The comparative osteology of Plesiochelys bigleri n. sp., a new coastal marine turtle from the Late Jurassic of Porrentruy (Switzerland)

General description

The cranium of Plesiochelys bigleri is known from two specimens. The first one is the holotype specimen (MJSN TCH007-252) and consists of the posterior part of a skull (Fig. 4) associated with a relatively complete shell and some limb and girdle elements. The parts of the skull anterior to the pterygoids (including the nasal, palatal, and orbital regions) and the lateral part of the left otic chamber are missing. For this reason, the following bones are missing from that specimen: nasal, prefrontal, frontal, postorbital, premaxilla, maxilla, vomer, and palatine. Post-mortem deformation is minor, but the basicranium is partly disarticulated along the basisphenoid–pterygoid suture. The preservation of this specimen is not optimal. The skull was initially heavily encrusted by ferruginous mineralizations. Although most of these mineralizations were skillfully removed during preparation, sutures remain rather difficult to see in this specimen. The length of the skull as measured from the pterygoid–vomer suture to the stem of the condylus occipitalis (the condyle itself is missing) is 38.5 mm, whereas the width at the level of the condyli mandibularis is 60.6 mm (Table 2).

The second specimen (MJSN TCH006-1451) is an isolated, partial skull missing all of the skull roof, the ethmoid region, the orbital area, the anterior part of the snout, and most of the palate (Fig. 5). As a result, the following bones are missing from that specimen: nasal, prefrontal, frontal, postorbital, and premaxilla. The skull has been severely flattened dorsoventrally during fossilization. Crushing forces were not oriented exactly dorsoventrally and resulted in a slight lean toward the right hand side, mostly apparent in posterior view (Figs. 5E and 5F). As preserved, the skull is 59.8 mm in length from the anteriormost part of the maxilla to the condylus occipitalis (34.4 mm from pterygoid–vomer suture to condylus occipitalis; see Table 2). The width taken at the level of the condyli mandibularis is 60 mm. MJSN TCH006-1451 is therefore slightly smaller than the holotype specimen (Table 2). Compared to other plesiochelyids, MJSN TCH006-1451 is about the same size as NMS 8738 and NMS 9145, both referred to Plesiochelys etalloni, but it is much smaller than OUMNH J.1582, the holotype skull of Plesiochelys planiceps (Anquetin, Püntener & Billon-Bruyat, 2015: Table 1).

Parietal

The parietals are best preserved in the holotype specimen (MJSN TCH007-252), but fragments of the ventral part of the processus inferior parietalis are preserved on the right hand side of the paratype specimen (MJSN TCH006-1451). The parietals form the posterior part of the skull roof and meet one another medially (Fig. 4). The anterior and lateral contacts of the parietals are not preserved. Most of the posterior margin of each parietal is natural. The temporal skull roof is relatively low compared to other Plesiochelys spp. The upper temporal emargination largely exposes the foramen stapedio-temporale in dorsal view, but does not extend as far anteriorly as to expose the processus trochlearis oticum. This is similar to the condition in Plesiochelys etalloni (NMB 435, NMS 40870). The development of the upper temporal emargination is not well known in other plesiochelyids. The preservation of the dorsal surface of the parietals in the holotype (MJSN TCH007-252) does not allow to identify scute sulci with confidence.

As usual in plesiochelyids (Anquetin, Püntener & Billon-Bruyat, 2015), the anterior braincase wall formed mostly by the processus inferior parietalis is shorter than in most turtles as a result of the great development of the foramen interorbitale. As preserved, the ventral contacts of the parietal in Plesiochelys bigleri are as follows: epipterygoid and pterygoid anterior to the foramen nervi trigemini; pterygoid, quadrate and prootic posterior to the foramen nervi trigemini; and supraoccipital posteroventrally (Figs. 4I–4L). The parietal forms the dorsal half of the anterior margin of the foramen nervi trigemini. There, the parietal and pterygoid have a broad contact that excludes the epipterygoid from the margin of that foramen. The posterior margin of the foramen nervi trigemini is formed entirely by a posteroventral process of the parietal that reaches the quadrate and prevents a contact between the pterygoid and prootic. This configuration of the region of the foramen nervi trigemini is characteristic of plesiochelyids (except for the parietal–quadrate contact that is absent in Plesiochelys planiceps) and J. oleronensis (Anquetin, Püntener & Billon-Bruyat, 2015). In Plesiochelys bigleri, the foramen nervi trigemini is somewhat more rounded than in other plesiochelyids, in which the foramen is usually taller than wide and oval in shape. In most specimens of Plesiochelys etalloni (NMB 435, NMS 8738, NMS 40870, NMS 40871, and NHMUK R3370), the foramen nervi trigemini is very narrow and forms a slit-like opening.

Jugal

In the holotype specimen (MJSN TCH007-252), the anterior part of the right jugal is preserved as an isolated fragment. A small flake of bone (probably from the maxilla) is still attached to the ventromedial margin of the isolated jugal. In the paratype specimen (MJSN TCH006-1451), only the ventral part of the jugal is preserved, but the bone is more complete on the right hand side (Fig. 5). The jugal forms the posteroventral corner of the orbit and contacts the maxilla ventrally and the quadratojugal posteriorly. The other contacts of the jugal with surrounding elements are unclear. As in Plesiochelys etalloni, Plesiochelys planiceps, and J. oleronensis, the jugal lacks a medial process extending to meet the pterygoid and/or palatine. Similarly, the maxilla lacks a corresponding posteromedial process and the foramen palatinum posterius remains open posterolaterally. The ventral margin of the jugal and the posterior border of the maxilla show that the lower temporal emargination was relatively well developed in Plesiochelys bigleri, much like the condition in Plesiochelys etalloni, Plesiochelys planiceps and J. oleronensis (note that the condition in Portlandemys spp. is unknown).

