Type species

Eurysternum? neuquinum Fernández and de la Fuente, Reference Fernández and de la Fuente1988.

Neusticemys neuquina (Fernández and de la Fuente, Reference Fernández and de la Fuente1988)
Figures 2–5, 7, 8

Reference Fernández and de la Fuente1988

Eurysternum? neuquinum Fernández and de la Fuente, p. 129, pls. 1, 2.

Reference Fernández and de la Fuente1993

Neusticemys neuquina; Fernández and de la Fuente, p. 284, pl. 1.

Reference de la Fuente, Gasparini, Coria and Salgado2007

Neusticemys neuquina; de la Fuente, p. 57, fig. 3.2, E–G.

Reference de la Fuente and Fernández2011

Neusticemys neuquina; de la Fuente and Fernández, p. 15, figs. 1–4.

Figure 2.

Neusticemys neuquina from the Titthonian level of Vaca Muerta Formation, photographs and drawings of the skull MOZ-PV-064 in: (1, 2) dorsal view, (3, 4) lateral right view, (5, 6) lateral left view, and (7, 8) anterior view. ane = apertura narina externa; ap = antrum postoticum; cso = crista supraoccipitalis; ct = cavum tympani; fr = frontal; fst = foramen stapedio-temporale; ica = incisura collumellae auris; ju = jugal; mx = maxilla; na = nasal; op = opisthotic; pa = parietal; pf = prefrontal; po = postorbital; pr = prootic; qj = quadratojugal; qu = quadrate; so = supraoccipital; sq = squamaosal.

Figure 3.

Neusticemys neuquina from Tithonian levels of Vaca Muerta Formation, photographs and drawings of the skull MOZ-PV-064 in: (1, 2) ventral view, and (3, 4) posterior view. ani = apertura narium interna; bo = basioccipital; bs = basisphenoid; co = condylus occipitalis; cso = crista supraoccipitalis; dbs = depression basisphenoidal; ex = exoccipital; fm = foramen magnum; fnh = foramen nervi hypoglossi; fpo = fenestra post-otica; fpp = foramen palatinum posterius; fsbt = fenestra subtemporalis; fti = fossa temporalis inferior; mx = maxilla; op = opisthotic; pa = parietal; pal = palatine; po = postorbital; pt = pterygoid; qu = quadrate; so = supraoccipital; sq = squamaosal; tb = tuberculum basioccipitale; vo = vomer.

Type material

Holotype, MLP 86-III-30-2, posterior third of a carapace, hyo- and hypoplastra, and other postcranial fragments (Fernández and de la Fuente, Reference Fernández and de la Fuente1988). Paratypes MLP 86-III-30-1, anterior part of a carapace, hyo- and hypoplastra, and pelvic girdle; and MOZ-PV-1106, anterior part of a carapace (Fernández and de la Fuente, Reference Fernández and de la Fuente1988).

Emended diagnosis

Neusticemys neuquina belongs to the clade Testudinata because it has a complete carapace and plastron enclosing the pectoral girdle. It is included inside Angolachelonia according to: the posterior orientation of the articular process of the quadrate; the presence of a prominent, ventrally infolding ridge of the posterior surface of the processus articularis of the quadrate; the long interpterygoid contact; and a pterygoid contact with the mandibular articular surfaces of the quadrate. It can be identified as a member of Thalassochelydia by: the anterolateral recess of the anterior surface of the quadrate lateral to the processus trochlearis oticum; the presence of a fossa on the supraoccipital-opisthotic-exoccipital contact area; the foramina anterius caroticus cerebralis found close together, but as independent foramina in the basisphenoid; the presence of the splenial in the mandible; lack of lateral ossifications of the costals, allowing the dorsal exposure of the distal end of ribs and the development of fontanelles only at the most anterior and posterior costals; the presence of rib-free peripherals; two to four vertebral scutes significantly broader than pleurals; and the central articulation of cervical vertebrae not formed. It differs from other Thalassochelydia by possessing a round depression on the ventral surface of the basisphenoid; a relatively larger oval foramen nervi trigemini; reduced and steepened triturating surfaces in both the maxilla and dentary; a flattened and wide carapace of moderate to large size (maximum carapace length 501 mm); a bound keel in the posterior third of the carapace; anterolateral margins of the carapace that are rectilinear and elongated, resulting in an anteroposterior elongation of the carapace; relatively elongated anterior and posterior limbs with flattened carpal and tarsal elements; and an elongated V digit in the pes.

Ocurrence

Cerro Lotena (Zapala, Neuquén Province, Argentina) (Fernández and de la Fuente, Reference Fernández and de la Fuente1988), 39°11′22.35″S, 69°38′29.57″W, lower section of the Vaca Muerta Formation (Weaver, Reference Weaver1931), V. andesensis Biozone, early Tithonian (two-fold division) (Riccardi, Reference Riccardi2015; Vennari, Reference Vennari2016).

