon bullockii (Evans & Kemp, 1975), but is reduced to absent in Compsemys victa (Lyson & Joyce, 2011), Glyptops ornatus (Gaffney, 1979a) and Dorsetochelys typocardium (Evans & Kemp, 1976). The upper temporal emargination is moderately concave and is mainly bordered by the squamosals and parietals, with the supraoccipital only forming the posteromedial margin. In dorsal view, the skull roof covers most of the otic capsule and the entire foramen stapedio-temporale (Fig. 1A). This arrangement resembles that of Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon bullockii (Evans & Kemp, 1975), but contrasts with Compsemys victa (Lyson & Joyce, 2011), which likely lacks an upper temporal emargination. The crista supraoccipitalis extends only slightly posteriorly to the foramen magnum (Fig. 1A). No cranial scute sulci are visible on the dorsal skull surface. The skull surface is marked by small and irregular tubercles (Fig. 1A), which resemble those of Glyptops ornatus (Gaffney, 1979a), Pleurosternon moncayensis (Perez-Garcia et al., 2021), and Pleurosternon bullockii (Evans & Kemp, 1975), although the latter exhibits a more crenulated like pattern towards the back of the skull roof, and differ from the weakly crenulated pattern of Arundelemys dardeni (Lipka et al., 2006) and Compsemys victa (Lyson & Joyce, 2011).
Nasal
The nasal is moderately large, approximately as long as wide, and overall similar to that of other non-baenodd paracryptodires (Figs. 1A and 2C). In dorsal view, the nasal contacts its counterpart medially for two-thirds of its anteroposterior length. Although the nasals slightly diverge with their posterior margin from the midline, they are not fully separated by the anterior processes of the frontals, as is the case in Pleurosternon bullockii (Evers et al., 2020). On the dorsal skull surface, the nasal contacts the frontal posteromedially, the prefrontal posterolaterally, and the maxilla posteroventrally (Fig. 1A). At its posterior end, the nasal forms a transverse facet that braces the frontal dorsally and ventrally for a short distance. The nasal forms the dorsal margin of the apertura narium externa and roofs the nasal cavity (Fig. 2C). The ventral exposure of the nasal within the nasal cavity is smaller than its external exposure on the dorsal skull roof, as the nasal is partially underlain by the prefrontal. Within the nasal cavity, the nasal contacts its counterpart anteromedially and forms a shallow median ridge that separates the nasal cavity into right and left nasal valves.
Prefrontal
The prefrontal contributes to the formation of the nasal cavity and the anterodorsal margin of the orbit (Figs. 2A and C). The dorsal plate of the prefrontal is exposed dorsally but does not contact its counterpart medially. The ventral exposure of the prefrontal within the nasal cavity is greater than its dorsal exposure, as the prefrontal broadly underlies a portion of the frontal. The dorsal plate contacts the maxilla anteroventrally, the nasal anteromedially, and the frontal posteromedially and posteriorly (Figs. 1A, 2A and C). Within the orbit, the suture between the prefrontal and frontal is strongly interdigitated, forming a W-shaped suture in ventral view. The thin, descending process of the prefrontal extends ventrally to form the anteromedial wall of the fossa orbitalis. The descending process of the prefrontal likely formed the anterodorsal margin of the large foramen orbito-nasale, but damage obscures this region. The descending process contacts the maxilla anterolaterally, the palatine ventrally, and the vomer ventromedially. Other non-baenodd paracryptodires exhibit similar contacts and contributions of the prefrontal to various cranial structures as UCM 53971.
Frontal
The frontal is a subtriangular bone that is about twice as long antero-posteriorly as it is wide mediolaterally at its widest part, which is located at its posterior contact with the parietal (Fig. 1A). The frontal forms an orbital process at its mid-length that inserts between the prefrontal anteriorly and the postorbital posteriorly, thereby preventing these bones from contacting each other (Figs. 1A and 2A). This process contributes to the dorsal margin of the orbit. Compsemys victa (Lyson & Joyce, 2011) differs from that condition as the prefrontal and postorbital contact each other along the dorsal margin of the orbit, preventing the frontal from contributing to the formation of the latter. Anterior to the orbital process of UCM 53971, the anterior processes of the frontal have parallel margins and fully separate the prefrontals from one another (Figs. 1A and 2C). At the anterior end, the frontal margin tapers medially along its contact with the nasal (Fig. 1A). This taper has a similar extent in Dorsetochelys typocardium (Evans & Kemp, 1976) and Glyptops ornatus (Gaffney, 1979a), is more extensively developed in Pleurosternon bullockii (Evans & Kemp, 1975), but is lacking in Compsemys victa (Lyson & Joyce, 2011). The anterior process of the frontal tapers more gradually within the roof of the fossa orbitalis of UCM 53971. Each frontal has a shallow, but well-developed ventral ridge, the crista cranii, which collectively form the sulcus olfactorius.
Parietal
The parietal is about twice as wide posteriorly as anteriorly and consists of a dorsal and a ventral plate (Figs. 1A and 2B). The dorsal plate roofs the braincase and forms most of the weakly developed upper temporal emargination and the posterior limit of the skull roof. In dorsal view, the parietal fully conceals the otic capsule, including the foramen stapedio-temporale (Fig. 1A). A weakly developed upper temporal emargination is also found in Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon bullockii (Evans & Kemp, 1975). On the dorsal skull roof of UCM 53971, the parietal contacts its counterpart along the midline, the frontal anteriorly, the postorbital anterolaterally and laterally, the squamosal posterolaterally, and the supraoccipital posteromedially (Fig. 1A). The ventrally descending vertical plate of the parietal, the processus inferior parietalis, has a broad contact with the epipterygoid anterior to the foramen nervi trigemini, a short contact with the pterygoid posterior to the foramen nervi trigemini, a broad contact with the prootic within the upper temporal fossa, and a broad posterior contact with the supraoccipital within the upper temporal fossa (Fig. 2B). The processus inferior parietalis forms the anterior part of the lateral wall of the cavum cranii, the medial margin of the fossa temporalis, the posterior margin of the foramen interorbitale, and the dorsal and posterodorsal margins of the foramen nervi trigemini (Fig. 2B). The parietal forms a minor anterolateral process that partially underlies the postorbital and that forms a mediolateral ridge visible in the roof of the temporal fossa. This ridge forms the posterior limit of the sulcus palatino-pterygoideus, which is anteriorly bordered by the posterodorsal margin of the fossa orbitalis and which is developed similarly to that of Pleurosternon bullockii (Evers et al., 2020). Posterior to the foramen nervi trigemini, the processus inferior parietalis bears a posteroventrally oriented process that contacts the pterygoid along the posterior margin of the foramen nervi trigemini (Fig. 2B). This process prevents the prootic from participating in the formation of the foramen nervi trigemini, a condition that is also observed in Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021), but not in Compsemys victa (Lyson & Joyce, 2011) and Glyptops ornatus (Gaffney, 1979a).
