A taxonomic review of the Late Jurassic eucryptodiran turtles from the Jura Mountains (Switzerland and France)

TESTUDINES Batsch, 1788 EUCRYPTODIRA Gaffney, 1975c PLESIOCHELYIDAE Baur, 1888

Plesiochelys solodurensis Rütimeyer, 1873

1873 Plesiochelys solodurensis. Rütimeyer [new species]

1975a Plesiochelys etalloni. Gaffney [subjective synonymy]

2014 Plesiochelys etalloni. Anquetin, Deschamps & Claude [subjective synonymy]

Taxonomic assessment. Invalid name, subjective synonym of Plesiochelys etalloni (Pictet & Humbert, 1857).

Type material. NMS 59, a distorted sub-complete shell. Lectotype designated by Bräm (1965: 81).

Type horizon and locality. Solothurn Turtle Limestone, uppermost member of the Reuchenette Formation (Kimmeridgian, Late Jurassic), vicinity of Solothurn, Canton of Solothurn, Switzerland (Fig. 1).

Illustrations of type. Rütimeyer (1873: plate XII, Figs. 1 and 2); Figs. 2E–2H.

Remarks. Rütimeyer (1873) and Bräm (1965) referred most of the Plesiochelys material from Solothurn either to P. solodurensis or P. etalloni. The main difference they recognized between the two species was the presence of a central plastral fontanelle in P. etalloni. Bräm (1965: 60–62) even concluded that the two species were very similar, to the point of being hardly differentiable if hyo- and hypoplastron were not preserved. Considering the fact that the retention of small shell fontanelles is intraspecifically variable in some extant species (e.g., Pelomedusa subrufa, Graptemys barbouri, and Macroclemys temminckii; see Pritchard, 2008), Gaffney (1975a) assumed this was also the case in Plesiochelys and referred P. solodurensis, P. jaccardi and P. etalloni to the same species. Lapparent de Broin, Lange-Badré & Dutrieux (1996) also considered the persistence of a small central plastral fontanelle in adults as an intraspecific variation of P. solodurensis, which they considered as a distinct species. Based on an extensive review of the relevant material, we reached a conclusion similar to that of Gaffney (1975a) and Lapparent de Broin, Lange-Badré & Dutrieux (1996), although we ultimately disagree on the delimitation and inclusiveness of Plesiochelys species (Anquetin, Deschamps & Claude, 2014; see also Table 1 and Discussion).

Craspedochelys picteti Rütimeyer, 1873

1873 Craspedochelys Picteti. Rütimeyer [new species]

1975a Plesiochelys etalloni. Gaffney [subjective synonymy]

1988 Craspedochelys jaccardi. Antunes, Becquart & Broin [subjective synonymy]

Taxonomic assessment. Valid name.

Synonymy. Craspedochelys crassa Rütimeyer, 1873.

Type material. NMS 129, anterior half of a shell with plastron poorly preserved and right part of the carapace missing. Holotype by monotypy (Bräm, 1965: 137).

Type horizon and locality. Solothurn Turtle Limestone, uppermost member of the Reuchenette Formation (Kimmeridgian, Late Jurassic), vicinity of Solothurn, Canton of Solothurn, Switzerland (Fig. 1).

Illustrations of type. Rütimeyer (1873: plate V, Fig. 1); Figs. 3A–3D.

Referred specimens. Specimens listed in Bräm (1965); NMS 130 (holotype of Craspedochelys crassa Rütimeyer, 1873.

Revised diagnosis. Craspedochelys picteti can be diagnosed as a representative of Craspedochelys by a broad carapace, about as wide as long (as preserved), a weak nuchal notch, and a high length/width ratio of costal bones. Differing from C. jaccardi in: greater size (carapace length up to 550 mm); carapace heraldic shield-shaped and more quadrangular anteriorly; slightly lower length/width ratio of costal bones (4.3 as opposed to 4.8 or more for the fourth costal in C. jaccardi); relatively small pygal; contact between peripheral 11 and costal 8 limited or absent; hyoplastron slightly longer than wide.

Remarks. Bräm (1965) notably characterized C. picteti by the following suite of features: carapace as wide as long and shaped like a heraldic shield, with anterior rim extending only slightly convex up to the third peripheral then bending almost at right angle toward the rear; nuchal notch weak; vertebral scales moderately broad extending only about one third of the length of costals; free first thoracic rib; second thoracic rib contacting only the second thoracic vertebra. Gaffney (1975a) tentatively synonymized C. picteti with P. etalloni, considering the features proposed by Bräm (1965) as resulting either from postmortem deformation (carapace shape and width) or from biological variation (degree of nuchal emargination, width of vertebral scales). Furthermore, Gaffney (1975a) argued that the first and second thoracic ribs are only visible in NMS 608, and that their condition is ambiguous due to incomplete preparation and postmortem damage. NMS 608 is currently mounted on a wall in the NMS exhibition, and we were therefore unable to confirm Bräm’s (1965) observations. Antunes, Becquart & Broin (1988) rejected Gaffney’s (1975a) conclusions and synonymized C. picteti with C. jaccardi, though without directly studying the Swiss material. Finally, Lapparent de Broin, Lange-Badré & Dutrieux (1996) re-established C. jaccardi and C. picteti as distinct species, considering the first as a smaller form with thinner shell plates.

NMS 129, NMS 608, and NMS 130 (holotype of C. crassa) share a number of features that clearly distinguish them from other species from the Jura Mountains: anterior part of the carapace broad with anterior rim almost straight up to the level of the p3–p4 suture; reduced nuchal notch restricted to the nuchal plate; second and third vertebral scales extending about one third of the length of the costals (Fig. 3). These are the same characters (Gaffney, 1975a) dismissed as resulting from postmortem deformation or biological variation. However, our review of the Solothurn material indicates that these features are never found in any other specimen, no matter how deformed or variable it may be (see Discussion). Therefore, we consider Bräm’s (1965) conclusions on C. picteti as valid. However, this species is relatively poorly known and more material is needed.