Descriptions and comparisons: identification as Neusticemys neuquina.—Neusticemys neuquina was originally diagnosed based on postcranial features (Fernández and de la Fuente, Reference Fernández and de la Fuente1988, Reference Fernández and de la Fuente1993; de la Fuente, Reference de la Fuente, Gasparini, Coria and Salgado2007). However, several years later, two specimens (MACN-PV-105 and MHNSR-PV-1195) with skull and associated postcranial remains, including diagnostic features, were recovered in the Vaca Muerta Formation (de la Fuente et al., Reference de la Fuente, Sterli and Fernández2014; Fernández et al., Reference Fernández, Herrera, Vennari, Campos, de la Fuente, Talevi and Aguirre Urreta2019).

Comparison of the postcranial material associated with MACN-PV-105 and MHNSR-PV-1195, together with the holotype MLP 86-III-30-2 (Fernández and de la Fuente, Reference Fernández and de la Fuente1988) and previously described specimens MLP 92-IV-10-1 and MOZ-PV-6094 (Fernández and de la Fuente, Reference Fernández and de la Fuente1993; de la Fuente, Reference de la Fuente, Gasparini, Coria and Salgado2007; de la Fuente and Fernández, Reference de la Fuente and Fernández2011), allows assignment of the Vaca Muerta material to Neusticemys neuquina. MOZ-PV-064 and the Vaca Muerta skulls share: relatively well-developed temporal emargination; large orbits; an infolding ridge on the posterior surface of the quadrate below the incisura columella auris and over the mandibular condyle of the quadrate; a keeled dorsal surface of the squamosal; a constriction of the pterygoid plate at the base of the quadrate process of the pterygoid; and a depression on the basisphenoid ventral surface. Because MHNSR-PV-1195 is more complete than MACN-PV-105 (a juvenile), the former will be emphasized in comparisons. Similarly sized MHNSR-PV-1195 and MOZ-PV-064 share: possession of a deep pterygoid fossa where the foramen posterius canalis carotici interni is located; a sharp, steep ridge in the lateral margin of the pterygoids that runs from the posterior end of the pterygoid to the basioccipital; and an open foramen palatinum posterius.

General observations

Four skulls and mandibles have been identified as remains of Neusticemys neuquina: three adults (MOZ-PV-064, 151, MHNSR-PV-1195) and one juvenile (MACN-PV-105). MOZ-PV-064 is the most complete; it preserves most of the skull bones and most of the posterior mandibular elements, lacking only the two premaxillae, and most of the right quadratojugal, right squamosal, and right postorbital. The right quadrate lacks the area articularis mandibularis. The skull is filled with sediments that proved hard to remove, so the CT scans were instrumental in clarifying the morphology of hidden parts. General skull shape is rectangular in lateral aspect and trapezoidal in dorsal and ventral aspects. The skull has a large orbit that occupies slightly less than half of the total length of the skull and a well-developed temporal emargination. The orbits are located laterally and have a laterodorsal orientation allowing dorsal exposure of the jugal and maxilla. The temporal emargination is such that it does not prevent the exposure of the foramen stapedio-temporale in dorsal view.

Nasals

Nasals are present in Neusticemys neuquina (Fig. 2.1–2.8), as in Plesiochelys etalloni (Pictet and Humbert, Reference Pictet and Humbert1857) and other thalassochelydians (Gaffney, Reference Gaffney1975a, Reference Gaffney1976; Rieppel, Reference Rieppel1980). The nasals are in contact for their whole length along the midline and have a subtriangular shape (Fig. 2.1, 2.2). They contact the prefrontal posteriorly (Fig. 2.1, 2.2) and the maxilla lateroventrally (Fig. 2.3, 2.4). In Portlandemys gracilis Anquetin, Püntener, and Billon-Bruyat, Reference Anquetin, Püntener and Billon-Bruyat2015 and Jurassichelon oleronensis Pérez-Garcia, Reference Pérez-Garcia2015, the nasals contact the frontal posteromedially (Gaffney, Reference Gaffney1976; Rieppel, Reference Rieppel1980; Anquetin et al., Reference Anquetin, Püntener and Billon-Bruyat2015; Evers and Benson, Reference Evers and Benson2019) by means of an anterior process of the latter that prevents the medial joining of the prefrontals. Other thalassochelydians, e.g., Plesiochelys etalloni, Solnhofia parsonsi (Gaffney, Reference Gaffney1975b), and Portlandemys mcdowelli (Gaffney, Reference Gaffney1975a), present the condition seen in Neusticemys neuquina in which the nasals do not meet the frontal. In Plesiochelys planiceps Owen, Reference Owen1842, it is possible that there was nasofrontal contact, but because the nasals are missing in the only known specimen (OUMNH J1582), this possibility cannot be assessed.