Quadratojugal

The morphology of the quadratojugal is poorly known in Plesiochelys bigleri. Only a small part of the right quadratojugal is preserved anterior to the cavum tympani in the holotype (MJSN TCH007-252). A larger portion of the right quadratojugal is preserved in the paratype (MJSN TCH006-1451), but the bone is fragmented in several pieces and misses its dorsal and anterior margins. Anteriorly, the quadratojugal articulates with the jugal. Its ventral margin forms the posterior half of the lower temporal emargination. Posteriorly, the quadratojugal braces the external margin of the cavum tympani along an extended, curved suture with the quadrate (Figs. 4 and 5). It seems that the posteroventral process of the quadratojugal along the processus articularis of the quadrate is proportionally shorter in Plesiochelys bigleri than in Plesiochelys etalloni and Portlandemys gracilis (unknown in other plesiochelyids). Posterodorsally, the quadratojugal has a broad vertical contact with the squamosal just dorsal and slightly anterior to the level of the incisura columellae auris.

Squamosal

The squamosal is best preserved in the holotype (MJSN TCH007-252), but the paratype (MJSN TCH006-1451) provides additional information on the morphology of this element. As usual in turtles, the squamosal forms the posterolateral corner of the otic chamber, notably contributing to the formation of the antrum postoticum. The anterodorsal part of the bone is unfortunately not preserved in any specimen. The contacts of the squamosal in the temporal roof (notably with the postorbital and parietal) are therefore unknown in Plesiochelys bigleri. The other contacts of the squamosal are as follows: quadrate anteromedially and ventrally; quadratojugal anterolaterally; and opisthotic posteromedially. Compared to other turtles, the antrum postoticum of plesiochelyids is usually described as moderately developed (Gaffney, 1976; Rieppel, 1980; Anquetin, Püntener & Billon-Bruyat, 2015). In Plesiochelys bigleri, the antrum postoticum is also moderately developed, but, compared to other plesiochelyids, the cavity is deeper both medially and posterodorsally. Remarkably, the anterior margin of the antrum postoticum is formed entirely by the quadrate (MJSN TCH007-252; Fig. 4). This differs from the condition observed in Plesiochelys etalloni and Portlandemys gracilis (as well as J. oleronensis and most basal pan-cryptodires) in which the squamosal contributes to the anterolateral margin of the antrum postoticum. The posterodorsal parasagittal crest of the squamosal is well developed in Plesiochelys bigleri, notably the posterior part that forms an extended and somewhat pointed lamina (MJSN TCH007-252; Figs. 4I and 4J). A wide concavity is present on the lateral surface of the squamosal just posterior to the opening of the antrum postoticum. Posteroventrally, the squamosal contributes only to a limited extent to the rugose area for the attachment of the M. depressor mandibulae (Werneburg, 2011).

Maxilla

The maxilla is not preserved in the holotype specimen. Only the posterior part of the bone is preserved in the paratype (MJSN TCH006-1451), but about half of the original bone is present on the right hand side in that specimen (Fig. 5). As preserved, the maxilla contacts the jugal posterodorsally and the palatine posteromedially. As for the jugal, the maxilla lacks a posteromedial process that would close the foramen palatinum posterius posterolaterally. The maxilla seems to participate to the anterolateral margin of the foramen palatinum posterius both in dorsal and ventral views. The labial ridge is slender and very high, notably posteriorly. Anteriorly, the labial ridge is somewhat blunted in MJSN TCH006-1451, but this is possibly a preservational artifact. The labial ridge is separated from the lingual ridge by a deep trough. As in Plesiochelys etalloni, Plesiochelys planiceps, and Portlandemys mcdowelli (unknown in Portlandemys gracilis), the lingual ridge is broad and high. In these species, the lingual ridge is rugose, which contrasts with the condition in J. oleronensis where the summit of the lingual ridge is acute and smooth. In Plesiochelys bigleri, the lingual ridge is closer to the condition in other plesiochelyids, but the bone surface is slightly eroded. In contrast to Portlandemys mcdowelli, the lingual ridge is formed entirely by the maxilla. Due to poor preservation and the presence of markings on the bone surface, we were unable to locate the foramen supramaxillare in MJSN TCH006-1451.

Vomer

The vomer is missing in the holotype (MJSN TCH007-252) and only the posteroventral part of the bone is preserved in MJSN TCH006-1451 (only visible in ventral view). Posteriorly, the vomer reaches the pterygoids and fully separates the palatines on the ventral surface of the palate, but not on the dorsal surface (Fig. 5). This configuration is similar to that of Plesiochelys etalloni and Portlandemys mcdowelli, but differs from that of J. oleronensis. The condition in Plesiochelys planiceps is uncertain because the posteroventral part of the vomer is broken in the type and only known specimen (OUMNH J.1582).