Craspedochelys jaccardi (Pictet, 1860)

1860 Emys jaccardi. Pictet [new species]

1873 Plesiochelys Jaccardi. Rütimeyer [new combination]

1975a Plesiochelys etalloni. Gaffney [subjective synonymy]

1988 Craspedochelys jaccardi. Antunes, Becquart & Broin [new combination]

Taxonomic assessment. Valid name.

Synonymy. None.

Type material. MHNN FOS 977, a complete shell. Holotype by monotypy.

Type horizon and locality. Les Hauts-Geneveys, Canton of Neuchâtel, Switzerland (Fig. 1), “Virgulien supérieur”, possibly corresponding to the early Tithonian (see Lapparent de Broin, Lange-Badré & Dutrieux, 1996: 552). According to Pictet (1860), the specimen was collected from a quarry near Les Brenets, whereas for Jaccard (1860) the specimen came from a different quarry near Les Hauts-Geneveys. Jaccard (1870) confirmed the locality as being Les Hauts-Geneveys (Ayer, 1997).

Illustrations of type. Pictet (1860: plates I–III); Lapparent de Broin, Lange-Badré & Dutrieux (1996: plate IV); Figs. 4A–4D.

Referred specimens. See Bräm (1965).

Revised diagnosis. Craspedochelys jaccardi can be diagnosed as a representative of Craspedochelys by a broad carapace, about as wide as long (as preserved), a weak nuchal notch, a high length/width ratio of costal bones, and a hyoplastron wider than long. Differing from C. picteti in: smaller size (carapace length up to 420 mm); carapace more evenly rounded anteriorly; higher length/width ratio of costal bones (4.8 or more, as opposed to 4.3 for the fourth costal); wider pygal bone; contact between peripheral 11 and costal 8 present; hyoplastron wider than long (slightly longer than wide in C. picteti).

Remarks. Emys jaccardi Pictet, 1860 was referred to the genus Plesiochelys by Rütimeyer (1873), a conclusion followed by Bräm (1965), who differentiated this species based mainly on the following features: carapace about as wide as long; nuchal notch evenly rounded; plastron oval in outline; small xiphiplastral notch; plastron length about 73% that of the carapace; small central plastral fontanelle, mainly formed by hypoplastron; vertebrals relatively narrow. Gaffney (1975a) synonymized P. jaccardi with P. etalloni, notably explaining the broad shell of the former by postmortem compression. Antunes, Becquart & Broin (1988) were the first to refer the species jaccardi to the genus Craspedochelys based on the following characters: broad carapace (width/length ratio exceeding 90%); pentagonal outline with anterior part quadrangular; small central plastral fontanelle; and vertebral scales reduced in width. This position was later confirmed by Lapparent de Broin, Lange-Badré & Dutrieux (1996), although, in contrast to Antunes, Becquart & Broin (1988), they recognized C. picteti and C. jaccardi as two distinct forms, based primarily on size difference and variation in the thickness of the shell bones.

The characteristics exhibited by the holotype of C. jaccardi (MHNN FOS 977) are inconsistent with a referral to P. etalloni, as suggested by Gaffney (1975a). For example, postmortem compression or individual variation cannot explain the significant reduction of the plastron length in C. jaccardi (Table 2). The proportions of costals, hyoplastron, and xiphiplastron are also markedly different in the two species (see Discussion). Therefore, C. jaccardi is considered as a valid species. However, this species is only known by a limited number of specimens and some questions remain regarding the attribution of the Solothurn specimens to this species (see Discussion).

Table 2:

Comparison of the ratio between the length of the plastron and the length of the carapace in selected specimens referred to P. etalloni, C. picteti, and C. jaccardi.

	Plastron length (mm)	Carapace length (mm)	Ratio
Plesiochelys etalloni
NMS 59	400	474	0.84
NMS 78	–	361a	–
NMS 79	–	–	–
NMS 116	–	–	–
NMS 669	363	410	0.89
NMS 675	369	445	0.83
MAJ 2005-11-1	431	471	0.92
Craspedochelys jaccardi
NMS 101	300b	413	0.73
NMS 102a	–	363	–
NMS 612	–	–	–
NMS 673	292	411	0.71
MHNN FOS 977	283	365	0.78
Craspedochelys picteti
NMS 608	–	540	–

DOI: 10.7717/peerj.369/table-2

Notes:

a Carapace missing about 20 mm.

b Estimated plastron length.

Tropidemys langii Rütimeyer, 1873

1873 Tropidemys Langii. Rütimeyer [new species]

Taxonomic assessment. Valid name.

Synonymy. Tropidemys expansa Rütimeyer, 1873, Tropidemys gibba Rütimeyer, 1873, and Craspedochelys plana Rütimeyer, 1873.

Type material. NMS 16, posterior part of a carapace. Lectotype designated by Bräm (1965: 176).

Type horizon and locality. Solothurn Turtle Limestone, uppermost member of the Reuchenette Formation (Kimmeridgian, Late Jurassic), vicinity of Solothurn, Canton of Solothurn, Switzerland (Fig. 1).

Illustrations of type. Rütimeyer (1873: plate VII, Fig. 1); Figs. 5A and 5B.

Referred specimens. See Püntener et al. (2014).

Revised diagnosis. See Püntener et al. (2014).

Remarks. Rütimeyer (1873) initially described three species of Tropidemys in Solothurn: Tr. langii, Tr. expansa and Tr. gibba. Bräm (1965), who had access to a sub-complete carapace (NMS 15; Figs. 5C and 5D), concluded that there was no reason to differentiate three species based on the available material. Püntener et al. (2014) recently revised the Solothurn material and described new specimens from the Kimmeridgian in the vicinity of Porrentruy, Switzerland (Fig. 1). They confirmed Bräm’s (1965) conclusions.