Prefrontal

The prefrontal is composed of a horizontal plate dorsally and a vertical plate ventrally, as is common in turtles (Fig. 2.1–2.8) The vertical plate of the prefrontal in Neusticemys neuquina forms the anterolateral margin of the foramen orbito-nasale. The dorsal plates of the prefrontals contact each other in the midline, and the nasals prevent contact between prefrontals and the apertura narina externa (Fig. 2.1, 2.2, 2.7, 2.8). The prefrontal contacts are medially with the other prefrontal, anteriorly with the nasals, posteriorly with the frontals (Fig. 2.1, 2.2), anterolaterally with the maxilla (Fig. 2.1–2.8), ventrally with the vomer, and ventrolateraly with the palatines. As discussed above, Neusticemys neuquina is comparable with Plesiochelys etallloni, Solnhofia parsonsi, and Portlandemys mcdowelli in which the prefrontals meet medially and prevent the type of frontal-nasal contact seen in Portlandemys gracilis and Jurassichelon olenorensis. In all thalassochelydians including Neusticemys neuquina, frontal and prefrontal have a medial constriction over the orbits at the suture level.

Frontal

The frontal in Neusticemys neuquina is a plate-like element that has a roughly trapezoidal outline (Fig. 2.1, 2.2), being wider posteriorly than anteriorly and not fused with the other frontal. Ventrally, the frontal forms a moderately deep parasagittal ridge, which forms a medial, ventrally open trough that connects the anterior margin of the cavum cranni posteriorly with the fissura ethmoidalis anteriorly (sulcus olfactorius). The frontal participates on the dorsal margin of the orbit and contacts anteriorly with the prefrontal, posteriorly with the parietal, lateroposteriorly with the postorbital, and medially with the other frontal (Fig. 2.1, 2.2). The morphology of both frontals has a trapezoidal outline, whereas on Plesiochelys etalloni the outline is more rectangular (Gaffney, Reference Gaffney1976, fig 8).

Parietal

The parietal is formed by two bone plates that are perpendicular to one another, a horizontal dorsal plate and a ventral parasagittal one (Fig. 2.1–2.8). The dorsal plate has a transverse suture with the frontal and an anteromedially trending one with the postorbital, which starts converging medially almost at the midline of the parietal's total length (Fig. 2.1, 2.2). The dorsal plate contacts the frontal anteriorly, the postorbitals laterally, the supraoccipital posteroventrally, and the other parietal medially. The parasagittal plate of the parietal descends at an acute angle from the anterior end of the parietal to its ventral contact with the epipterygoid and pterygoid. Posteriorly, it is less steepened and contacts the supraoccipital and prootic along its descending arm. The parasagittal plate forms the dorsal margin of the foramen nervi trigemini and seems to prevent the participation of the prootic in the foraminal margin, as is the case in most Thalassochelydia. However, this condition cannot be assessed fully because our CT scan lacked sufficient resolution to determine bone limits in this section. The foramen nervi trigemini is proportionally larger than in other known thalassochelydians, and reaches farther ventrally due to its a very elongated oval outline. Temporal emargination resembles that of Plesiochelys spp. and Portlandemys spp., in which the foramen stapedio-temporale is exposed in dorsal view. In Jurassichelon oleronensis, the foramen is not fully exposed (Rieppel, Reference Rieppel1980); in Solnhofia parsonsi, this condition has not been observed due to preservation biases (Parsons and Williams, Reference Parsons and Williams1961; Gaffney, Reference Gaffney1976; Joyce, Reference Joyce2000), although it has been argued that it is present but hidden (Parsons and Williams, Reference Parsons and Williams1961; Evers and Benson, Reference Evers and Benson2019).

Postorbital

The postorbital is a plate-like bone that roofs most of the posterolateral portion of the skull. In MOZ-PV-064, the postorbital is preserved almost completely on the left side and has a roughly subrectangular shape (Fig. 2.1–2.6). It is limited by the orbital fenestra anteriorly, the frontal anterodorsally, the jugal anteroventrally, the parietal medially, the quadratojugal posteroventrally, and the squamosal and temporal emargination posteriorly (Fig. 2.1–2.8). The arrangement of postorbitals is the same as in Solnhofia parsonsi, Plesiochelys spp., and Portlandemys spp. In Jurassichelon oleronensis, the postorbital does not participate in the temporal emargination because parietal-squamosal contact prevents its exposure.

Jugal

The jugal is a slender, relatively long element that is exposed on the lateral margin of the skull (Fig. 2.3–2.6). It contacts the maxilla anteriorly, the postorbital dorsally, and the quadratojugal posteriorly. It is limited anteriorly by the orbit and ventrally by the maxilla, cheek emargination, and quadratojugal. The jugal lacks the medially directed jugal process, leaving an open foramen palatinum posterius (Fig. 3.1, 3.2). Jugal morphology is proportionally similar to those of Plesiochelys spp. and Jurassichelon oleronensis. In Portlandemys mcdowelli, Portlandemys gracilis, and Solnhofia parsonsi, the foramen palatinum posterius is closed by a medial process of the jugal.