Palatine

The palatine is only preserved in the paratype (MJSN TCH006-1451). The bone is not complete and only the posterior and lateral parts can be observed (Fig. 5). As preserved, the palatine contacts the pterygoid posteriorly, the vomer medially on the ventral surface of the palate, the other palatine medially on the dorsal surface of the palate, and the maxilla anterolaterally. Laterally, the palatine defines the anteromedial border of the foramen palatinum posterius. The outline of the foramen palatinum posterius is impossible to assess in Plesiochelys bigleri since the lateral margin of the palatine and anterolateral part of the pterygoid are incomplete in all of the specimens. The palatine was initially described as forming a small portion of the lingual ridge in Plesiochelys and Portlandemys (Gaffney, 1976). In fact, the palatine really contributes to the lingual ridge only in Portlandemys mcdowelli (condition unknown in Portlandemys gracilis). In Plesiochelys etalloni, Plesiochelys bigleri, Plesiochelys planiceps, and J. oleronensis, the palatine–maxilla suture extends along the medial base of the lingual ridge, i.e., on its dorsomedial edge, and the palatine therefore does not take part into the formation of the ridge or triturating surface.

Quadrate

On the dorsal surface of the otic chamber, the quadrate contacts the prootic anteromedially, the opisthotic posteromedially, and the squamosal posterolaterally. As usual, quadrate and prootic contribute relatively equally to the formation of the foramen stapedio-temporale, but variability exists (see right side of the holotype MJSN TCH007-252). Anterolaterally, the quadrate has a long curved suture with the quadratojugal. This suture is mostly parallel to the anterior margin of the cavum tympani, but lies anterior to it (the quadratojugal does not enter the margin of the cavum tympani). The cavum tympani of Plesiochelys bigleri is deeper than in other plesiochelyids. This is notably apparent anteroventral and posterodorsal to the incisura columellae auris. In contrast to other plesiochelyids, the lateral margin of the cavum tympani faces more posterolaterally than laterally in Plesiochelys bigleri (Figs. 4 and 5). The quadrate forms the entire anterior margin of the antrum postoticum, which is remarkable (see Squamosal). The incisura columellae auris remains open posteroventrally. The processus articularis of the quadrate is damaged in the holotype (MJSN TCH007-252), which may give the misleading impression that this process is short. This structure is better preserved in the paratype (MJSN TCH006-1451). A prominent ventrally infolding ridge occurs on the posterior surface of the processus articularis. Starting from the posterolateral corner of the condylus mandibularis, this ridge extends dorsomedially toward the incisura columellae auris increasing in height. Ventral to the incisura, the ridge thickens, and bends sharply medially. Pursuing its medial course, the ridge thins progressively, and finally merges with the posterolateral border of the pterygoid. This ventrally infolding ridge is found in all plesiochelyids, but also in J. oleronensis, Solnhofia parsonsi Gaffney, 1975b and Parachelys eichstaettensis Meyer, 1864, and is a strong argument to support the monophyly of these Late Jurassic coastal marine turtles from Europe (Anquetin, Püntener & Billon-Bruyat, 2015). The condylus mandibularis of Plesiochelys bigleri is remarkably narrow anteroposteriorly compared to other plesiochelyids. The condyle consists of two slightly concave facets separated by a wide, but shallow parasagittal furrow.

The quadrate forms about half of the moderately developed processus trochlearis oticum, which is less prominent than in Plesiochelys etalloni. On the anterior surface of the otic chamber, the quadrate contacts the prootic dorsomedially, the parietal medially, and the pterygoid ventromedially. This region is best preserved on the left hand side of the holotype MJSN TCH007-252. Inside the cavum acustico–jugulare, the quadrate forms the lateral part of the aditus canalis stapedio-temporalis and canalis stapedio-temporalis, as well as the lateral half of the posterior opening of the canalis cavernosus. Below the antrum postoticum, a rugose area occurs on the ventral surface of the skull and probably served for muscular attachment. In Plesiochelys bigleri, this area is mostly formed by the quadrate, with minor contributions from the opisthotic and squamosal.

Epipterygoid

The epipterygoid is best preserved in the holotype (MJSN TCH007-252), whereas only the ventral part of the bone is preserved in the paratype (MJSN TCH006-1451). In lateral view, the epipterygoid is a trapezoidal element located between the crista pterygoidea of the pterygoid and the processus inferior parietalis of the parietal. The epipterygoid is exposed on the medial surface of the anterior braincase wall, albeit to a more moderate extent. Posteriorly, a broad contact between the parietal and pterygoid excludes the epipterygoid from the anterior margin of the foramen nervi trigemini (Figs. 4I–4L). The same configuration seemingly occurs on the medial surface of the anterior braincase wall. Posteroventrally, a fossa cartilaginis epipterygoidei is present and prevents a contact between the epipterygoid and the quadrate. The anterolateral process of the epipterygoid that extends onto the dorsal surface of the pterygoid is well developed. A shallow furrow prolongs this process on the dorsal surface of the pterygoid in Plesiochelys bigleri (MJSN TCH006-1451). A similar process occurs in Plesiochelys etalloni and Portlandemys mcdowelli. This process is more reduced in Plesiochelys planiceps and J. oleronensis.

Pterygoid

Except for each processus pterygoideus externus, the pterygoids are complete in the paratype (MJSN TCH006-1451), but their ventral surface is somewhat abraded. Longitudinal striae extend from the pterygoid fossae posteriorly to the palatines and vomer anteriorly (Fig. 5C). In the holotype (MJSN TCH007-252), the anterior part of the pterygoids is more poorly preserved, although the left processus pterygoideus externus is complete. In contrast, the posterior part of the pterygoids (pterygoid fossa and quadrate process) is better preserved. The pterygoids are disarticulated from the basicranium in the holotype.