Postorbital
The postorbital is an elongate, plate-like bone that is about twice as long as broad and that contributes to the formation of the dorsolateral skull roof (Figs. 1A and 2A). The postorbital contacts the jugal anteroventrally along a sinusoid-shaped suture, the quadratojugal posteroventrally, the frontal anterodorsally, the parietal mediodorsally, and the squamosal posteriorly (Figs. 1A and 2A). The broad contact between the parietal and squamosal posterior to the postorbital prevents the latter from contributing to the formation of the upper temporal emargination (Fig. 1A). The postorbital forms the posterior margin of the orbit and the posterior wall of the fossa orbitalis (Fig. 2A). Within the fossa orbitalis, the postorbital has a flat, slightly oblique contact with the jugal. The medial process of the jugal prevents any contact of the postorbital with the maxilla, pterygoid, or palatine. Together with the jugal, the postorbital forms a robust posterior wall of the fossa orbitalis, reminiscent of the condition seen in pleurodires and trionychids, although the postorbital reaches the pterygoid and palatine bones in these taxa (Gaffney, 1979b). This extended posterior wall of the fossa orbitalis is also present in Compsemys victa (Lyson & Joyce, 2011), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon bullockii (Evers et al., 2020).
Jugal
The jugal forms the posteroventral portion of the fossa orbitalis and makes a small contribution to the posteroventral margin of the orbit (Figs. 2A and C). This contribution to the orbit is also present in Compsemys victa (Lyson & Joyce, 2011), but absent in Dorsetochelys typocardium (Evers et al., 2020), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon bullockii (Evers et al., 2020). Within the fossa orbitalis of UCM 53971, the jugal contacts the maxilla anteriorly and broadly rests on the posterior part of this bone along a horizontal suture (Fig. 2C). The medial process of the jugal contacts the palatine medially and the pterygoid posteromedially. On the dorsolateral skull surface, the jugal otherwise contacts the postorbital dorsally and the quadratojugal posteriorly (Fig. 2A). Lastly, the jugal forms the anterodorsal margin of the lower temporal emargination, which only rises just above the lower margin of the orbit, but appears accentuated by the brevity of the skull and the depth of the maxilla (Fig. 2A).
Quadratojugal
The quadratojugal is a thin, triradiate element. The posterior rim of the quadratojugal is concave and forms the anteroventral margin of the cavum tympani (Fig. 2A). The quadratojugal also forms the posterior half of the cheek emargination. The anterior process of the quadratojugal contacts the jugal anteriorly. The quadratojugal otherwise contacts the postorbital anterodorsally, the squamosal posterodorsally, and the quadrate posteriorly and posteroventrally (Fig. 2A). The quadratojugal of UCM 53971 is overall similar to that of Pleurosternon bullockii (Evans & Kemp, 1975) and to Dorsetochelys typocardium (Evans & Kemp, 1976), which, however, has a taller anterior process.
Squamosal
The squamosal is a complex element that lies at the posterodorsolateral margin of the skull (Figs. 1A and 2). It forms the antrum postoticum, the posterodorsal margin of the cavum tympani, and the lateral margin of the upper temporal emargination. On the skull roof, the squamosal contacts the postorbital anteriorly, likely the quadratojugal anteroventrally, the quadrate ventrally, and the parietal anteromedially (Figs. 1A and 2A). Within the temporal fossa, the squamosal contacts the quadrate anteromedially and the opisthotic posteromedially (Figs. 1A and 2D). The squamosal forms a distinct, posteromedially directed curved flange that represents the posterior-most aspect of the skull (Fig. 1A). A broad concavity is developed along the lateral margin of this flange, which is likely the attachment site of the m. depressor mandibulae (Werneburg, 2011).
Premaxilla
The premaxillae are not preserved.
Maxilla
The maxilla forms the anterior and ventral margins of the orbit and the anterolateral aspects of the floor of the fossa orbitalis (Figs. 2A and C). In ventral view, the maxilla contacts the palatine medially along the margin of the triturating surface, the pterygoid posteromedially, and the jugal posterolaterally (Fig. 1B). Although the vomer is slightly displaced, a contact between the maxilla and the vomer was likely present anterior to the apertura narium interna. The ascending process of the maxilla forms the lateral margin of the apertura narium externa and contacts the nasal anterodorsally and the prefrontal posterodorsally (Figs. 2A and C). The foramen alveolare superius is formed as an unusually large opening located at the medial base of the ascending process, ventrally underneath the margin of the foramen orbito-nasale. On the lateral surface of the skull, the maxilla contributes to the formation of the anteroventral margin of the cheek emargination and contacts the jugal along a z-shaped suture (Fig. 2A). Within the fossa orbitalis, the maxilla forms the anterolateral margin of the foramen orbito-nasale, broadly contacts the palatine medially, and is posterodorsally covered by the jugal (Figs. 2A and C). The maxilla forms along with a minor contribution from the palatine a relatively narrow triturating surface that expands towards the posterior and that is bordered laterally by a high labial ridge (Fig. 1B). High labial ridges are also present in Compsemys victa (Lyson & Joyce, 2011) and Dorsetochelys typocardium (Evans & Kemp, 1976). The labial ridge and the triturating surface of UCM 53971 are strongly convex along the anteroposterior length of the maxilla. This curvature is intermediate in Pleurosternon bullockii (Evans & Kemp, 1975) and only weakly developed in Compsemys victa (Lyson & Joyce, 2011) and Glyptops ornatus (Gaffney, 1979a). Carpenter and Bakker (1990) noted that the triturating surfaces of UCM 53971 are much wider than in Glyptops ornatus and Pleurosternon bullockii. We agree with their comparison with Glyptops ornatus (Gaffney, 1979a), but suggest that the triturating surfaces of Pleurosternon bullockii have similar proportions to those of UCM 53971 (Evers et al., 2020). The triturating surfaces of UCM 53971 otherwise are narrower than those of Compsemys victa (Lyson & Joyce, 2011) and have similar proportions to those of Dorsetochelys typocardium (Evans & Kemp, 1976). The foramen supramaxillare is entirely formed by the maxilla and located within the orbit close to the suture with the jugal. Within the maxilla, the foramen supramaxillare leads into the canalis infraorbitalis, which extends anteriorly. A multitude of canals connects the ventral surface of the maxilla to the canalis infraorbitalis along its path. At the level of the ascending process of the maxilla, the canalis alveolaris superior splits from the canalis infraorbitalis. The canalis alveolaris superior, which contained the superior alveolar artery, then extends dorsomedially and exits the maxilla through the foramen alveolare superius to join the fossa nasalis.