THALASSEMYDIDAE Zittel, 1889

Thalassemys hugii Rütimeyer, 1873

1873 Thalassemys Hugii. Rütimeyer [new species]

Taxonomic assessment. Valid name.

Synonymy. Thalassemys Gresslyi Rütimeyer, 1873 and Eurysternum ignoratum Bräm, 1965.

Type material. NMS 1, a large carapace plus associated plastron fragments and postcranial remains. Lectotype designated by Bräm (1965: 143).

Type horizon and locality. Solothurn Turtle Limestone, uppermost member of the Reuchenette Formation (Kimmeridgian, Late Jurassic), vicinity of Solothurn, Canton of Solothurn, Switzerland (Fig. 1).

Illustrations of type. Rütimeyer (1873: plate I); Bräm (1965: plate 7); Figs. 6A–6D.

Referred specimens. Specimens listed in Bräm (1965); NMS 5 (holotype of Eurysternum ignoratum Bräm, 1965), NMS 12 (holotype of Th. gresslyi Rütimeyer, 1873), NMS 124, NMS 412, NMS 20981, NMS 22325-22327 (and associated remains), NMS 37251.

Revised diagnosis. Thalassemys hugii can be diagnosed as a representative of Thalassemys by the great widening of neural 1, the retention of costo-peripheral fontanelles, the presence of clearly visible linear striations perpendicular to sutures between most shell elements, the presence of a lateral plastral fontanelle, the absence of sutural connection of the epi- and entoplastron with the hyoplastron, and the presence of a small xiphiplastral fontanelle. Differing from Th. marina in: narrower vertebral scales with anterolateral and posterolateral margins of equal length (as opposed to posterolateral margin shorter in Th. marina); smaller lateral plastral fontanelle.

Remarks. Because Thalassemys hugii was typified based on a relatively completed shell and partial associated post-cranial remains, its validity has never been questioned. This turtle is nonetheless not very well known and remains are relatively rare in contemporaneous deposits. This is the largest turtle in Solothurn reaching more than 630 mm (the pygal is missing in NMS 1). Bräm (1965) diagnosed Th. hugii by the following features: presence of a large longitudinal central plastral fontanelle, sometimes closed anteriorly by the hyoplastra and posteriorly by the xiphiplastra; vertebral scales relatively narrow (as opposed to wide in ‘Th.’ moseri); very large size.

During our review of the Solothurn material, we were able to identify a set of characters that prompted a revision of the traditional concept of Th. hugii (see Discussion). Perhaps the most important of these characters is the presence of a lateral plastral fontanelle, the purported absence of which was used by Bräm (1965) to differentiate Thalassemys from Eurysternum. Based on this review of the material, E. ignoratum is synonymized with Th. hugii (see below).

‘Thalassemys’ moseri (Bräm, 1965)

1965 Thalassemys moseri. Bräm [new species]

1996 Plesiochelys solodurensis. Lapparent de Broin, Lange-Badré & Dutrieux [subjective synonymy]

Taxonomic assessment. Valid name.

Synonymy. None.

Type material. NMS 618, partial carapace and plastron. Holotype (Bräm, 1965: 155).

Type horizon and locality. Solothurn Turtle Limestone, uppermost member of the Reuchenette Formation (Kimmeridgian, Late Jurassic), vicinity of Solothurn, Canton of Solothurn, Switzerland (Fig. 1).

Illustrations of type. Bräm (1965: plate 8, Figs. 2 and 3); Figs. 7A–7D.

Referred specimens. Specimens listed in Bräm (1965); PMZH A/III 514, a nearly complete skull and partial shell from the early Tithonian of La Morelière (Isle of Oléron, Department of Charente-Maritime, France) referred by Rieppel (1980).

Revised diagnosis. Species of dubious affinity characterized by the combination of the following features: medium-sized shell (carapace length about 350 mm); three cervical scales; wide vertebrals covering about half of the costals laterally; pattern of carapacial scales recalling that of Plesiochelys and Craspedochelys; presence of costo-peripheral fontanelles; costal bones relatively thin distally; rib tips easily disarticulated from peripherals; large central plastral fontanelle, oval in outline; absence of lateral plastral fontanelle; epi- and entoplastron not sutured to hyoplastron; xiphiplastron possibly forming a small xiphiplastral notch. See Rieppel (1980) for cranial characters. Differing from Plesiochelys etalloni, Craspedochelys picteti, C. jaccardi, and Tropidemys langii in: retention of costo-peripheral fontanelles; epi- and entoplastron not sutured to hyoplastron. Differing from Thalassemys hugii in: smaller size; wide vertebrals covering about half of the costals laterally; absence of lateral plastral fontanelle.

Remarks. Bräm (1965) originally diagnosed ‘Thalassemys’ moseri as a representative of Thalassemys by the retention of costo-peripheral fontanelles and the absence of lateral plastral fontanelles. ‘Thalassemys’ moseri was furthermore differentiated from Th. hugii by its smaller size, the presence of broad vertebral scales, and the presence of a large central plastral fontanelle closed anteriorly by the hyoplastra and posteriorly by the hypoplastra (as opposed to the very extensive central fontanelle he considered to be present in Th. hugii; but see Discussion). Lapparent de Broin, Lange-Badré & Dutrieux (1996) argued that the holotype of ‘Th.’ moseri (NMS 618) with its three cervicals, wide vertebrals and oval central plastral fontanelle was probably a young individual of P. solodurensis, the only Plesiochelys species they recognized in Solothurn. According to them, this specimen could not be referred to Thalassemys because the type species of this genus is a very large form with only one cervical scale. Bräm (1965: 155) himself was aware of the apparent similarities between ‘Th.’ moseri and what he described as P. etalloni (i.e., Plesiochelys specimens with a central plastral fontanelle). This is especially true for the pattern of carapacial scales. However, ‘Th.’ moseri and P. etalloni diverge on characters that are traditionally used to differentiate plesiochelyids from thalassemydids, such as the presence of carapacial fontanelles between costals and peripherals.