In ventral view, the pterygoid contacts the vomer anteromedially, the palatine anteriorly, the quadrate posterolaterally, and the basisphenoid posteromedially. Posteriorly, the pterygoid probably also contacted the basioccipital, but the sutures in this region are poorly preserved in both specimens. As preserved, the pterygoid does not seem to contact the exoccipital posteriorly, but this region is rarely well preserved in plesiochelyids and the presence/absence of this contact is probably of poor systematic value (Anquetin, Püntener & Billon-Bruyat, 2015). In ventral view, the processus pterygoideus externus is similar in development and shape to that of Plesiochelys etalloni and Plesiochelys planiceps. However, the parasagittal plate on the lateral margin of the processus pterygoideus externus is more developed in Plesiochelys etalloni than in the two other species. A distinct ridge extends posteromedially from the posterior edge of the processus pterygoideus externus to the posterolateral part of the ventrally open canalis caroticus internus. This ridge forms the medial margin of the pterygoid fossa. Compared to other plesiochelyids, J. oleronensis and eurysternids (Anquetin, Püntener & Billon-Bruyat, 2015), the pterygoid fossa of Plesiochelys bigleri stands out as being remarkably shallow. The configuration of the canalis caroticus internus is similar to the condition in Plesiochelys etalloni and J. oleronensis (see Anquetin, Püntener & Billon-Bruyat, 2015 for a review). The canalis caroticus internus is superficial and open ventrally at least along its anterior half (Figs. 4 and 5). The posterior half of the canalis caroticus internus may have been floored by a thin ventromedial flap of the pterygoid, but the preservation of the paratype (MJSN TCH006-1451) prevents a definitive conclusion. Since no flooring is preserved in any specimen, the position of the foramen posterius canalis carotici interni cannot be determined with precision. The anterior part of the canalis caroticus internus follows the basisphenoid–pterygoid suture. By comparison with Plesiochelys etalloni, the split between the palatine and cerebral branches of the internal carotid artery was probably not floored by bone, but the preservation of the specimens prevents a clear observation of the foramen posterius canalis carotici palatinum (possibly visible on the left-hand side of MJSN TCH006-1451) and foramen posterius canalis carotici cerebralis. The flooring of the cavum acustico–jugulare by the posterolateral part of the pterygoid is not very extensive. The processus interfenestralis of the opisthotic therefore remains largely visible in ventral view. For that matter, Plesiochelys bigleri is closer to the condition observed in Plesiochelys planiceps. Although this contact is now disarticulated in the holotype, there was a contact between the processus interfenestralis of the opisthotic and the pterygoid. Based on the disarticulated surface, this contact was probably not sutural. Lateral to this contact, the pterygoid forms the medial part of the floor of the posterior opening of the canalis cavernosus.

In the ethmoid region, the pterygoid forms the ventral margin of the foramen nervi trigemini, the rest of the foramen margin being formed by the parietal (Figs. 4I–4L). The pterygoid–parietal contact anterior to the foramen nervi trigemini excludes the epipterygoid from the margin of this foramen. Medial to the crista pterygoidea, the pterygoid forms the floor of the sulcus cavernosus. This region is best preserved in the paratype specimen (MJSN TCH006-1451). The foramen anterius canalis carotici palatinum opens in the anterior part of the sulcus cavernosus about halfway between the level of the foramen anterius canalis carotici interni and the tip of the trabecula. After exiting the foramen anterius canalis carotici palatinum, the palatine branch of the internal carotid artery continues forward in a groove within the floor of the sulcus cavernosus and is not obstructed anteriorly by a crest extending from the crista pterygoidea to the midline shelf of the pterygoid, as seen in some plesiochelyids (Anquetin, Püntener & Billon-Bruyat, 2015). Anterolaterally, a small foramen occurs on the dorsal surface of the pterygoid medial to the anterolateral process of the epipterygoid. This foramen may correspond to the foramen nervi vidiani, but this should be further investigated (see also Anquetin, Püntener & Billon-Bruyat, 2015). Anterior to this small foramen, the dorsal surface of the pterygoid forms a shallow, rounded depression, which likely served for the attachment of one of the eye muscles (Gaffney, 1976). Finally, as noted above (see Epipterygoid), a shallow furrow prolongs the anterolateral process of the epipterygoid on the dorsal surface of the pterygoid.

Supraoccipital

The supraoccipital contacts the parietal anteriorly, the prootic anterolaterally, the opisthotic posterolaterally, and the exoccipital posteriorly. A broad contact between the prootic and opisthotic separates the supraoccipital from the quadrate on the floor of the fossa temporalis superior (Figs. 4 and 5). This contrasts with the condition observed in Portlandemys mcdowelli and most specimens referred to Plesiochelys etalloni. Most of the crista supraoccipitalis is preserved in the holotype MJSN TCH007-252. This structure is relatively low, especially compared to Plesiochelys planiceps and Portlandemys mcdowelli. The condition in Plesiochelys etalloni is more difficult to appreciate based on the available material, but it seems to be intermediate in development between Plesiochelys bigleri and the two aforementioned species. The posterior part of the crista supraoccipitalis is broken in MJSN TCH007-252, but it is unlikely that this structure projected far behind the level of the condylus occipitalis. As usual, the supraoccipital forms the dorsal part of the foramen magnum.

Exoccipital

The exoccipital contacts the supraoccipital dorsomedially, the opisthotic laterodorsally, the basioccipital ventrally, and the processus interfenestralis of the opisthotic anterolaterally. As preserved, there is no contact between the exoccipital and pterygoid, which may be a difference with Plesiochelys etalloni, but this contact is often difficult to observe in this taxon (Anquetin, Püntener & Billon-Bruyat, 2015). An exoccipital–pterygoid contact is otherwise present in Portlandemys ssp. and J. oleronensis. In contrast to Plesiochelys etalloni, Plesiochelys planiceps, Portlandemys mcdowelli (condition unknown in Portlandemys gracilis), and J. oleronensis, the exoccipitals apparently do not meet in the floor of the foramen magnum, but this feature can present some intraspecific variability (Anquetin & Chapman, 2016). The contribution of the exoccipital to the condylus occipitalis is uncertain, but probably reduced. There are two foramina nervi hypoglossi on each side and they are formed exclusively by the exoccipital.