Quadratojugal

The quadratojugal is a triradiate bone with an anterior branch that contacts the jugal anteriorly and the postorbital dorsally; a posterodorsal branch that contacts the squamosal posteriorly, the quadrate ventromedialy, and the postorbital dorsally; and a narrower posteroventral branch that contacts the quadrate posteromedially (Fig. 2.5, 2.6). The quadratojugal is not exposed on the temporal emargination (Fig. 2.1, 2.2), but it constitutes part of the dorsal and posterior margin of the cheek emargination (Fig. 2.5, 2.6). The quadratojugal is only known in some thalassochelydian turtles (i.e., Plesiochelys etalloni, Plesiochelys bigleri Püntener et al., Reference Püntener, Anquetin and Billon-Brullat2017, Portlandemys gracilis, and Jurassichelon oleronensis). In Plesiochelys etalloni, the posteroventral branch of the quadratojugal almosts reaches the area condylus mandibularis, whereas in Neusticemys neuquina, Plesiochelys bigleri, J. oleronensis, and Portlandemys gracilis, it does not reach that far. In thalassochelydian turtles including Neusticemys neuquina, the quadratojugal displays similar robustness with the exception of J. oleronensis in which this element seems to be slender (Rieppel, Reference Rieppel1980, fig. 3; Anquetin et al., Reference Anquetin, Püntener and Joyce2017, fig.1).

Squamosal

The squamosal in Neusticemys neuquina is roughly cone-shaped, forming a well-developed antrum postoticum (Fig. 2.1, 2.2, 2.5, 2.6) as is typical in thalassochelydian turtles (Anquetin et al., Reference Anquetin, Püntener and Joyce2017), with the exception of Plesiochelys bigleri in which the antrum postoticum is formed only by the quadrate (Püntener et al., Reference Püntener, Anquetin and Billon-Brullat2017). In Neusticemys neuquina, the squamosal is gently sloped dorsally with a strong dorsal keel (Fig. 2.5, 2.6), but this keel is rougher than the one found in Plesiochelys bigleri (see Püntener et al., Reference Püntener, Anquetin and Billon-Brullat2017). In Neusticemys neuquina the squamosal also forms the posterodorsal enclosure of the cavum tympani. The squamosal contacts the postorbital anterodorsally, the quadrate anteroventromedially, the quadratojugal anteriorly, and the opisthotic posteromedially. The overall morphology of the squamosal of Neusticemys neuquina is similar to those of most thalassochelydian turtles with the exception of Plesiochelys bigleri for the reasons mentioned above.

Premaxilla

The premaxilla in MOZ-PV-064 is preserved as a cast (Figs. 2.1–2.8, 3.1, 3.2). It is limited by the apertura narina externa dorsally and the maxilla laterally. The cast replicates the dorsal surface of the ventral plate of the premaxilla; it preserves the posteroventral suture with the vomer (Fig. 3.1, 3.2). The foramen prepalatinum is not preserved. Compared to other adult and juvenile specimens of Neusticemys neuquina (MOZ-PV-5804, MHNSR-PV-1195, MOZ-PV-6094, MACN-PV-105), the snout was presumably acute, as commonly seen in thalassochelydian turtles with the exception of Plesiochelys etalloni, in which the snout is somewhat stouter.

Maxilla

The maxilla in Neusticemys neuquina, as in most turtles, is a combination of a ventrohorizontal and dorsovertical bone plates. The dorsovertical plate is exposed in lateral view and forms the lateral wall of the apertura narina externa. The dorsovertical plate meets the premaxilla medially, the jugal posterodorsally, and the prefrontal dorsally (Fig. 2.1–2.8). The ventrohorizontal plate forms the triturating surface (Fig. 3.1, 3.2). In the case of Neusticemys neuquina, this triturating surface is not as developed as in other thalassochelydian turtles, e.g., Jurassichelon oleronensis (see Rieppel, Reference Rieppel1980), and seems to be more steepened than the one preserved in Plesiochelys etalloni and Plesiochelys planiceps. It also appears to lack or exhibit the reduced medial ridge that is developed on the aforementioned species (Gaffney, Reference Gaffney1975a). This plate contacts the premaxilla and the vomer anteromedially and the palatines medially (Fig. 3.1, 3.2). However, the plate leaves an open foramen palatinum posterius to the fenestra subtemporalis due to the lack of a medially directed process of the maxilla.

Vomer

The vomer of Neusticemys neuquina has a nail-like shape with a bulkier anterior end and an acute posterior one. It contacts the premaxilla anteriorly, the maxilla anterolaterally, the prefrontal anterodorsally, the palatine dorsolaterally, and the pterygoid posteriorly (Fig. 3.1, 3.2). The vomer separates the palatines preventing them from joining medially. The anterior contact of the vomer with the maxilla and premaxilla is Y-shaped. The vomer also does not participate in the triturating surface or the foramen orbito-nasale as in Jurassichelon oleronensis, Plesiochelys spp., and Portlandemys spp. In J. oleronensis, there is no contact between the pterygoid and the vomer, whereas in other thalassochelydians, this contact is present. Solnhofia parsonsi exhibits a special condition among thalassochelydians because of the development of a secondary palate, resulting in a palatal mosaic that differs from those of other thalassochelydian turtles.