Our review of the material clearly indicates that ‘Th.’ moseri is unique among Solothurn turtles. The presence of costo-peripheral fontanelles and the absence of sutural contact between the hyoplastron and the epiplastron and entoplastron are inconsistent with an attribution to P. etalloni, C. picteti, or C. jaccardi. The elongated neurals, the absence of a neural keel, and the broad vertebral scales clearly differentiate ‘Th.’ moseri from Tr. langii. Finally, the smaller size, the broad vertebrals, and the absence of a lateral plastral fontanelle distinguish ‘Th.’ moseri from Th. hugii. Therefore, we confirm the conclusions of Bräm (1965) and consider ‘Th.’ moseri as a distinct species. However, the generic attribution to Thalassemys is rejected. The pattern of carapacial scales and the absence of lateral plastral fontanelle suggest that this species is more closely related to Plesiochelys than to Thalassemys. However, the skull described by Rieppel (1980) is clearly different from that of P. etalloni, which prevents a tentative referral of ‘Th.’ moseri to Plesiochelys (see Discussion). The material described by Rieppel (1980) should be duly revised.

EURYSTERNIDAE Dollo, 1886

Plesiochelys etalloni

Plesiochelys etalloni is known from about 30 relatively complete shells and uncountable shell fragments, most of which from the quarries in the vicinity of Solothurn, Switzerland (Fig. 1). This material provides a good opportunity to grasp the level of intraspecific variability in this fossil species. A general description of the shell morphology of P. etalloni can be found in Anquetin, Deschamps & Claude (2014).

The carapace of P. etalloni is usually evenly oval, but some specimens have a more quadrangular anterior rim (e.g., NMS 78 and NMS 116; Fig. 8). Carapaces that have been flattened during fossilization tend to be characterized by a more pronounced angulation of their anterior margin, resulting from the partial disarticulation of some peripherals. In P. etalloni and Craspedochelys jaccardi (MHNN FOS 977; Fig. 4), this angulation is always located at the level of the p2–p3 suture, whereas C. picteti (NMS 129 and NMS 608; Fig. 3) is unique in showing an angulation at the level of the p3–p4 suture (see below).

The posteromedial region of the carapace is relatively variable in P. etalloni, as generally common in turtles (Zangerl, 1969). The seventh and eighth neurals are usually shorter and more variable in shape than the preceding ones. These two neurals even fuse in some specimens (e.g., NMS 79 and NMS 669; Fig. 8). The eighth neural might even be much reduced or absent in certain individuals allowing a midline contact of the eighth costals (e.g., MAJ 2005-11-1; Fig. 2; see Anquetin, Deschamps & Claude, 2014). In most specimens, there is an intermediate element of varying size and shape between the eighth neural and the first suprapygal (Figs. 2 and 8). We are uncertain of the identity of this additional element (ninth neural, additional suprapygal, or neomorphic bone). Its shape and size are quite variable, from a small quadrangular element about the size of preceding neurals to a large triangular or trapezoidal element about the size of the following suprapygal. This extreme variation of size and shape is probably inconsistent with an identification as a ninth neural, but this intermediate element is also articulated with the vertebral series (at least partially), which is incongruent with an identification as a suprapygal. For the time being, we prefer to simply refer to this element as the ‘intermediate’ element. It is particularly interesting to note that a similar element in known in C. picteti (Fig. 3), C. jaccardi (Fig. 4), Tropidemys langii (Fig. 5), and Thalassemys hugii (Fig. 6). The fourth intervertebral sulcus always runs on this intermediate element, or on the first suprapygal if the intermediate element is absent. Posterior to the intermediate element, there are usually two suprapygals, which sometimes fuse into one single element. The first suprapygal is generally larger and wider than the second, preventing a contact between the latter and the eighth pair of costals, but the actual size of each suprapygal is relatively variable from one individual to another (Fig. 8).

The three cervical scales are visible in all specimens in which this area is sufficiently preserved, but it should be noted that the cervical sulci are lost relatively quickly once the area is slightly damaged. The presence of three cervicals has long been though to represent a unifying character of Plesiochelyidae sensu stricto, including Plesiochelys, Craspedochelys, and Tropidemys (e.g., Bräm, 1965; Lapparent de Broin, Lange-Badré & Dutrieux, 1996; Slater et al., 2011; Püntener et al., 2014; Pérez-García, in press). However, the presence of three cervicals has been reported in some Eurysternidae (Joyce, 2003; Anquetin & Joyce, in press), and three cervicals may also have been present in Th. hugii (see below).

The pattern of carapacial scales of Plesiochelys etalloni is similar to that of C. picteti, C. jaccardi, and ‘Th.’ moseri. In P. etalloni, this pattern is subject to a certain degree of variability. This informs us on the variability that may be expected in the other aforementioned species, which are currently represented by considerably less specimens. The vertebral sulci are generally sinuous, but to a variable extent from one individual to another. Vertebrals 2–4 are wide, hexagonal scales. However, if the second and third vertebrals consistently cover about half of the costal length laterally, the lateral extent of the fourth vertebral is more variable. In some specimens (e.g., NMS 79 and NMS 118; Figs. 2 and 8), the fourth vertebral extends as far as the peripherals laterally, significantly reducing the width of the fourth pleural in the process. The marginals are generally restricted to the peripherals, but in some specimens the fourth and/or seventh marginals extend very slightly on the costals (e.g., NMS 59, NMS 60, and NMS 669; Figs. 2 and 8). Finally, it is interesting to note that, in all specimens in which this area is known, the twelfth pair of marginals extends anteriorly on the second suprapygal, whereas it is restricted to the pygal in C. picteti and C. jaccardi (not known in ‘Th.’ moseri).