Basioccipital

The ventral aspect of the basioccipital is poorly preserved in all specimens. However, it is apparent that the basioccipital contacts the basisphenoid anteriorly and the pterygoid anterolaterally in this area. The tubercula basioccipitale are only moderately developed and correspond to what is known in similarly sized individuals of Plesiochelys etalloni, but not in larger specimens (Anquetin & Chapman, 2016). Posterodorsally, there is an extensive contact with the exoccipital, but the basioccipital nevertheless enters the ventral margin of the foramen magnum. It appears that the basioccipital forms most of the condylus occipitalis, which is only preserved (poorly) in MJSN TCH006-1451. In dorsal aspect, the basioccipital offers little remarkable features. The basis tuberculi basalis is relatively low, and an oval depressed area occurs in front of it. Finally, there is also a contact with the ventromedial margin of the processus interfenestralis of the opisthotic in the floor of the recessus scalae tympani.

Prootic

On the dorsal surface of the otic chamber, the prootic contacts the parietal anteromedially, the quadrate laterally and anteroventrally, the opisthotic posteriorly, and the supraoccipital posteromedially. The prootic forms the medial half of the processus trochlearis oticum, which is reduced compared to that of Plesiochelys etalloni. Anteromedially, the prootic is excluded from entering the posterior margin of the foramen nervi trigemini by an elongate descending process of the parietal (Figs. 4I–4L). A broad contact between this descending process of the parietal and the quadrate prevents a prootic–pterygoid contact in this area. This configuration is found in all plesiochelyids but Plesiochelys planiceps, as well as in J. oleronensis and possibly also in eurysternids (Anquetin, Püntener & Billon-Bruyat, 2015).

Inside the cavum acustico–jugulare, the prootic forms the anterior half of the fenestra ovalis, the medial part of the aditus canalis stapedio-temporalis, and most of the roof of the posterior opening of the canalis cavernosus. As in other plesiochelyids, the aditus canalis stapedio-temporalis is located more posteriorly than the posterior opening of the canalis cavernosus rather than in the roof of the latter as in many turtles. In this area, the prootic contacts the opisthotic posterodorsally, the pterygoid ventrally, and the quadrate laterally. A C-shaped furrow occurs on the surface of the prootic dorsolateral to the fenestra ovalis. Inside the cavum cranii, the prootic contacts the supraoccipital posterodorsally, the parietal anterodorsally, the basisphenoid ventromedially, and the pterygoid anteroventrally. In contrast to Plesiochelys etalloni, the pila prootica is not ossified. Medial to the foramen nervi trigemini, the prootic forms a well-defined recess that accommodated the ganglion of the trigeminal (V) nerve.

Opisthotic

On the dorsal surface of the otic chamber, the opisthotic contacts the prootic anteriorly, the quadrate anterolaterally, the squamosal posterolaterally, the supraoccipital medially, and the exoccipital posteromedially. The anterior contact with the prootic is relatively wide and clearly separates the supraoccipital and quadrate (Figs. 4 and 5). This condition is found also in Plesiochelys planiceps, Portlandemys gracilis, and J. oleronensis. In contrast, the prootic–opisthotic contact is reduced or absent in Portlandemys mcdowelli and most specimens of Plesiochelys etalloni. The posterior aspect of the opisthotic is complete only on the right hand side of the paratype (MJSN TCH006-1451). The extremity of the processus paroccipitalis forms a distinct crest for muscular attachment. In Plesiochelys bigleri, as well as in Portlandemys mcdowelli and J. oleronensis, the processus paroccipitalis extends posterolaterally. This contrasts with Plesiochelys etalloni, Plesiochelys planiceps, and Portlandemys gracilis in which the processus paroccipitalis extends more posteriorly. This difference changes the occipital outline as seen in dorsal view from a broad arch in Plesiochelys bigleri to a more narrow arch in Plesiochelys etalloni.

Ventrally, the opisthotic forms a large part of the roof of the cavum acustico–jugulare. In this area, the opisthotic contacts the prootic anteriorly, the quadrate anterolaterally, the squamosal posterolaterally, and the exoccipital posteromedially. The extremity of processus interfenestralis is triangular in shape and contacts the exoccipital, basioccipital, and pterygoid. The medial margin of the processus interfenestralis is pierced by a large fenestra perilymphatica, which is apparently entirely contained in bone thanks to a ventromedial contribution from the basioccipital (MJSN TCH007-252). A well-defined foramen externum nervi glossopharyngei opens at the base of the processus interfenestralis. It is noteworthy that this foramen opens more laterally than in other plesiochelyids and J. oleronensis. Lateral to the base of the processus interfenestralis and foramen externum nervi glossopharyngei, the opisthotic forms a strong curving ridge oriented anteroposteriorly. As far as we know, this ridge is not present in other plesiochelyids.