Vomer
The vomer is a single, narrow bone, which floors the posteromedial part of the fossa nasalis and forms the medial margins of the apertura narium interna (Fig. 2B–C). The vomer is relatively flat, but its anterior part is significantly broadened, similar to the vomer of some pleurodires (Gaffney, 1979b). The vomer contacts the maxilla anterolaterally along a short suture and the palatine posterolaterally along an elongate suture (Figs. 1B and 2A). The vomer reaches the pterygoid posteriorly and prevented the palatine from contacting its counterpart medially (Fig. 1B), as observed in Compsemys victa (Lyson & Joyce, 2011) and Dorsetochelys typocardium (Evans & Kemp, 1976). At about mid-length, the vomer of UCM 53971 bears extremely low dorsolateral processes for articulation with the prefrontals (Fig. 4). The bar of bone that connects the vomer with the prefrontal is mostly formed by the prefrontal (Fig. 4). This condition is similar to what is observed in testudinoids, but contrasts with trionychids, in which the vomer forms most of this bony connection (Loveridge & Williams, 1957), and pleurodires, in which the vomer–prefrontal contact is absent (Gaffney, 1979b). A similar vomer–prefrontal contact as in UCM 53971 is present in Compsemys victa (Lyson & Joyce, 2011), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon bullockii (Evans & Kemp, 1975), but the extent of this contact is unclear in Dorsetochelys typocardium (Evans & Kemp, 1976). The overall shape and contacts of the vomer in UCM 53971 resemble the condition seen in some xinjiangchelyids (see Annemys levensis, Rabi et al., 2014) and thalassochelydians (Anquetin & André, 2020; Anquetin et al., 2015, 2017). The sulcus vomeri is a shallow, narrow groove and extends along the posterodorsal part of the vomer and deeply into the interorbital space (Fig. 4A and C), as is the case in a diverse mix of cryptodires (e.g., kinosternoids, testudinoids, Carettochelys, some cheloniids; Evers et al., 2019).
Palatine
The palatine is a plate-like bone that forms the posterior margin of the apertura narium interna, the posterior margin of the foramen orbito-nasale, and the entire margins of the small foramen palatinum posterius (Figs. 1B and 2A). The portion of the palatine between the foramen orbito-nasale and the foramen palatinum posterius floors the medial aspect of the fossa orbitalis and contacts the maxilla laterally. The palatine contacts the descending process of the prefrontal anterodorsally, the vomer medially, the pterygoid posteromedially and posteriorly, and the jugal posterolaterally lateral to the foramen palatinum posterius (Figs. 1B and 2A). The palatine has minor contributions to the triturating surfaces in the form of a low lingual ridge formed along the contact with the maxilla (Fig. 1B).
Pterygoid
The anterior part of the pterygoid contacts its counterpart medially along the midline, the vomer anteromedially, the palatine anteriorly along a V-shaped suture, the maxilla anterolaterally at the posterior end of the triturating surface, and the jugal laterally at the anterior margin of the lower temporal fossa (Fig. 1B). The posterior part of the pterygoid consists of a posterior process that floors the cavum acustico-jugulare and reaches the basioccipital at the same level as the parabasisphenoid (Figs. 1B and 2D). The posterior process of the pterygoid does not extend beyond the posterior limit of the parabasisphenoid (Fig. 1B), as in Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), Pleurosternon bullockii (Evans & Kemp, 1975), and Pleurosternon moncayensis (Perez-Garcia et al., 2021). The posterior process of the pterygoid contacts the parabasisphenoid posteromedially along a nearly straight suture, the basioccipital posteriorly along a small, concave suture, and the quadrate posterolaterally, but clearly lacks a contact with the exoccipital (Fig. 1B). The pterygoid fossa on the ventral surface of the posterior pterygoid process is moderately deep. The pterygoid possesses a well-developed processus pterygoideus externus that extends posteroventrally into the lower temporal fossa (Fig. 2A). The processus pterygoideus externus of UCM 53971 is dorsoventrally expanded with its lateral surface. The vertical plate of the lateral surface is large, slightly medially recurved along its dorsal margin, and therefore resembles the processus trochlearis pterygoidei of pleurodires, as was recently also observed for Pleurosternon bullockii (Evers et al., 2020). A well-developed processus pterygoideus externus is also present in Glyptops ornatus (Gaffney, 1979a). The processus pterygoideus externus is anteriorly sutured to the jugal. At mid-length of the suture between the parabasisphenoid and the pterygoid, the pterygoid forms a socket that holds the basipterygoid process of the parabasisphenoid, as in Glyptops ornatus (Gaffney, 1979a), Pleurosternon bullockii (Evans & Kemp, 1975; Evers et al., 2020), and Pleurosternon moncayensis (Perez-Garcia et al., 2021). Jointly with the basipterygoid process of the parabasisphenoid, the pterygoid of UCM 53971 forms a cavity midway along the parabasisphenoid–pterygoid suture, which leads into several foramina dorsally and anteriorly and is posteriorly confluent with a narrow sulcus on the ventral skull surface along the posterior part of the parabasisphenoid–pterygoid suture (Figs. 1B and 5–6). The same morphology has been described for Pleurosternon bullockii (Evans & Kemp, 1975; Evers et al., 2020), and grossly similar morphologies seem to be present in Glyptops ornatus (Gaffney, 1979a), Pleurosternon moncayensis (Perez-Garcia et al., 2021), and Arundelemys dardeni (Lipka et al., 2006), but not baenodds (e.g., Gaffney, 1972a; Rollot et al., 2018) or Compsemys victa (Lyson & Joyce, 2011). The posterior sulcus present in Pleurosternon bullockii and Uluops uluops likely held the internal carotid artery, as already proposed by Carpenter and Bakker (1990). The anterior cavity is similar to the fenestra caroticus observed in some turtle groups like xinjiangchelyids, sinemydids, sandownids, or plesiochelyids (Evers & Joyce, 2020; Rabi et al., 2013b; Raselli & Anquetin, 2019), the main difference in Pleurosternon bullockii and Uluops uluops to the aforementioned turtles being that the posterior course of the internal carotid artery is not encased in bone, but ventrally open. The following foramina can be identified within the anterior region of the cavity of UCM 53971: the foramen posterius canalis nervi vidiani of the facial nerve system (Rollot et al., 2018) and two foramina for the carotid arterial branches, namely the foramen posterius canalis carotici lateralis, and the foramen posterius canalis carotici basisphenoidalis (see Nomenclature above regarding