Bräm (1965) mentioned the presence of epiplastral bulbs in P. etalloni. We confirm that two pairs of epiplastral bulbs are present in specimens with undamaged epiplastra (e.g., NMS 59, NMS 94, NMS 629, NMS 669; Figs. 2 and 8). The entoplastron is usually diamond-shaped, but in some specimens its posterior half is more or less elongated. Finally, the presence/absence of a central plastral fontanelle is interpreted as an intraspecific variation of P. etalloni (Gaffney, 1975a; Lapparent de Broin, Lange-Badré & Dutrieux, 1996; Anquetin, Deschamps & Claude, 2014). Like their carapacial counterparts, the plastral scales exhibit a certain degree of variability in their shape and relations with underlying bony elements. In some specimens (e.g., MAJ 2005-11-1 and NMS 94; Figs. 2 and 8), the plastral midline sulcus is irregularly sinuous. The length of the pectoral compared to that of the humeral is quite variable in P. etalloni. The pectoral may be shorter (e.g., MAJ 2005-11-1, NMS 59, NMS 94), about equal (e.g., NMS 66, NMS 669), or longer than the humeral (e.g., NMS 629, NMS 675). Most commonly, there are four pairs of inframarginals, except in NMS 78 where there are five pairs (Fig. 8). These inframarginals are either entirely restricted to plastral elements (e.g., NMS 59, NMS 79), or some of them, usually the third and/or fourth, may extend slightly laterally on the peripherals (e.g., MAJ 2005-11-1, NMS 94). Finally, the anal scales very rarely extend anteriorly on the hypoplastra (e.g., NMS 59, NMS 79), otherwise the anals are restricted to the xiphiplastra (Figs. 2 and 8).

Craspedochelys picteti

Craspedochelys picteti is known mainly from two specimens from Solothurn (Fig. 1). The holotype (NMS 129) is relatively incomplete, consisting only of the anterior left quarter of the carapace and associated hyoplastra, but NMS 608 consists of a large, sub-complete carapace (Fig. 3). Craspedochelys picteti is mainly characterized by a heraldic shield-shaped carapace. Anteriorly, the carapace rim is almost straight transversally from the nuchal to the third peripheral. The carapace margin then bends abruptly posteriorly at the level of the p3–p4 suture. As discussed above for P. etalloni, variations in the degree of angulation of the anterior part of the carapace are probably the result of postmortem compression in these turtles, but the shift in the location of this angulation in C. picteti indicates that the anterior outline of the carapace was truly broader in this taxon. From peripherals 4 to 7, the margin is almost straight and parallel to the anteroposterior axis of the carapace. At the level of the p7–p8 suture, the margin bends abruptly medially and continues obliquely toward the pygal. The width of the carapace decreases rapidly from the eighth peripheral to the pygal.

The nuchal is a wide, trapezoidal element with a shallow nuchal notch, which does not extend on the first peripheral. Specimens referred to P. etalloni usually have a more pronounced and more laterally extended nuchal notch. There are eight neurals. The first neural is more rectangular. Neurals 2–6 are elongate, hexagonal elements with shorter sides facing anteriorly. As in P. etalloni, there was probably a certain amount of intraspecific variability in the morphology of the seventh and eighth neurals. In NMS 608, neural 7 is a short hexagonal element, whereas neural 8 is an irregularly shaped, wider than long element. Posterior to neural 8, there is a large trapezoidal element that corresponds to the intermediate element described in P. etalloni (see above). In NMS 608, the two suprapygals may have been fused together, but poor preservation prevents a definitive conclusion on the matter. The pygal is a relatively small, almost square-shaped element. In P. etalloni and C. jaccardi, the pygal is usually much wider (Figs. 2 and 4). Probably as a result of the reduced size of the pygal, the eleventh peripheral does not contact the eighth costal in NMS 608. It is uncertain whether this unique configuration of the pygal area is a true characteristic of C. picteti or an individual variation of NMS 608, but NMS 61 (an indeterminate carapace fragment) exhibits the exact same arrangement. There are eight pairs of costals. The length of costals 6–8 decreases rapidly posteriorly. Proportionally to their length the costals of C. picteti are thinner than those of P. etalloni, but not as much as those of C. jaccardi (Table 3). The arrangement and shape of carapacial scales remind that of P. etalloni, to the notable exception that vertebral scales are narrower and cover about a third to half of the costal length. However, this character appears to be subject to a significant amount of variation in P. etalloni. There are three cervical scales. The twelve pairs of marginals never extend on the costals. In contrast to P. etalloni, the twelfth marginals do not extend anteriorly on the second suprapygal.

Our knowledge of the plastron of C. picteti is limited to the hyoplastron of NMS 129 (Figs. 3C and 3D). This element is slightly longer than wide, which contrasts with the condition in C. jaccardi (see below). Based on the shape of its sutural contact with the hyoplastron, the entoplastron was probably a small element. A central plastral fontanelle was present in NMS 129. There are no further indications on the shape and size of the plastron in this species, which prevents comparison with other taxa from the Jura Mountains, notably P. etalloni and C. jaccardi.