Basisphenoid

In ventral aspect, the basisphenoid contacts the pterygoid anteriorly and laterally, and the basioccipital posteriorly (Figs. 4 and 5). As noted above, the basisphenoid–pterygoid suture is disarticulated in the holotype, but this contact is better preserved in the paratype (MJSN TCH006-1451). The posterior contact with the basioccipital is poorly preserved in both specimens. The general outline of the basisphenoid in ventral view is triangular, as in most plesiochelyids (Anquetin, Püntener & Billon-Bruyat, 2015). The ventral surface of the basisphenoid is apparently slightly concave. The morphology of the canalis caroticus internus is similar to the condition observed in Plesiochelys etalloni and J. oleronensis (Anquetin, Püntener & Billon-Bruyat, 2015). The canalis caroticus internus is superficial and runs along the basisphenoid–pterygoid suture as a ventrally open canal. The posterior part of the canalis caroticus internus may have been partly floored by a ventromedial flap formed by the pterygoid, but this region is not well preserved in any specimen. Based on the available material, the position of the foramen posterius canalis carotici interni cannot be determined. As in Plesiochelys etalloni, the split between the cerebral and palatine branches of the internal carotid artery was probably not floored by bone. However, the preservation prevents a clear observation of the foramen posterius canalis carotici palatinum. In the holotype, a portion of each canalis caroticus cerebralis is preserved on the anterior part of the basisphenoid. Anteriorly, these two canals penetrate deeply in the basisphenoid and exit in the sella turcica dorsally. A small unnamed foramen opens dorsomedially along the canalis caroticus cerebralis (only visible in MJSN TCH007-252; Fig. 4).

The dorsal aspect of the basisphenoid is better preserved in the paratype (MJSN TCH006-1451) and can be readily observed in that specimen thanks to the loss of the anterior part of the roof of the cavum cranii (Fig. 6). The basioccipital contacts the basioccipital posteriorly, the prootic posterolaterally, and the pterygoid anterolaterally and anteriorly. The part of the basisphenoid that floors the cavum cranii is slightly concave. A moderately raised area located sagittally on the posterior margin of the basisphenoid corresponds to the anterior part of the basis tuberculi basalis. The posterior foramen nervi abducentis opens about midway along the part of the basisphenoid that floors the cavum cranii. The anterior foramen nervi abducentis opens ventral and slightly anteromedial to the base of the processus clinoideus, relatively close to the basisphenoid–pterygoid suture. This is the condition usually found in plesiochelyids, with the exception of Plesiochelys etalloni (Anquetin, Püntener & Billon-Bruyat, 2015). The processus clinoideus is formed just medial to the basisphenoid–prootic suture. In contrast to Plesiochelys etalloni, the pila prootica is not ossified. The dorsum sellae is high and does not overhang the sella turcica, as usual in plesiochelyids. The surface below the dorsum sellae is devoid of ridge and slopes relatively gently anteriorly. In that matter, Plesiochelys bigleri is intermediate between Plesiochelys etalloni, in which the surface below the dorsum sellae is near vertical, and Portlandemys spp., in which that surface slopes very gently anteriorly resulting in a relatively long distance between the dorsum sellae and the foramina anterius canalis carotici cerebralis (Anquetin, Püntener & Billon-Bruyat, 2015). The foramina anterius canalis carotici cerebralis are separated by a broad bar of bone, which is unusual in plesiocheyids, and open slightly posterior to the level of the foramina anterius canalis carotici palatinum. The trabeculae are relatively short and straight, and frame a small, well-defined sella turcica. The rostrum basisphenoidale is short, and the anterior tip of the trabeculae actually represents the anteriormost extension of the basisphenoid in dorsal view.

General description

Elements of the carapace are preserved in 39 out of the 41 shells herein referred to Plesiochelys bigleri (Figs. 7 and 8). The largest specimens reach a carapace length of about 550 mm (MJSN BSY006-307, MJSN SCR011-30, and MJSN TCH005-21), whereas the smallest have a carapace length of about 220–250 mm (MJSN BSY008-848, MJSN SCR010-327, and MJSN TCH007-516). Measuring carapace length with precision in disarticulated and incomplete specimens is not possible, but we estimate that most specimens are about 450 mm in carapace length, which corresponds approximately to what is known for Plesiochelys etalloni in Solothurn. Most shells of Plesiochelys bigleri are disarticulated, but two shells (MJSN BSY006-307 and MJSN TCH007-519) were found articulated. Bones were prepared out of the marly matrix individually and we were able to reassemble them on a moldable sand bed and reconstruct the 3D shape of the shell. Post-mortem deformation occurred in most specimens and to variable extents. Our reconstructions are therefore tentative. The resulting carapace outlines are either evenly oval (e.g., MJSN TCH006-767), or roundish (e.g., MJSN TCH007-252; Figs. 7A and 7B). In some specimens the posterior carapace part is pointed (e.g., MJSN BSY009-815 and MJSN SCR011-30).

The shells of Plesiochelys bigleri and Plesiochelys etalloni are very similar. The following description therefore primarily focuses on the few differences between the two species and the intraspecific variations observed in Plesiochelys bigleri. A general description of the shell morphology of Plesiochelys etalloni and a discussion of its intraspecific variability can be found elsewhere (Anquetin, Deschamps & Claude, 2014; Anquetin, Püntener & Billon-Bruyat, 2014).

Nuchal

The shape of the nuchal in Plesiochelys bigleri varies from almost rectangular (e.g., MJSN SCR011-140; Figs. 8I and 8J) to trapezoidal (e.g., MJSN BSY007-257). This element can be about as wide as long (e.g., MJSN BSY009-815) or clearly wider than long (e.g., MJSN SCR011-160). The nuchal notch is usually shallow, but can also be much reduced (e.g., MJSN SCR011-160) or strongly pronounced (e.g., MJSN BSY007-257). In some specimens (e.g., MJSN TCH006-1420; Figs. 8A and 8B), the posteromedial part of the nuchal that rticulates with the first neural projects posteriorly.