terminology; Fig. 6D–E). Additionally, the foramen distalis nervi vidiani can be identified at the posterior end of the cavity (see below). The foramen posterius canalis nervi vidiani is located on the anterolateral margin of the cavity, anterior to the basipterygoid process (Fig. 6D). It can be unambiguously identified based on the course of the attached canal, the canalis nervus vidianus, which extends anteriorly through the pterygoid and exits the skull through a foramen positioned close to the anterior suture with the palatine, anteroventrolaterally to the epipterygoid (Rollot et al., 2018; Fig. 3). Uluops uluops shows unequivocal evidence of osteological correlates for both subordinate branches of the internal carotid artery that are commonly recognized in turtles (Rabi et al., 2013b; Rollot et al., 2021). In addition to the foramen posterius canalis carotici basisphenoidalis, the entrance foramen for the cerebral artery which is located on the medial margin of the cavity within the parabasisphenoid (see Parabasisphenoid) and universally present in turtles, UCM 53971 also shows evidence for the presence of a canal and associated foramina for the palatine artery. The foramen posterius canalis carotici lateralis is located on the anteromedial margin of the cavity and leads to the canalis caroticus lateralis that transmits the palatine branch of the internal carotid artery until its exit foramina within the cavum cranii, the foramen anterius canalis carotici lateralis (Figs. 5 and 6E). This identification is unequivocally supported by the separate coexistence of canals for the vidian nerve and palatine artery (see also Rollot et al., 2021). The pterygoid of UCM 53971 forms the ventral aspects of the foramen posterius canalis carotici lateralis, canalis caroticus lateralis, and foramen anterius canalis carotici lateralis. A summary of apparent variation to the presence of the palatine canal in paracryptodires is included below (see Discussion). A small foramen pierces the pterygoid within the internal carotid groove of UCM 53971, along the parabasisphenoid–pterygoid suture (Fig. 6D). This is identified as the foramen distalis nervi vidiani, which is the ventromedial exit foramen of the posterior portion of the vidian nerve, which is transmitted from the geniculate ganglion to the ventral surface of the skull via the canalis pro ramo nervi vidiani. The foramina for the vidian nerve along the carotid groove and cavity imply that a short section of the vidian nerve was transmitted together with the internal carotid artery in a ventrally exposed position.
At its posteromedial end, the pterygoid contributes to the formation of anterior tuberculum basioccipitale, which is otherwise mainly formed by the parabasisphenoid (see Parabasisphenoid; Fig. 1B). Within the cavum acustico-jugulare, the pterygoid contacts the prootic anterodorsomedially, the opisthotic dorsomedially, and the quadrate laterally. The pterygoid forms the ventral and anterolateral margins of the canalis cavernosus that extends posteriorly from the foramen cavernosum to the cavum acustico-jugulare, and the ventral and lateral margins of the foramen cavernosum and sulcus cavernosus. The sulcus cavernosus is well defined as a deep groove between the parabasisphenoid medially and the crista pterygoidei of the pterygoid laterally. The crista pterygoidei forms a relatively low crest, which forms a pointed dorsal projection along the anterior margin of the trigeminal foramen, but becomes anteriorly low. In this anterior region of the crista, the pterygoid contacts the epipterygoid anterolaterally along a nearly vertical suture. The pterygoid also frames the ventral and posterior margins of the trigeminal foramen, with a small dorsal process that contacts the parietal in the posterodorsal margin of the trigeminal foramen (Fig. 2B). This parietal–pterygoid contact excludes the prootic from contributing to the trigeminal foramen, as in Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021). The exclusion of the prootic from the trigeminal foramen has also been reported for several baenids (e.g., Brinkman & Nicholls, 1991; Lyson & Joyce, 2009a). However, this is the only contact between the pterygoid and parietal in UCM 53971, as the epipterygoid fully separates the two along the dorsal margin of the crista pterygoidei. Posterior to the trigeminal foramen, the pterygoid contacts the quadrate posteriorly and the prootic posterodorsally (Fig. 2B). The foramina and canalis nervus abducentis have an unusual topological arrangement in UCM 53971. The foramen posterius canalis nervi abducentis is located on the anterodorsal surface of the parabasisphenoid, as in all turtles (Fig. 7A). In all other turtles that we are aware of, the canalis nervus abducentis then traverses the parabasisphenoid in a roughly anterior trajectory, to exit on the anterior surface of the parabasisphenoid, usually ventrally or ventrolaterally to the clinoid processes (e.g., Gaffney, 1979b). In UCM 53971, however, the canalis nervus abducentis extends anterolaterally through the parabasisphenoid, then enters the pterygoid and connects with the posterior portion of the sulcus cavernosus through the foramen anterius canalis nervi abducentis (Fig. 7). Thus, the foramen anterius canalis nervi abducentis is entirely formed by the pterygoid and located in the floor of the sulcus cavernosus posterolateral to the base of the clinoid process of the parabasisphenoid (Fig. 7B–C). The contribution of the pterygoid to the formation of the canalis nervus abducentis and foramen anterius canalis nervi abducentis is highly unusual among turtles as the foramina and canalis nervus abducentis are in most cases entirely formed by the parabasisphenoid. The foramen anterius canalis nervi abducentis is also generally located just lateral to the base of the clinoid process and medial to the sulcus cavernosus (Gaffney, 1979b), although generally close to the pterygoid–parabasisphenoid suture (Anquetin et al., 2015). A few exceptions to this conformation have been highlighted in some taxa. The thalassochelydians Plesiochelys and Portlandemys have a foramen anterius canalis nervi abducentis in a more posteroventral position relative to the base of the clinoid process than usually observed in turtles (Anquetin et al., 2015; Gaffney, 1976). The condition observed in UCM 53971 is also similar to the baenid Eubaena cephalica, which has the lateral margin of the foramen anterius canalis nervi abducentis formed by the pterygoid (Rollot et al., 2018), but differs from Pleurosternon bullockii, in which the foramen anterius canalis nervi abducentis is located in the parabasisphenoid within the retractor bulbi pits ventral to the clinoid process (Evers et al., 2020).