Craspedochelys jaccardi

Craspedochelys jaccardi was originally described based on a single shell (MHNN FOS 977) from the vicinity of Neuchâtel, Switzerland (Fig. 1). Subsequently, additional specimens from Solothurn have been referred to this species (Rütimeyer, 1873; Bräm, 1965), although they are characterized by a slightly divergent morphology. Therefore, the following discussion is primarily based on the morphology of the holotype (Figs. 4A–4D). Craspedochelys jaccardi is a moderately sized turtle (carapace length up to 420 mm) characterized notably by a shortened plastron representing less than 80% of the carapace length (as opposed to 85–90% in Plesiochelys etalloni; see Table 2). As preserved, the shell is broad, even as wide as long in some specimens. Postmortem compression may affect our perception of shell width, but none of the many Solothurn specimens referred to P. etalloni has a shell as wide as long, no matter how flattened it is. In contrast to what Gaffney (1975a) suggested, the specimens referred to C. jaccardi are not more dorsoventrally flattened than specimens referred to P. etalloni. Anteriorly, the carapace is evenly rounded with only a weak nuchal notch mostly restricted to the nuchal bone. The carapace is slightly pentagonal in outline. The nuchal is a broad, trapezoidal element. The first neural is rectangular, whereas following neurals tend to be elongate and hexagonal with shorter sides anteriorly. There are up to eight neurals, but several specimens exhibit a reduction or loss of the seventh and/or eighth neurals allowing a midline contact of the seventh and/or eighth costals. As in P. etalloni and C. picteti notably, there is usually an intermediate element between the last neural and the first suprapygal (see above). As in other species, this element is relatively variable in shape and size. There are usually two suprapygals, the first larger than the second. The pygal is a wider than long element, larger than the same bone in C. picteti. There are eight pairs of costals, which are proportionally thinner and longer (higher length/width ratio; see Table 3) than those of P. etalloni and C. picteti. There are 11 pairs of longer than wide peripherals greatly increasing in width posteriorly. The posteriormost peripherals may have been slightly wider than long. The arrangement and shape of carapacial scales remind that of P. etalloni, but there seems to be a greater variability in the outline of vertebral scales in C. jaccardi (see below). There are three cervical scales. The twelve pairs of marginals never extend on the costals. In contrast to P. etalloni, the twelfth marginals do not extend anteriorly on the second suprapygal.

As noted above, the plastron of C. jaccardi is greatly reduced in length compared to that of P. etalloni. This reduction seems to result mainly from the shortening of the posterior lobe, which is apparent from the long post-xiphiplastral space (Lapparent de Broin, Lange-Badré & Dutrieux, 1996). The exact outline of the anterior plastral lobe is uncertain because the epiplastra are damaged in all known specimens. The posterior lobe is broad and rounded. There is a small central plastral fontanelle. The epi-hyoplastral suture is mostly transversal. Similarly to P. etalloni, the entoplastron is a diamond-shaped, longer than wide element with its anterior sides shorter than the posterior ones, but there appears to be a great variation in the size of this element between individuals (Fig. 4). The hyoplastron is remarkable in being wider than long (see Table 4), which probably reflects both the increased width of the shell and the reduced length of the plastron. Similarly, the xiphiplastron is as wide as long, which contrast with the longer than wide element found in most other turtles. There is a weak xiphiplastral notch, barely visible in some specimens. The extragular scales are restricted to the epiplastra. It is uncertain whether or not the gulars extended onto the anteromedial part of the entoplastron. The pectoral is reduced in length compared to the humeral. The anal scales are restricted to the xiphiplastra. There are four inframarginals increasing in length posteriorly. All inframarginals but the first extend slightly over the peripheral laterally (only visible in NMS 673).

As mentioned above, there is a certain number of differences between the holotype of C. jaccardi (MHNN FOS 977) and specimens from Solothurn referred to this species (notably NMS 101 and NMS 673). On the carapace, the most obvious differences concern the vertebral scales. The vertebral pattern of MHNN FOS 977 is somewhat unusual (Figs. 4A and 4B). The first vertebral is narrower than the nuchal bone posteriorly and it widens greatly anteriorly to reach the sulcus between the first and second marginal. The second vertebral is similarly narrow anteriorly and its anterolateral margin curves greatly toward the midline. The second and third intervertebral sulci are displaced anteriorly lying just anterior to the middle of neural 3 and neural 5, respectively (instead of just posterior to the middle of neural 3 and over the posterior part of neural 5 in most other turtles). Consequently, the third vertebral is shorter, whereas the fourth vertebral is significantly longer (Fig. 4). An unusually long fourth vertebral is also known in a referred specimen from the Kimmeridgian of Murat (Department of Lot, France; Lapparent de Broin, Lange-Badré & Dutrieux, 1996: Figs. 3 and 4). As a result of this unusual arrangement of the vertebrals, the second pleural of MHNN FOS 977 is shortened, whereas the third pleural is greatly lengthened. The vertebral pattern of NMS 101 is also relatively unusual (Figs. 4E–4F). Vertebral sulci are irregularly sinuous. The outline of vertebrals 2–4 is particularly odd, with notably a narrower, sub-quadrangular third vertebral. The fifth vertebral is significantly shorter than in other specimens referred to C. jaccardi. The vertebral pattern of NMS 673 is less unusual (Figs. 4I and 4J). The first vertebral is wide and trapezoidal. Vertebrals 2–4 are wide, hexagonal elements. Laterally, vertebrals 2–3 extend slightly less than the mid-length of the costals, which is slightly less than in MHNN FOS 977. In NMS 101 and NMS 673, the sulcus between the fifth vertebral and the twelfth marginals is located just posterior to the suture between the second suprapygal and the pygal, whereas the sulcus is positioned around the mid-length of the pygal in MHNN FOS 977. Finally, the two Solothurn specimens are unique in having a first interpleural sulcus reaching the fourth marginal on the third peripheral, instead of the fourth as in most turtle, including the holotype of C. jaccardi (Fig. 4).

MHNN FOS 977, NMS 101, and NMS 673 also exhibit differences regarding their plastral morphology. The plastron is proportionally shorter in the Solothurn specimens (about 70% of the carapace length, as opposed to 78% in MHNN FOS 977; see Table 2). Their entoplastron is larger. In the holotype, the central plastral fontanelle is formed equally by the hyo- and hypoplastra, whereas in the Solothurn specimens it is formed mostly or entirely by the hypoplastra. The central plastral fontanelle is rounded in MHNN FOS 977 and NMS 101, but it is oval and narrow in NMS 673.