Neurals, suprapygals, and pygal

Like Plesiochelys etalloni, Plesiochelys bigleri usually has eight neurals, one intermediate element, two suprapygals, and one pygal. However, this condition may vary. Neural bones can be split in two (e.g., neural 2 in MJSN SCR010-342), fused together (e.g., neural 8 with the intermediate element in MJSN TCH005-21), or be reduced (e.g., neural 7 in MJSN SCR011-140) or completely obliterated (e.g., neural 7 in MJSN SCR011-413 and MJSN TCH006-1420; Figs. 8A and 8B) by costals with midline contact. The first neural is rectangular to oval in shape. It usually tapers posteriorly (not in MJSN BSY007-257 and MJSN TCH005-42; Figs. 8M and 8N). Neurals 2–6 are elongated hexagons with shorter sides facing anteriorly. However, these hexagons are often deformed (e.g., MJSN BSY007-257, MJSN SCR010-342, MJSN SCR011-30, MJSN SCR011-276, MJSN SCR011-413, MJSN TCH005-42, and MJSN TCH006-767). Neurals 7 and 8 are shorter and more irregular in shape than the preceding neurals. Their growth is often constrained by a midline contact of the costals. Neural 8 is usually the smallest bone in the series. In some specimens, it is pentagonal with shorter sides facing posteriorly (e.g., MJSN BSY009-815, MJSN TCH007-252; Figs. 7A and 7B). The thickness of the neural bones in Plesiochelys bigleri is significantly reduced compared to Plesiochelys etalloni (Fig. 9). This is especially obvious for neurals 2–5. We statistically demonstrate this difference herein (see below).

As relatively common in turtles (Zangerl, 1969), the posteromedial region of the carapace is characterized by a great deal of intraspecific variation in Plesiochelys bigleri. The element that follows neural 8 corresponds to the intermediate element of Anquetin, Püntener & Billon-Bruyat (2014). This intermediate element varies much in shape, but usually tapers anteriorly (e.g., MJSN SCR011-140, MJSN SCR011-160, and MJSN SCR011-276). In one specimen (MJSN TCH006-1420; Figs. 8A and 8B) the intermediate element is fused to the first suprapygal.

Plesiochelys bigleri usually has two suprapygals that are clearly wider than long. Although irregular in shape (usually from trapezoidal to sub-triangular), the first suprapygal usually tapers anteriorly and the second posteriorly (e.g., MJSN BSY009-815, MJSN SCR011-160, MJSN TCH006-1420; Figs. 8A and 8B). The two suprapygals can fuse into a single element (e.g., MJSN TCH007-519 and MJSN SCR011-148). In some specimens, the first suprapygal is divided in two (e.g., MJSN SCR011-30, MJSN TCH007-252; Figs. 7A and 7B). The pygal is a trapezoidal element that is wider than long. Its relative size varies substantially from one individual to another.

Costals and peripherals

Plesiochelys bigleri has eight pairs of costals and eleven pairs of peripherals. Their shape and arrangement are relatively stable within the species. The length–width ratio of costal four corresponds to that of Plesiochelys etalloni and differs notably from that of C. jaccardi (Anquetin, Püntener & Billon-Bruyat, 2014). The costals in Plesiochelys bigleri are not as thick as the same elements in Plesiochelys etalloni. In the latter, the proximal part of the costals is very thick, matching the thick neural bones, and the costals usually remain relatively thick on their distal margin. In Plesiochelys bigleri, both the proximal and distal parts of the costals are thinner. A midline contact occurs frequently between costals 7 (e.g., MJSN SCR011-140; Figs. 8I and 8J), sometimes also between costals 6 (e.g., MJSN SCR011-30, MJSN TCH006-1420; Figs. 8A and 8B), and rarely between costals 8 (e.g., MJSN TCH005-42; Figs. 8M and 8N). Such a reduction of posterior neurals and midline contacts of costals can also occur in Plesiochelys etalloni (e.g., MAJ 2005-11-1 and MJSN TCH006-574), but more rarely than in Plesiochelys bigleri. This condition appears to occur more commonly in C. jaccardi (Anquetin, Püntener & Billon-Bruyat, 2014).

Scutes of the carapace

There are three cervical scutes of about equal size in Plesiochelys bigleri (best preserved in MJSN CRT007-2 and MJSN TCH007-519). However, as common in plesiochelyids (see Anquetin, Püntener & Billon-Bruyat, 2014), cervical scutes are difficult to discern in many specimens.

There are usually five vertebrals, four pairs of pleurals, and 12 pairs of marginals. Vertebrals 1 and 5 are always the shortest in the series. The second and third vertebrals are rectangular (e.g., MJSN BSY009-815) to hexagonal (e.g., MJSN SCR011-30) in shape and cover one third (e.g., MJSN TCH007-519) to about half (e.g., MJSN TCH006-767) of the costal length laterally. Vertebral 4 is usually hexagonal in shape. In some specimens it almost extends as far as the peripherals laterally, significantly reducing the size of pleural 4 (e.g., MJSN BSY006-326 and MJSN SCR011-160). The intervertebral scute sulci usually run on neurals 1, 3, and 5. The scute sulcus between vertebrals 4 and 5 most often runs on the intermediate element (e.g., MJSN BSY009-815), but can also cross neural 8 (e.g., MJSN SCR011-148) or the first suprapygal (e.g., MJSN TCH007-252; Fig. 7B). The 12th pair of marginals is either restricted to the pygal and peripheral 11 (e.g., MJSN BSY006-326 and MJSN TCH006-767), or extends on the suprapygal 2 (e.g., MJSN TCH006-1420, MJSN SCR011-140; Figs. 8B and 8J).