Epipterygoid
The epipterygoid is a triradiate element that forms the anteroventrolateral wall of the cavum cranii and the anterior margin of the foramen nervi trigemini (Fig. 2B). The epipterygoid contacts the pterygoid ventrally along its entire ventral surface, has a short contact posteriorly with the epipterygoid process of the quadrate, and is tightly sutured with the descending process of the parietal along the dorsal process of the epipterygoid (Fig. 1B). Whereas the epipterygoid is essentially a flat element in Pleurosternon bullockii (Evers et al., 2020), the epipterygoid of UCM 53971 has a lateral bulge at its dorsal process, just anterior to the dorsal margin of the trigeminal foramen. The bulge extends as a thick ridge over nearly the entire lateral surface of the epipterygoid, paralleling the anteroventral margin of the obliquely oriented trigeminal foramen. Anterior to the trigeminal foramen, the epipterygoid fully separates the parietal and pterygoid, and is therefore firmly integrated into the secondary lateral braincase wall. In many cryptodires and thalassochelydians (e.g., Evers & Joyce, 2020), the epipterygoid is a more surficial element that only covers the lateral surface of the crista pterygoidei, thereby allowing a broad contact between the crista with the parietal. An ossified epipterygoid is present in Compsemys victa (Lyson & Joyce, 2011), Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), Pleurosternon bullockii (Evans & Kemp, 1975), and Pleurosternon moncayensis (Perez-Garcia et al., 2021), but is absent in baenodds (Gaffney, 1972a).
Quadrate
The quadrate forms the condylus mandibularis below the cavum tympani and most of the middle ear, including most of the cavum tympani. In lateral view, the quadrate contacts the quadratojugal anteriorly along a curved, convex suture, and the squamosal posterodorsally (Fig. 2A). The condylus mandibularis does not extend deeply below the cavum tympani, as in all non-baenid paracryptodires. The cavum tympani is a deep cavity that is fully confluent with the antrum postoticum. The quadrate only contributes to its anterodorsal margin. Within the lower temporal fossa, the quadrate forms a short epipterygoid process, which contacts the epipterygoid anteromedially (Fig. 2B). On the dorsal surface of the otic capsule, the quadrate contacts the prootic anteromedially, the supraoccipital medially along a short suture directly posterior to the foramen stapedio-temporale, the opisthotic posteromedially, and the squamosal posterolaterally (Figs. 2B–C and 8A). A short, medial contact with the supraoccipital directly posterior to the foramen stapedio-temporale is preserved on the intact right side of the skull. The quadrate forms the lateral margin of the aditus canalis stapedio-temporalis and canalis stapedio-temporalis, and the posterolateral margin of the canalis cavernosus (Fig. 8B). The foramen stapedio-temporale is exposed on the dorsal surface of the otic capsule (Fig. 2B), and is bordered laterally by the quadrate and medially by the prootic, as in Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021). The foramen stapedio-temporale of Compsemys victa is also formed by the supraoccipital in addition to the quadrate and prootic (Lyson & Joyce, 2011). Two facets separated by a shallow sulcus are present on the condylus mandibularis for the articulation with the mandible. The quadrate does not contribute to the foramen nervi trigemini (Fig. 2B). The quadrate contributes to the lateral half of the processus trochlearis oticum and forms the lateral wall of the cavum acustico-jugulare (Fig. 8A–B). The quadrate forms the incisura columella auris (Fig. 8B), which is opened posteroventrally as in Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), Pleurosternon bullockii (Evans & Kemp, 1975), and Pleurosternon moncayensis (Perez-Garcia et al., 2021).
Prootic
The prootic forms the anteromedial portion of the otic capsule. It contacts the parietal anteromedially, the supraoccipital posteromedially, the quadrate laterally, the pterygoid ventrolaterally, the parabasisphenoid ventromedially, and the opisthotic posteriorly (Figs. 2B–C and 8). A small contact between the parietal and pterygoid posterior to the foramen nervi trigemini prevents the prootic from contributing to the margin of the latter (Fig. 2B), as is also the case in Pleurosternon bullockii (Evers et al., 2020). The prootic of UCM 53971 is broadly exposed on the dorsal surface of the otic capsule, where it forms the medial half of the foramen stapedio-temporale (Fig. 2B). The medial surface of the canalis stapedio-temporalis is also formed by the prootic, and leads ventrally through the otic capsule into the cavum acustico-jugulare (Fig. 8B). Here, the prootic borders the canalis cavernosus medially, which is the passage between the foramen cavernosum and the cavum acustico-jugulare posteriorly. The foramen cavernosum is also formed laterally by the prootic. The anterior part of the prootic significantly extends anterodorsally beyond the level of the foramen cavernosum, forming a roof-like extension above the trigeminal area including the cavum epiptericum for the trigeminal ganglion. Medially to the cavum epiptericum, the ventral base of the prootic that rests against the parabasisphenoid forms a deep notch for the trigeminal nerve stem (see Evers et al., 2019). Additionally, the prootic forms the anterior parts of the lateral wall of the cavum cranii, and forms the fossa acustico-facialis on its medial margin at the interface with the cavum cranii (Fig. 8D). From this fossa, the canals of the facial nerve (VII) or canalis nervus facialis and for the acoustic nerve (VIII) enter the prootic (Fig. 8D). The canal for the acoustic nerve connects the fossa acustico-facialis to the cavum labyrinthicum and the canalis nervus facialis connects the fossa acustico-facialis to the ventromedial part of the canalis cavernosus (Fig. 3A). The prootic forms the anterior portion of the cavum labyrinthicum, including the anterior halves of the canalis semicircularis anterior and horizontalis (Fig. 8B and D). The foramen aquaducti vestibuli is ossified within the prootic–supraoccipital contact, and anterior to the hiatus acusticus. The prootic and opisthotic form a fully ossified (= surrounded by bone) fenestra ovalis (Figs. 2D and 8C), as also found in Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021). The posterior surface of the prootic just lateral to the fenestra ovalis has a deep perilymphatic fossa (Fig. 8C), as in Pleurosternon bullockii (Evers et al., 2020).