Palatine

The palatine of Neusticemys neuquina is a plate-like element that contacts the vomer ventrolaterally along its entire length (Fig. 3.1–3.2). A dorsal anterior contact with the prefrontal is not now identifiable but might have been present. Posteriorly the palatine contacts the pterygoid along a convex suture. Anterolaterally, it contacts the maxilla. They form the mediolateral and dorsal margins of the foramen palatinum posterius and the lateromedial margin of the orbito-nasal foramen. The palatines also form most of the floor of the orbital capsule. In Neusticemys neuquina, as in other thalassochelydians without a secondary palate, e.g., Plesiochelys etalloni, Plesiochelys bigleri, Portlandemys mcdowelli, and Portlandemys gracilis, the vomer separates the palatines medially and contacts the pterygoids, whereas in Jurassichelon oleronensis, the palatines are partially separated in the anterior portion by the vomer only. The palatines in MOZ-PV-064 are more developed anterolaterally than in other thalassochelydians, due to a narrower triturating surface mostly confined to a labial ridge.

Pterygoid

The pterygoid ventral surface shape in Neusticemys neuquina is similar to those of plesiochelyids and some extant and extinct cryptodires (e.g., Chelydra serpentina Linnaeus, Reference Linnaeus1758), exhibiting a medial constriction as well as expansion at both the anterior and posterior ends (Fig. 3.1, 3.2). The pterygoid is a complex bone that contacts a great number of skull bones. In ventral view, the pterygoid contacts the palatine anteriorly, the vomer anteromedially, the other pterygoid medially, the basisphenoid and basioccipital posteromedially, and the quadrate posterolaterally (Fig. 3.1, 3.2). The pterygoid contacts the epipterygoid and the parietal close to its midline dorsolaterally, and the opisthotic and prootic posterodorsally. The quadrate and pterygoid form the fenestra postotica and the pterygoid does not contact the exoccipital. This condition can also be observed in plesiochelyid turtles but not in protostegids in which contact between these elements is present.

The processus pterygoideous externus is well developed with a very robust end. It meets neither the maxilla nor the jugal, in a condition similar to that exhibited by Plesiochelys planiceps, leaving an open foramen palatinum posterius. The crista pterygoidea is as developed as in plesiochelyids, and their overall morphology is similar and comparable. Ventrally, each pterygoid has a ridge that runs from the posterolateral margin of the processus pterygoideus externus to the level of the basioccipital contact where it runs to the basioccipital tuberosities (Fig. 3.1, 3.2). This ridge seems to be more developed in Neusticemys neuquina than in Plesiochelys planiceps, but it is missing in Plesiochelys etalloni. This feature is also recognized in Portlandemys mcdowelli, Portlandemys gracilis, Solnhofia parsonsi, and Jurassichelon oleronensis, in which these ridges are present and well developed.

The pterygoid floors most of the cavum acustico-jugulare, but leaves a good portion of it exposed in ventral view. This flooring is achieved by the means of the posterior process of the pterygoid, which is proportionally similar in size to those of Plesiochelys planiceps, Plesiochelys etalloni, Solnhofia parsonsi, and Jurassichelon oleronensis.

A deep and well-developed pterygoid fossa can be found on the ventral surface of the skull of Neusticemys neuquina. This deep pterygoid fossa is similar to equivalent fossae in Portlandemys mcdowelli, Portlandemys gracilis, and Jurassichelon oleronensis. In Plesiochelys planiceps and Plesiochelys bigleri, the fossa is shallower; in Plesiochelys etalloni and Solnhofia parsonsi, it is absent.

In Neusticemys neuquina, the foramen posterius canalis carotici interni opens ventromedially into the pterygoid fossa, and the foramen is formed only by the pterygoid. In Portlandemys mcdowelli and Portlandemys gracilis, this foramen is also formed only by the pterygoid, whereas in Plesiochelys etalloni, Plesiochelys planiceps, Plesiochelys bigleri, and Jurassichelon oleronensis, it is formed by both the pterygoid and the basisphenoid. The carotid system in Neusticemys neuquina features a ventrally covered split between the carotid subbranches. This condition is different from the one found in Plesiochelys etalloni and Plesiochelys bigleri, in which the split is ventrally exposed (Raselli and Anquetin, Reference Raselli and Anquetin2019). In contrast, Plesiochelys planiceps, Solnhofia parsonsi, and J. oleronensis have a ventrally covered split.

Epipterygoid

The epipterygoid is a laminar bone that contacts the pterygoid posteroventrolaterally and the parietal dorsally. It has a roughly triangular outline, and seems to contribute to part of the foramen nervi trigemini. Its overall morphology does not seem to differ from the epipterygoids of other thalassochelydians.

Basisphenoid

The basisphenoid of Neusticemys neuquina has an elongated triangular outline in ventral view (Fig. 3.1, 3.2). It is slightly longer than the length of the medial suture between the pterygoids. It contacts the pterygoids anterolaterally and the basioccipital posteriorly. Contacts with the opisthotic and prootic are not clearly defined, but it might be an artifact of CT scan data resolution and interpretation rather than a lack of contact between these elements. A distinct blind depression is recognized on the ventral surface of the basisphenoid, close to the basioccipital suture. This depression is relatively deep, occupying a quarter of the total length of the basisphenoid and almost all of the maximum width of the basisphenoid. This blind depression has a subcircular outline, and has not been identified in other thalassochelydian turtles.