One specimen (MJSN TCH005-42; Figs. 8M and 8N) shows an anomalous scute pattern. It has eight partially wedged vertebrals and five irregularly sized pleurals (maybe even six on the right side). This scute pattern shows no symmetry and we consider it as an abnormal condition in Plesiochelys bigleri.

General description

Elements of the plastron are preserved in 40 out of the 41 shells herein referred to Plesiochelys bigleri. With 481 mm, MJSN SCR10-1279 has the longest preserved plastron. In contrast, the well-preserved juvenile specimen MJSN SCR010-327 has a plastron length of only 194 mm. The anterior plastral lobe of Plesiochelys bigleri is most often somewhat quadrangular in outline (e.g., MJSN SCR010-1196, MJSN SCR011-140; Figs. 8K and 8L), but can exceptionally also be rounded (e.g., MJSN CRT007-2), or even pointed (e.g., MJSN TCH006-1420; Figs. 8C and 8D). In Plesiochelys etalloni, the anterior plastral lobe usually has a rather rounded, sometimes pointed, anterior outline (Anquetin, Püntener & Billon-Bruyat, 2014). Like in Plesiochelys etalloni, a central plastral fontanelle (always longer than wide) is occasionally present (e.g., MJSN TCH006-1420; MJSN BSY006-307; Figs. 8C, 8D, 8G and 8H).

Epiplastra and entoplastron

The shape of the epiplastra varies from rather rectangular (MJSN SCR010-1196) to triangular (e.g., MJSN TCH006-1420; Figs. 8C and 8D). The anterolateral border of the epiplastra is often somewhat angular, mirroring the anterior outline of the anterior plastral lobe. Epiplastral bulbs, as described in Plesiochelys etalloni (Bräm, 1965; Anquetin, Püntener & Billon-Bruyat, 2014), are absent or only weakly expressed (e.g., MJSN TCH007-252; Fig. 7C). The entoplastron is usually roundish (e.g., MJSN SCR011-140; Figs. 8K and 8L) to roughly kite-shaped (e.g., MJSN TCH007-252; Figs. 7C and 7D), but its relative size and length–width proportion varies greatly from one individual to another. The entoplastron of the juvenile specimen MJSN SCR010-327 is elongated and kite-shaped.

Hyo-, hypo-, and xiphiplastra

The hyoplastra are always longer than wide, although only by a small amount in a few individuals (e.g., MJSN BSY009-815). This corresponds to what is known in Plesiochelys etalloni and many other turtles, and contrasts with the condition in C. jaccardi, where the hyoplastra are clearly wider than long (Anquetin, Püntener & Billon-Bruyat, 2014). The suture between the hyo- and hypoplastra is generally straight, but sometimes shows individual symmetric anteroposterior projections (e.g., MJSN CRT007-2, MJSN BSY006-307; Figs. 8G and 8H). There is a small supernumerary bone between the hyoplastra of the juvenile specimen MJSN SCR010-327, but the sutures of this roundish bone are absent on the visceral side of the plastron. The posterolateral borders of the hypoplastra often project so as to articulate with the corresponding notches of the xiphiplastra (e.g., MJSN SCR011-413). The xiphiplastra are always longer than wide and often asymmetric. In most specimens, the xiphiplastra are relatively long elements, possibly more elongated in proportion than in Plesiochelys etalloni. However, in some individuals the xiphiplastra appear to be significantly reduced in length, but still longer than wide (e.g., MJSN BSY009-815).

Scutes of the plastron

The gular and extragular scute sulci are relatively shallow. In Plesiochelys etalloni, the gular and extragular sulci are usually deeper, notably anteriorly, as a result of the presence of the epiplastral bulbs. As in Plesiochelys etalloni, the gular scute is either restricted to the epiplastra (e.g., MJSN CRT007-2), or extends to the entoplastron (e.g., MJSN TCH007-252; Fig. 7D). In those specimens where the plastral midline sulcus is discernable, it is irregularly sinuous, sometimes creating small supernumerary scutes (e.g., MJSN SCR011-30). A similar condition is also known in Plesiochelys etalloni.

Generally, the sulcus between the humeral and pectoral scutes is straight, while the sulcus between the pectoral and abdominal scutes is curving anteriorly. The sulcus between the femoral and anal scutes is usually restricted to the xiphiplastron, but extends to the hypoplastron in some specimens. In the latter case, the anal scutes may form a rectangular anterior projection on the hypoplastra (e.g., MJSN SCR011-30). A similar variability is present in Plesiochelys etalloni (Anquetin, Püntener & Billon-Bruyat, 2014).

There are usually four pairs of inframarginal scutes. As in Plesiochelys etalloni, the third one is generally the longest in the series. It covers the suture between hyo- and hypoplastron. The juvenile specimen MJSN SCR010-327 shows a variation of this pattern by having three inframarginal scutes on the left hypoplastron. It is however unclear whether this resulted in a total of five inframarginal scutes on the left side and whether this was also the case on the right side. The inframarginal scutes usually extend laterally on the peripherals. Occasionally some of them are restricted to plastral elements (e.g., MJSN CRT007-2 and MJSN TCH006-767). This is also a condition known in Plesiochelys etalloni (Anquetin, Püntener & Billon-Bruyat, 2014).