Opisthotic
The opisthotic is largely exposed in dorsal view through the upper temporal emargination (Fig. 1A). It contacts the supraoccipital anteromedially, the quadrate anterolaterally, the squamosal posterolaterally, the exoccipital posteromedially, the prootic anteriorly, the parabasisphenoid ventromedially, and the pterygoid and basioccipital ventrally along the processus interfenestralis (Figs. 1A, 2D, and 8). The opisthotic does not contribute to the foramen stapedio-temporale and canalis stapedio-temporalis. The posterolaterally and slightly ventrally directed paroccipital process of the opisthotic forms the dorsal margin of the fenestra postotica and roofs the cavum acustico-jugulare (Fig. 2D and Fig. 8A). The dorsal surface of the paroccipital process bears a prominent ridge posteromedially, which bounds an elongated fossa or groove that parallels the posteromedial margin of the process. The cavum acustico-jugulare is separated from the cavum labyrinthicum by the processus interfenestralis (Fig. 8A and C). The processus interfenestralis contacts the basioccipital and pterygoid ventrally along a horizontal suture. The ventral base of the processus interfenestralis is anteriorly expanded to a slight footplate, which also contacts the prootic in the ventral margin of the fenestra ovalis, which is notably large in UCM 53971 (Fig. 8C). Within the cavum labyrinthicum, the opisthotic forms canals for the posterior and lateral semicircular ducts (Fig. 8D). The processus interfenestralis furthermore delimits the recessus scalae tympani anteriorly and forms the lateral and dorsal margins of the fenestra perilymphatica within the recess (Fig. 8A). Also within the recessus scalae tympani, the opisthotic forms the anterior margin of the foramen jugulare anterius. A tiny ventromedial contact with the parabasisphenoid is present along the processus interfenestralis, as in Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021), and an additional contact with this bone is observed posteriorly to the hiatus acusticus. The foramen externum nervi glossopharyngei (IX) and foramen internum nervi glossopharyngei are visible at the dorsal base of the processus interfenestralis (Fig. 8C).
Supraoccipital
The supraoccipital is a singular, unpaired element that forms the posterior tip of the skull roof, roofs the cavum cranii posteriorly, and forms the dorsal margin of the foramen magnum (Figs. 1A and 2D). The supraoccipital forms a large, vertical sheet of bone between the skull roof and foramen magnum. The great ventrodorsal depth of the adductor fossa/upper temporal fossa is similar to that seen in early turtles like Australochelys africanus, but the supraoccipital is mediolaterally thickened significantly within the fossa in these turtles. Posteriorly, the crista supraoccipitalis of UCM 53971 is short and barely protrudes beyond the foramen magnum (Fig. 1A). The supraoccipital is slightly exposed dorsally in the skull roof, forming a quadrangular surface that contacts the parietals anteriorly (Fig. 1A). A similar exposure is present in Dorsetochelys typocardium (Evans & Kemp, 1976), but not in Pleurosternon bullockii (Evans & Kemp, 1975; Evers et al., 2020), in which the dorsal exposure of the supraoccipital is much smaller. Within the floor of the upper temporal fossa, the supraoccipital contacts the parietal anteriorly, the prootic anterolaterally, the opisthotic posterolaterally, and the exoccipital posteriorly (Figs. 1A, 2B, and D). Along its most lateral extension, the undamaged right side of the supraoccipital has a short contact with the quadrate just posterior to the foramen stapedio-temporale, to which it does not contribute. The supraoccipital roofs the cavum labyrinthicum, forms the dorsal margin of the hiatus acusticus, the posterior portion of the canalis semicircularis anterior and the anterior portion of the canalis semicircularis posterior, and borders the foramen aquaducti vestibuli dorsally.
Basioccipital
The basioccipital of UCM 53971 is a single, unpaired element developed as a roughly rectangular block in the floor of the cavum cranii, as in other non-baenodd paracryptodires (Fig. 1B). The basioccipital forms the ventral margin of the foramen magnum and the complete articular surface of the condylus occipitalis as in Dorsetochelys typocardium (pers. comm. by Jérémy Anquetin about the holotype DORCM G.23), Pleurosternon moncayensis (Perez-Garcia et al., 2021), and very likely Glyptops ornatus (Gaffney, 1979a). The basioccipital contacts the exoccipitals dorsally along a horizontal suture (Fig. 2D). A low crista dorsalis basioccipitalis is present on the anterior portion of the dorsal basioccipital surface. Posterolaterally, the basioccipital bears a short, but thick tuberculum basioccipitale, which is formed without contributions of any other bones (Fig. 1B). The basioccipital is significantly broader at the level of the tubercula basioccipitale than the parabasisphenoid. Within the cavum acustico-jugulare, the basioccipital contacts the processus interfenestralis of the opisthotic anterodorsally along a horizontal suture. The basioccipital contacts the parabasisphenoid anteriorly along a convex suture, which is laterally framed by the anterior tubercula basioccipitale that are jointly formed by the parabasisphenoid and pterygoids, and overlap parts of the ventral basioccipital surface (Fig. 1B). The basioccipital forms a slight depression posterior to its contact with the parabasisphenoid and two foramina basioccipitale are located on the ventral surface of the basioccipital within this depression.
Exoccipital
The exoccipital contacts the supraoccipital dorsomedially, the opisthotic dorsolaterally, and the basioccipital ventrally (Figs. 1A and 2D). A contact with the pterygoid is clearly absent. The exoccipital forms the posterolateral wall of the cavum cranii, the lateral margin of the foramen magnum, the medial margin of the fenestra postotica, the posterior margin of the foramen jugulare anterius, and roofs the cavum acustico-jugulare posteromedially. Within the cavum acustico-jugulare, the exoccipital contacts the opisthotic anteriorly and forms the medial and ventral margins of the fenestra perilymphatica and the posterior margin of the foramen jugulare anterius (Fig. 8A). The exoccipital bears three foramina nervi hypoglossi (XII) that gradually increase in size towards the posterior (Figs. 2D and 8A). Only two foramina nervi hypoglossi are present in Dorsetochelys typocardium (Evans & Kemp, 1976), Glyptops ornatus (Gaffney, 1979a), and Pleurosternon moncayensis (Perez-Garcia et al., 2021). The anteriormost two foramina nervi hypoglossi already lie within the cavum acustico-jugulare, a condition observed for several relatively basal turtles, including meiolaniforms, Eileanchelys waldmani, or Kallokibotion bajazidi (Evers & Benson, 2019). The exoccipital reaches onto the dorsolateral part of the condylus occipitalis, but does not contribute to the condylar articular facet for the atlas (Figs. 2D and 8A).