The two foramen posterius canalis carotici interni cerebralis are located close to the suture between the basisphenoid and the pterygoid, and they are restricted to the pterygoid. They are located on the mediolateral walls of the pterygoid fossa just below the higher point of the pterygoid medial ridge. The foramen anterius canalis carotici interni cerebralis are found close together on the anterior end of the dorsum sellae and the posterior end of the sella turcica. This condition is shared by all thalassochelydian turtles.

In turtles, it is frequently seen that the basisphenoid is subdivided into two distinct regions separated by the dorsum sellae. In Neusticemys neuquina, the anteriormost region has the sella turcica and the rostrum basisphenoidale and is shorter than the posterior one, a condition shared with other thalassochelydians. The dorsum sellae on Neusticemys neuquina is tall and high and has a medial ridge with a slight anterodorsal projection. Well-defined ridges connect the dorsum sellae with the clinoid processes. The prootic foramen in Neusticemys neuquina is open, leaving the clinoid processes free, as in most thalassochelydians with the exception of Plesiochelys etalloni in which it is closed.

The rostrum basisphenoidale seems to be long, reaching the vomer and palatine suture of the pterygoid.

Prootic

The prootic in Neusticemys neuquina is similar in its overall morphology to those of thalassochelydians. Its contacts are: anterodorsally with the parietal, dorsally with the supraoccipital, posteriorly with the opisthotic, anterolaterally with the quadrate, lateroventrally with the pterygoid, and ventromedially with the basisphenoid. This element is well exposed on its dorsal and anterior surfaces. As in most thalassochelydians, the prootic seems to be excluded from the foramen nervi trigemini by a ventral process of the parietal. The prootic does not have any kind of ossification for the lateral semicircular canal, a condition shared with Plesiochelys etalloni and Plesiochelys planiceps but not with Jurassichelon oleronensis or Portlandemys mcdowelli (see Evers and Benson, Reference Evers and Benson2019, appendices S1, S4). Solnhofia parsonsi is coded by Evers and Benson (Reference Evers and Benson2019) as ambiguous for this character. The prootic ventral process is short and lacks an extensive posterior contact with the pterygoid, as in most thalassochelydians. A recess is present on the posterior surface of the prootic anterodorsolateral to the fenestra ovalis, as noted by Evers and Benson (Reference Evers and Benson2019, appendix S1). The prootic and opisthotic do not contact ventrally, thus preventing ventral enclosure of the fenestra ovalis.

Opisthotic

The opisthotic of Neusticemys neuquina is similar in shape to the opisthotic of other thalassochelydian turtles, enclosing the posterior half of the fenestra ovalis and having a long paroccipital process. It contacts the prootic anteriorly, the quadrate anterolaterally, the exoccipital posteromedially, the supraoccipital dorsally, and the squamosal posterodorsolaterally (Figs. 2.1, 2.2, 3.1–3.4). The processus interfenestralis is developed into a ventrally directed excrescence in a fashion similar to those of Plesiochelys etalloni, Plesiochelys planiceps, Portlandemys mcdowelli, and Solnhofia parsonsi. As a bony feature, this process seems not to have reached any basicranial elements in Neusticemys neuquina, although in Plesiochelys planiceps and Solnhofia parsonsi, it apparently did with the help of an inferred cartilaginous element. The perilymphatic fenestra is contained only by interfenestralis process of the ophistotic, and is similar in size to that of Plesiochelys planiceps. The paroccipital process of the opisthotic has a dorsoventrally convex surface, as is the case in most thalassochelydians (Solnhofia parsonsi differs in that this process has a slight dorsoventral concavity).

Quadrate

As in most turtles, the quadrate of Neusticemys neuquina forms the cavum tympani, the anterolateral wall of the middle ear, the processus articularis, and the condylus mandibularis. It contacts the pterygoid ventromedially, the opisthotic posteromedially, the prootic anteromedially, the quadratojugal anterolaterally, and the squamosal posterodorsally (Figs. 2.3–2.6, 3.1–3.4). The impression of the cavum tympani is kidney-shaped in lateral view (Fig. 2.3–2.6), and seems to have been as deep as those of Plesiochelys etalloni and Plesiochelys planiceps. Posteroventrally, the cavum exhibits the incisura columella auris, which opens posteriorly as in plesiochelyids and other thalassochelydians. Posterodorsally, the cavum tympani opens to the antrum postoticum formed by the squamosal. In occipital view, ventral to the incisura columella auris, and dorsal to the condylus mandibularis, the quadrate has a distinct ridge that has been proposed to be associated with the M. depressor mandibulae or the M. pterygoideus portio ventralis (Gaffney, Reference Gaffney1976). The condylus mandibularis has a rectangular shape, with its main axis transverse to the main axis of the skull (Fig. 3.1, 3.2). The condylus has two distinct surfaces, lateromedially oriented at an obtuse angle. The lateral surface is concave, whereas the medial one is slightly convex. On the anterior surface and close to the prootic suture, a large and well-developed processus trochelaris oticum can be found, forming a well-defined muscular scar. This process is limited to the medial part of the otic chamber and exhibits a deep recess laterally. The cranial articular process of the quadrate has a strong posterior inclination similar to those seen in Plesiochelys etalloni, Plesiochelys bigleri, and Plesiochelys planiceps.