The aforementioned differences can be diversely interpreted and may ultimately warrant the placement of the Solothurn specimens in a different species. However, intraspecific variability (notably sexual dimorphism in the case of the variation of the relative plastral length), ontogenetic stage (NMS 101 and NMS 673 are about 15% larger than the holotype specimen), and stratigraphic age (MHNN FOS 977 is possibly slightly younger than the Solothurn specimens) may also explain at least part of these differences. In order to avoid the unnecessary creation of a new species, we still tentatively refer NMS 101 and NMS 673 to C. jaccardi. Hopefully, new discoveries will eventually shed light on this particular question. In the meantime, comparisons should be made primarily with MHNN FOS 977, the holotype of C. jaccardi.

Lapparent de Broin, Lange-Badré & Dutrieux (1996) proposed a number of relative proportions of various shell measurements in order notably to discriminate between the different Plesiochelys and Craspedochelys species (carapace length/width ratio, ratio between the length of the second intercostal sulcus and the width of the third vertebral, ratio between the length of the posterior plastral lobe and the length of the bridge, posterior plastral lobe length/width ratio, ratio between the length of the bridge and the length of the carapace, ratio between the length of the post-xiphiplastral space and the length of the carapace). However, many of these proportions are not discriminative and the others are too much influenced by postmortem deformation. In the course of the present study, we have also been looking for ratios that would allow to discriminate between the species at hand. As discussed above the ratio between the length of the plastron and the length of the carapace clearly differentiate C. jaccardi from P. etalloni (Table 2). For the other ratios, we have focused on individual bones whose measurements are not extensively affected by postmortem deformation. The length/width ratios of the hyoplastron and xiphiplastron discriminate between C. jaccardi and P. etalloni (Tables 4 and 5). In C. jaccardi, the hyoplastron is wider than long and the xiphiplastron about as wide as long, whereas the hyoplastron and xiphiplastron are both longer than wide in P. etalloni. The ratio between the length of the carapace and the length of the fourth costal reveals that the shell is proportionally wider in C. jaccardi than in P. etalloni and C. picteti (Table 6). Finally, the length/width ratio of the fourth costal is probably the most interesting feature, because it clearly allows to discriminate between the three aforementioned species. This ratio is high in C. jaccardi, slightly lower in C. picteti, and much lower in P. etalloni (Table 3). This is also clearly visible directly on the specimens where costals 2–6 seem thinner and elongate in C. jaccardi (Fig. 4), whereas they are wider and shorter in P. etalloni (Figs. 2 and 8). It is also interesting to note that measurements taken from MHNN FOS 977, NMS 101, and NMS 673 are generally congruent, which suggests that these specimens truly belong to a single species.

Tropidemys langii

In the Jura Mountains, the shell of Tropidemys langii is known from 19 specimens (out of which five are relatively complete) from the localities of Solothurn and Porrentruy, Switzerland (Fig. 1). In addition to this material, a partial carapace is also known from the site of Sainte-Croix (Canton of Vaud, Switzerland), but this specimen is supposed to have been found in Valanginian (Early Cretaceous) deposits. The material from Porrentruy is particularly important because it provides additional information regarding the plastron and limb bones (humerus and femur) of this species. All this material has been recently revised and described by Püntener et al. (2014).

The carapace of Tr. langii is tectiform in the posterior part. Its outline varies from oval to roundish. The nuchal is relatively variable in Tr. langii. The nuchal notch can be more or less pronounced, and is even absent in some individuals. MJSN VTT006-563 exhibits a pair of small supernumerary bones on the anterolateral edges of the nuchal, which changes its usually trapezoidal outline (Püntener et al., 2014: Fig. 8). Tropidemys langii is mainly characterized by the presence of thick, wide, hexagonal, and keeled neurals. Although the angle formed by the keel and the geometry of the neurals are subjected to some intraspecific variation (see Püntener et al., 2014: Table 1), these characters clearly distinguish Tr. langii from other Late Jurassic turtles, including Plesiochelys etalloni, Craspedochelys picteti, Craspedochelys jaccardi, Thalassemys hugii, and ‘Thalassemys’ moseri. The midline keel is barely noticeable on the anterior neurals, then it becomes progressively more pronounced posteriorly before subsiding on the suprapygals. It nevertheless reaches as far as the pygal posteriorly. The pygal region itself is relatively poorly known. There are usually two suprapygals that vary in shape among specimens. In some specimens, there seems to be an intermediate element between the eighth neural and the first suprapygal (Fig. 5), as in P. etalloni, C. picteti, C. jaccardi, and Th. hugii. The pygal is a wide and rectangular element. There are eight pairs of costals. It is remarkable that costals 1 and 2 curve strongly anteriorly in their distal parts, whereas costals 4–8 curve posteriorly. The third costal is straight and widens distally, compensating for the diverging curvature of costals 2 and 4.

All sufficiently preserved specimens of Tr. langii have three cervical scales, but they vary in shape and proportion. Tropidemys langii is characterized by its very narrow vertebral scales. The intervertebral sulci are usually convex anteriorly in the midline. The third intervertebral sulcus is variably located on the fifth or the sixth neural (Püntener et al., 2014: Table 1). In contrast to P. etalloni, C. picteti, C. jaccardi, and Th. hugii, in which the fourth intervertebral sulcus is usually located posterior to the eighth neural on the intermediate element, if present, or on the first suprapygal, this sulcus extends medially on the eighth neural in Tr. langii. Furthermore, the arrangement of vertebral scales distinguishes Tr. langii from Tr. seebachi Portis, 1878. This species is only known from the Kimmeridgian of Hannover, Germany, and is characterized by the presence of up to eight vertebral scales and an additional row of paired scales intercalated between the vertebrals and pleurals (Karl, Gröning & Brauckmann, 2012). The pleural scales are very wide in Tr. langii. The interpleural sulci are usually located on the posterior part of costals 2, 4, and 6, but the first intercostal sulcus may extend onto the third costal (e.g., MJSN VTT006-253 and NMS 15; see Fig. 5). MJSN VTT006-176 exhibits paired supernumerary pleural scales immediately lateral to the first vertebral (Püntener et al., 2014: Fig. 4B). The marginals of Tr. langii are generally rectangular in outline and about twice as long as wide. The fourth and fifth marginals may extend slightly onto the costals in some specimens (e.g., MJSN VTT006-253 and MJSN VTT006-563).