Parabasisphenoid
The parabasisphenoid is a single, unpaired element exposed on the ventral surface of the skull that forms most of the ventral margin of the cavum cranii. The parabasisphenoid contacts the pterygoid laterally along its entire length, the prootic dorsolaterally, and the basioccipital posteriorly along a straight vertical suture (Fig. 1B). An additional, small contact with the opisthotic is present at the posterolateral end of the parabasisphenoid. The parabasisphenoid forms the rostrum basisphenoidale, the sella turcica, the dorsum sellae, and the processus clinoideus. The parabasisphenoid is dorsally concave from the dorsum sellae to its posterior end. This concavity is interrupted along the midline by a high crista basis tuberculi basalis. The rostrum basisphenoidale is a narrow sheet of bone anterior to the dorsum sellae that covers the anterior portion of the canalis caroticus lateralis and forms the dorsal margin of the foramina anterius canalis carotici lateralis (Fig. 5A–B). Thus, the position of these foramina is somewhat unusual, in that they are positioned more medially than in most turtles, where the anterior exiting foramina are situated within the pterygoid–parabasisphenoid suture along the floor of the sulcus cavernosus, and thus lateral, rather than ventral to the rostrum basisphenoidale. The sella turcica is a moderately deep depression that contains the foramina anterius canalis carotici basisphenoidalis, which are relatively widely spaced (Fig. 5A–B). The dorsum sellae overhangs the sella turcica anteriorly, but there is no vertical ridge on the anterior surface below the dorsum sellae. The processus clinoideus are partially damaged, but the preserved parts indicate that they were likely relatively short and had broad bases. Posteriorly, the bases of the clinoid processes form together with the prootic a relatively deep notch that would have allowed the trigeminal nerve stem to pass from the cavum cranii into the cavum epiptericum (Evers et al., 2019). The broad base of the clinoid processes also overhangs the sulcus cavernosus anterolaterally, almost contacting the secondary lateral braincase wall that is formed by the epipterygoid at this level. This morphology hypertrophies the depth of the retractor bulbi pits ventrally to the clinoid processes, on the surface that rises vertically medial to the sulcus cavernosus. Usually, the anterior abducentis nerve foramina are positioned in this surface. In UCM 53971, these foramina are unusual in that they are more laterally placed than seen in other turtles within the floor of the sulcus cavernosus (Fig. 7C). UCM 53971 is furthermore the only turtle of which we are aware that has these abducentis foramina positioned within the pterygoid, rather than the parabasisphenoid (Fig. 7C–D). The abducens canal can be traced posteriorly from these foramina through the pterygoid and then the parabasisphenoid, where the posterior foramina are located in their ‘regular’ positions within the cup-shaped, dorsal parabasisphenoid surface. The parabasisphenoid forms most of the medial margin of the sulcus cavernosus and the ventral margin of the hiatus acusticus and has a slight posterolateral contact with the processus interfenestralis of the opisthotic. At about midpoint along the parabasisphenoid–pterygoid suture on the ventral skull surface, the parabasisphenoid has a laterally projecting basipterygoid process on each side, which extends laterally and inserts into a respective socket formed by the pterygoid (Fig. 6C–D). The overall morphology of this area is very similar to that of Pleurosternon bullockii (Evers et al., 2020) and Pleurosternon moncayensis (Perez-Garcia et al., 2021), with differences mainly in the presence vs. absence of a palatine artery (see Pterygoid above). At the level of the basipterygoid process, the parabasisphenoid and pterygoid form a cavity that is mainly bordered by the basipterygoid process of the parabasisphenoid (Figs. 1B and 6C–D), which is also present in Glyptops ornatus (Gaffney, 1979a) and Pleurosternon bullockii (Evans & Kemp, 1975; Evers et al., 2020). The vidian nerve as well as the palatine and cerebral arteries enter the skull through this cavity, but only the foramen posterius canalis carotici basisphenoidalis is formed by the parabasisphenoid (Fig. 6D–E). Posterior to the cavity located along the parabasisphenoid–pterygoid suture is a groove that extends posteriorly and is mainly formed by the parabasisphenoid (Fig. 6C–E). This groove housed the internal carotid artery. The parabasisphenoid, along with minor lateral contributions of the pterygoid, forms posterior processes on each side that overlap the basioccipital posteroventrally (Fig. 1B). These have been called anterior tubercula basioccipitale (Evers et al., 2020), as they are likely homologous with such structures of helochelydrids (Joyce et al., 2014). The anterior tubercula basioccipitale are slightly raised posteriorly in UCM 53971, forming a shallow fossa between them. In addition to helochelydrids, the anterior tubercula basioccipitale are also present in Glyptops ornatus (Gaffney, 1979a), Pleurosternon bullockii (Evans & Kemp, 1975; Evers et al., 2020), Pleurosternon moncayensis (Perez-Garcia et al., 2021), and Dorsetochelys typocardium (DORCM G.00023).
Labyrinth morphology
The labyrinth of UCM 53971 (Fig. 9) conforms to the general morphology of turtle labyrinths (Evers et al., 2019; Lautenschlager et al., 2018), but notably is morphologically distinct from other paracryptodire labyrinths that have been described (Pérez-García et al. 2021; Evers et al., 2021). The endosseous labyrinth of Uluops uluops is formed by the prootic, opisthotic, and supraoccipital. The anterior semicircular canal is slightly longer than the posterior semicircular canal (Fig. 9A and C). Both vertical semicircular canals are straight along their central sections, and most of the curvature is achieved near the ampullae and the common crus, which is dorsally embayed between the tallest section of the anterior and posterior semicircular canals (Fig. 9A). This dorsal embayment contrasts with the morphology of the labyrinth of Arundelemys dardeni, which lacks a clear embayment (Evers et al., 2021). The strongest difference between the labyrinth of U. uluops with other paracryptodires, notably the pleurosternid Pleurosternon moncayensis (Pérez-García et al. 2021), is the relative thickness of the semicircular canals, which is high in U. uluops, whereas other paracryptodires have slender canals. Pérez-García et al. (2021) interpret the slender semicircular canals of P. moncayensis as indicative of freshwater habits of pleurosternids, but the thick canals in U. uluops and its ecological interpretation as a freshwater turtle show that labyrinth morphology may be a poor indicator of ecology, especially when interpreted without a comprehensive comparative sample (e.g., Bronzati et al., 2021).
The angle between the vertical semicircular canals is roughly 90° in Uluops uluops (Fig. 9C), which is similar to the angle reported for Pleurosternon moncayensis (Pérez-García et al. 2021). The lateral and anterior ampullae of U. uluops are not well delimited from one another, and the ventral section of the posterior semicircular canal and the posterior section of the lateral semicircular canal are confined to a single osseous cavity, the secondary common crus (Fig. 9B). The fenestra ovalis is a large opening (Fig. 9A) that is fully closed by the prootic and opisthotic. The fenestra perilymphatica is also well defined and completely closed (Fig. 9B), therefore not leaving a hiatus postlagenum. Unlike in most turtles including other paracryptodires (Pérez-García et al. 2021; Evers et al., 2021), the ventral section of the endosseous labyrinth is well defined by bone, so that the lagena part of the labyrinth housing the cochlea is formed as a small ventral protrusion between the opisthotic, basioccipital and parabasisphenoid (Fig. 9A–B).
Stapes
The stapes is not preserved in UCM 53971.