Basioccipital

The basioccipital in Neusticemys neuquina is a robust element that forms the posteriormost region of the skull floor, with a width/length ratio close to 1.5 in MOZ-PV-064. It contacts the basisphenoid anteriorly, the exoccipital laterodorsally, and the pterygoid anterolaterally, and forms the basal third of the condylus occipitalis (Fig. 3.1–3.4). The basioccipital has two well-developed tuberosities (basi-tuberes) on its ventral surface; they are posteroventrally oriented and are connected to the ridges on the pterygoid. These tuberosities delimit a trough that is deep posteriorly on the basioccipital and the basisphenoid, becoming shallow anteriorly on the pterygoid. The development of these tuberosities resembles that of Plesiochelys etalloni, whereas in other thalassochelydians, e.g., Plesiochelys bigleri, Plesiochelys planiceps, Portlandemys gracilis, and Portlandemys mcdowelli, they are not as developed. The basioccipital prevents the exposure of the interfenestralis process in ventral view, as is typical in thalassochelydians.

Exoccipital

The exoccipital of Neusticemys neuquina is a roughly triangular element that contacts the basioccipital ventrally, the opisthotic anterolaterally, and the supraoccipital dorsally (Fig. 3.3, 3.4). There are two foramina nervi hypoglossi in each exoccipital, but not a foramen jugulare posterius separate from the fenestra postotica. Neusticemys neuquina shares this condition with other thalassocheldyians and extant chelonioids. The foramina nervi hypoglossi are not exposed ventrally because an extension of the basioccipital covers them. The exoccipital participates in the condylus occipitalis and in the adult forms the entire ventral margin of the foramen magnum, seemingly preventing any contribution from the basioccipital. However, in the juvenile specimen of Neusticemys neuquina (MACN-PV-105), the basioccipital might have had some dorsal exposure in the margin. The condition seen in MOZ-PV-064 is shared by most thalassochelydians, with the exception of Plesiochelys bigleri and Solnhofia parsonsi in which the basioccipital does participate in defining the margin of the foramen magnum at least in the adult stage (the only one known for these taxa).

Supraoccipital

The supraoccipital of Neusticemys neuquina is, as in most turtles, a bone that can be divided into two distinct regions, the posterior crista supraoccipitalis and an anteroventral region that contributes on the cavum labyrinthicum and cavum cranii. It contacts the exoccipital posteroventrolaterally, the opisthotic ventrolaterally, the prootic anteroventrolaterally, and the parietal anterodorsally (Figs. 2.1–2.6, 3.3, 3.4). The crista supraoccipitalis extends posteriorly almost to the level of the squamosals, but most of its dorsal portion is not preserved. In occipital view, a shallow fossa can be identified on the lateral surface of the supraoccipital, dorsal to the contact of exoccipital, opisthotic, and supraoccipital.

Mandible

In MOZ-PV-064, the postdentary elements are adequately preserved, the dentary and splenial less so. The latter's preserved portions are mostly represented by a cast or covered by sediments (Fig. 4.1–4.10). In dorsal view, the mandible is U-shaped, but this is an artifact of preservation preparation: its anterior region is formed by sediments and a cast which does not represent the true symphysis. However, in other specimens (MOZ-PV-5804, MOZ-PV-6094), the symphysis is preserved. In these specimens, the true shape of the mandible is V-shaped as is typical of thalassochelydians. The mandibular rami meet at an angle close to 40°. No ramal triturating surface is preserved in MOZ-PV-064, but it is in MHNSR-PV-1195. In the latter, the surface is not as developed as in Plesiochelys etalloni, or most other thalassochelydians for that matter, and it is very steepened, although not reaching the extreme condition seen in Dermochelys coriacea Vandelli, Reference Vandelli1761. In lateral view, the mandible of Neusticemys neuquina is taller than that of Portlandemys gracilis.

Figure 4.

Neusticemys neuquina from Tithonian levels of Vaca Muerta Formation, photographs and drawings of the lower jaw MOZ-PV-064 in: (1, 2) dorsal view, (3, 4) ventral view, (5, 6) right lateral view, (7, 8) left lateral view, and (9, 10) left visceral view. ang = angular; art = articular; cor = coronoid; den = dentary; fdm = foramen dentofaciale majus; fic = foramen intermandibularis caudalis; fme = fossa meckelii; fna = foramen nervi auriculotemporalis; pra = prearticular; sp = splenial; sur = surangular.

Dentary

Only the posterior portion of the dentary is preserved in MOZ-PV-064. It preserves the foramen dentofacile majus and the contact with the postdentary elements (Fig. 4.1–4.8). The foramen dentofaciale majus is large and comparable to those of other thalassochelydians. The dentary contacts the splenial medially, the surangular posterolaterally, the coronoid posterodorsally, and the angular ventrally. In MOZ-PV-6094, the dentary forms the anterior end of the mandible including the mandibular symphysis, which seems to have been short (< 30% of the total mandibular length). The dentaries are fused midsagitally and there is no symphyseal hook. The main feature of the dentary of Neusticemys neuquina, as seen in MHNSR-PV-1195, is the lack of a well-developed triturating surface. There is a well-defined labial ridge, but the lingual ridge seems to have been absent or poorly developed, leaving a very steepened triturating surface.