Internal carotid artery circulation pattern
The internal carotid artery is ventrally exposed in Uluops uluops, albeit funneled along a ventrally open groove that extends nearly antero-posteriorly along parts of the parabasisphenoid–pterygoid suture, as observed by Carpenter and Bakker (1990). As the internal carotid artery is never fully enclosed by bone, a canalis caroticus internus and foramen posterius canalis carotici interni are absent. The internal carotid groove ends anteriorly in a pit that resembles the fenestra caroticus of xinjiangchelyids in its topological position and by housing the splitting point of the internal carotid artery (Rabi et al., 2013b), but differs by being posteriorly open and connected to the carotid groove (Fig. 6C–E). The carotid pit of Uluops uluops is mainly formed by the basipterygoid process of the parabasisphenoid and located midway along the parabasisphenoid–pterygoid suture (Figs. 1B and 6C–E). Although paracryptodires were previously believed to universally possess a palatine artery (e.g., Brinkman & Nicholls, 1993; Gaffney, 1982), newer studies based on µCT data suggest that the artery is generally absent (Lipka et al., 2006; Rollot et al., 2018; Evers et al., 2020, 2021; Pérez-Garcia et al., 2021). Uluops uluops is the only paracryptodire with µCT data available that shows clear evidence that its internal carotid artery splits into both a medially branching cerebral and anterolaterally branching palatine artery. Reports that the canal is also present in Dorsetochelys typocardium (Anquetin & André, 2020) require verification with µCT data. The foramina posterius canalis carotici lateralis are located within the carotid pit and can be traced forward through their canals and anterior foramina (Fig. 6E). The canalis caroticus lateralis is long, has a small diameter, and extends anteriorly along the parabasisphenoid–pterygoid suture (Fig. 5). The anterior portion of the canalis caroticus lateralis is covered by the rostrum basisphenoidale. The foramen anterius canalis carotici lateralis is located at the anterior end of the rostrum basisphenoidale and formed by the pterygoid ventrally and the parabasisphenoid dorsally (Fig. 5A–B). The canalis caroticus basisphenoidalis extends anteromedially within the parabasisphenoid and enters the sella turcica by way of the relatively widely spaced foramina anterius canalis carotici basisphenoidalis (Fig. 5A–B).
The palatine artery appears to be greatly reduced compared to the cerebral and stapedial arteries. We measured the cross-sectional areas of the canalis caroticus basisphenoidalis, canalis caroticus lateralis, and canalis stapedio-temporalis close to their exit foramina of the skull, namely the foramen anterius canalis carotici basisphenoidalis, foramen anterius canalis carotici lateralis, and foramen stapedio-temporale, respectively. The surface of the cross section of the canalis caroticus lateralis is 0.090 mm2 on the left and 0.096 mm2 on the right. The surface of the cross section of the canalis caroticus basisphenoidalis is 0.258 mm2 on the left and 0.251 mm2 on the right. The surface of the canalis stapedio-temporalis was only measured with confidence on the right side, as the left otic capsule is damaged and some parts of the bones forming the margin of the left canalis stapedio-temporalis are missing. The surface value on the right side is 1.076 mm2. These results highlight that the stapedial artery is the largest and that the palatine artery is the smallest among the three canals measured. The palatine artery is less than half the size of the cerebral artery and only one tenth of the size of the stapedial artery, and may have been insignificant for blood supply, therefore potentially explaining its possibly repeated loss in paracryptodires (see Discussion).
Canalis cavernosus
The canalis cavernosus is an antero-posteriorly directed canal that contains the lateral head vein and connects the cavum cranii to the cavum acustico-jugulare (Fig. 5). The anterior portion of the canalis cavernosus is formed by the prootic dorsally and dorsomedially and by the pterygoid laterally and ventrally. The posterior portion of the canalis cavernosus is formed by the prootic medially and dorsally, by the pterygoid ventrally, and by the quadrate laterally. The canalis cavernosus can be subdivided into two sections: a ventral one and a dorsal one, separated by a constriction located on the medial side of the canalis cavernosus. The constriction is particularly strong at the level of contact between the canalis cavernosus and canalis nervus facialis, and has been suggested to be an osteological correlate for the separation between the lateral head vein and the mandibular artery (Rollot et al., 2021). In UCM 53971, the lateral head vein is housed in the ventral part of the canalis cavernosus and the mandibular artery in the dorsal part. The canalis cavernosus contacts the canalis nervus facialis along its ventral portion (Fig. 5A).
Facial nerve system
The facial nerve extends laterally from the fossa acustico-facialis to the canalis cavernosus through a long and small canalis nervus facialis, which is located in the prootic (Fig. 5A and C). From within the canalis cavernosus, the facial nerve divides into two branches at the geniculate ganglion: the hyomandibular and vidian nerves. A sulcus for the hyomandibular nerve is present and extends posteriorly to the cavum acustico-jugulare within the ventromedial margin of the canalis cavernosus, formed by the prootic and pterygoid (Fig. 5A). The vidian nerve enters the canalis pro ramo nervi vidiani slightly anterior to the contact between the canalis nervus facialis and canalis cavernosus (Fig. 5A–B). The canal is long, and extends ventromedially through the pterygoid, where it exits via the foramen distalis nervi vidiani and into the carotid groove that houses the internal carotid artery (Fig. 6D). The vidian nerve is inferred to first follow the path of the internal carotid artery into the carotid pit, and then the path of the palatine artery for a short distance within this cavity. The palatine artery and vidian nerve courses become separate in the anterior margin of the carotid pit, where they enter separate canals via distinct foramina (Fig. 5B and Fig. 6C–E). The canalis nervus vidianus for the vidian nerve starts lateral to the palatine artery canal, extends anteriorly through the pterygoid, and exits the skull by way of the foramen anterius canalis nervi vidiani, which is located on the dorsal surface of the pterygoid, anteroventrolaterally to the anterior end of the epipterygoid (Fig. 5). A nearly identical facial, hyomandibular, and vidian nerves pattern is present in Pleurosternon moncayensis (Perez-Garcia et al., 2021) and Pleurosternon bullockii (Evers et al., 2020), the main difference with UCM 53971 being that the vidian nerve does not follow the path of the palatine artery in Pleurosternon bullockii as this branch is absent.
Canalis basioccipitalis
The canalis basioccipitalis are paired canals that emerge from the ventral surface of the basioccipital by means of the small foramen basioccipitale. Owing to their very small size, the canalis basioccipitalis can only be followed for a very short distance within the basioccipital. The path of the canalis basioccipitalis is unclear but they might merge within the basioccipital.