Splenial

Only the left splenial can be identified, situated on the left ramus of the mandible. It is a sheet-like bone and poorly preserved, only the posteroventral portion being present (Fig. 4.9, 4.10). The only recognizable contacts are posteriorly with the prearticular and ventrally with the angular. The topological position of the splenial seems to be in agreement with the known position of this element in other thalassochelydians.

Angular

The angular in MOZ-PV-064 is well preserved on both rami of the mandible. It is an elongated element that forms the posteroventral portion of the mandible (Fig. 4.3–4.10). It contacts the surangular dorsolaterally, the prearticular dorsomedially, and the splenial dorsally. The angular does not seem to have reached the mandibular symphysis, as it does in Portlandemys gracilis (see Anquetin et al., Reference Anquetin, Püntener and Billon-Bruyat2015), but it extends farther anteriorly in this species than its homolog in Plesiochelys etalloni (see Gaffney, Reference Gaffney1976, fig. 48).

Surangular

The surangular is a sheet-like bone that forms the posterolateral wall of the mandible, most of the lateral surface of the area articularis, and the lateral side of the fossa meckelii (Fig. 4.1–4.8). It contacts the articular posteromedially, the dentary anteriorly, the angular ventrally, and the coronoid dorsomedially. The surangular in Neusticemys neuquina is relatively taller than the surangular found in most thalassochelydians. A single foramen auriculotemporalis opens on the lateral wall of the surangular, close to the area articularis.

Coronoid

Most of the coronoid is preserved, although the processus coronoideus is broken at its base and its anteriormost portion is broken and missing on both rami (Fig. 4.9, 4.10). It contacts the prearticular posteroventraly, the surangular posterolaterally, and the dentary anterodorsolaterally. It forms the anteromedial margin of the dorsal opening of the fossa meckelii. Because the anterior portion is not preserved, it is not possible to identify its participation in bounding either the foramen intermandibularis or the unnamed foramen found in all thalassochelydians (see Evers and Benson, Reference Evers and Benson2019).

Articular

The articular of Neusticemys neuquina is a roughly subrectangular element that respectively contacts the surangular and prearticular laterally and medially (Fig. 4.1, 4.2, 4.5–4.10). Together with the prearticular and surangular, it forms the area articularis mandibularis, as in Plesiochelys etalloni. The contributions of the prearticular and surangular are relatively larger than in other species, e.g., Toxochelys latiremis Cope, Reference Cope1873. The area articularis mandibularis displays two distinct and slightly concave areas separated by a medial ridge. The lateral surface of the area mandibularis is formed principally by the surangular, whereas the medial one is formed mainly by the articular and to a lesser degree by the prearticular.

Prearticular

The prearticular in Neusticemys neuquina is a plate-like element that is quite comparable to those of most thalassochelydians. It composes the posteromedial wall of the mandibular ramus and a small portion of the medial surface of the area articularis mandibularis (Fig. 4.1, 4.2, 4.9, 4.10). Posteriorly, it contacts the articular, ventrally the angular, dorsally the coronoid, and anteriorly the splenial. The fossa meckelii is large and well defined. The prearticular forms the medial margin of the fossa and seems to have made a greater participation to its outline than the surangular. The foramen intermandibularis caudalis can be identified in the contact surface between prearticular and angular on the posterior end of the prearticular.

Cervical vertebra

The right half of a cervical vertebra (Fig. 5.1–5.12) preserves most of the neural spine and neural arch, both postzygapophyses, most of the right portion of the centrum, and part of the right transverse process. The centrum preserves the right side and posterior condyle; the left side and anterior articular surface are missing. The posterior condyle is relatively deep and seems to have been taller than wide. The neural spine is high and bears two posteroventrally oriented postzygapophyses. The neural spine and neural arch are slightly higher than the centrum. A strongly developed transverse process is located at the anterior portion of the base of the neural arch. The centrum is strongly ventrally keeled.

Figure 5.

Neusticemys neuquina from Tithonian levels of Vaca Muerta Formation, photographs and drawings of the cervical vertebra MOZ-PV-064 in: (1, 7) right lateral view, (2, 8) left lateral view, (3, 9) anterior view, (4, 10) posterior view, (5, 11) ventral view, and (6, 12) posterior view fob = foreign object; nc = neural crest; prz = prezygapophysis; tp = transverse process; vc = ventral crest.

Material

MOZ-PV-064, an almost complete skull with lower jaw and a cervical vertebra.

Remarks

The most complete cranial remains referred to Neusticemys neuquina are described, allowing us to emend and expand the diagnosis of this species previously, mostly known from postcranial materials. The new description allowed comparison with other turtles of the Thalassochelydia clade.