The plastral anatomy of Tr. langii was poorly known until Püntener et al. (2014) described some articulated material from Porrentruy. The connection between the carapace and plastron is relatively strong, as indicated by the extensive attachment sites for the plastral buttresses on the ventral surface of the first and fifth costals. The epi-, ento-, and xiphiplastron of Tr. langii are unknown. Similar to the condition observed in C. jaccardi (see Table 4), the hyoplastron of Tr. langii varies in proportion from about as wide as long (MJSN VTT006-290) to wider than long (MJSN VTT006-563). There is a central plastral fontanelle mainly formed by the hyoplastra (e.g., MJSN VTT006-290 and MJSN VTT006-563), but it is possible that the central plastral fontanelle was reduced or absent in some individuals, as suggested by MJSN VTT006-52 (Püntener et al., 2014).

As in C. jaccardi, the humeral scale is significantly longer than the pectoral scale. Similar to the condition observed on the carapace, the arrangement of plastral scales exhibits a certain amount of variability, such as the presence of supernumerary scales. For example, in MJSN VTT006-563, a small triangular scale is intercalated between the hyoplastra (Püntener et al., 2014: Fig. 12B). A similar supernumerary scale is known in MAJ 2005-11-1, the holotype of Plesiochelys etalloni (Fig. 2). Based on the available material, the anal scale was probably restricted to the xiphiplastron. There were apparently four inframarginals on each side, the second inframarginal being the longest in the series.

Thalassemys hugii

Bräm (1965) listed 15 specimens of Thalassemys hugii in the historic Solothurn collection. At the beginning of the 1990s, the Geological Institute of Bern, Switzerland, collected additional turtle remains from the locality of St Niklaus (Meyer & Thüring, 2009; and references therein). This rich material is now housed in the NMS. If most of this material remains undetermined up until today, we have been able to identify a few specimens as Th. hugii. However, a detailed review of all the Solothurn material assignable to Th. hugii goes beyond the scope of the present study, and we will simply provide important additional information on the plastral morphology of this species.

Bräm (1965) described the carapace of Th. hugii as heart-shaped, but most peripherals are missing in the holotype (NMS 1; Figs. 6A–6D). Lapparent de Broin, Lange-Badré & Dutrieux (1996) revealed that posterior peripherals were actually relatively wide and that the carapace was oval. The nuchal is a broad, trapezoidal element, much similar to that of Plesiochelys and Craspedochelys, but without nuchal notch. There are eight neurals. The first neural is quadrangular and notably broadened anteriorly. Neural 2–6 are elongate, hexagonal elements with shorter sides facing anteriorly. In NMS 1, the sixth neural is subdivided into two elements, but this is not interpreted as having any systematic value. The seventh and eighth neurals are shorter, hexagonal elements. Historically, authors have described three suprapygals in Th. hugii (Rütimeyer, 1873; Bräm, 1965; Lapparent de Broin, Lange-Badré & Dutrieux, 1996). Comparisons suggest that the arrow-shaped element located directly posterior to the eighth neural in NMS 1 may actually correspond to the ‘intermediate’ element described in Plesiochelys etalloni, Craspedochelys picteti, and Craspedochelys jaccardi (see above). The ventral aspect of this element indicates that it was articulated to the vertebral series, but only for the anterior half of its length. As discussed above for P. etalloni, identifying this element is rather difficult. Posterior to this arrowhead-shaped element, there are two suprapygals. The pygal is not preserved in NMS 1. There are eight pairs of costals. Costals 1 and 2 are sutured to peripherals 1–3 in adult individuals. Small costo-peripheral fontanelles are retained between remaining costals and peripherals.

Bräm (1965) and Lapparent de Broin, Lange-Badré & Dutrieux (1996) described only one cervical scale in Th. hugii, but examination of the type specimen suggests that three may have been present. A more detailed review of the available material would be necessary in order to determinate the number of cervical scales in this species. The first vertebral is trapezoidal and greatly enlarged anteriorly. Posteriorly, its width is similar to that of the nuchal, but the first vertebral reaches the middle of the second marginal scale anterolaterally. Vertebrals 2–4 are significantly narrower than the same elements in P. etalloni, C. picteti, and C. jaccardi. Vertebral 2 is the narrowest and shortest of these three scales, whereas vertebral 4 is the widest and longest. The outlines of vertebrals 2–4 are characteristic. Their anterior and posterior borders are mostly straight and transverse. Their anterolateral borders are always slightly concave laterally, whereas their posterolateral borders are usually straight. These anterolateral and posterolateral borders are usually of about the same length for a given vertebral scale. As a consequence of the reduced width of the vertebrals, pleurals 1–3 are clearly wider than long.

The reconstruction of the plastron proposed by Bräm (1965: Fig. 30) is mainly based on the plastron of the holotype (NMS 1), which is poorly preserved and gives a misleading image of the true plastral morphology of Th. hugii (Figs. 6C and 6D). Referred specimens, such as NMS 20, NMS 593, and NMS 22325, provide important indications (Fig. 9). The central plastral fontanelle is not as extensive as to prevent a median contact of the hypoplastra, as suggested by Bräm’s (1965) reconstruction. In contrast, the hypoplastra do meet posteriorly for about half of their length along a strongly interdigitating contact. Behind this contact there is a small xiphiplastral fontanelle that prevents the xiphiplastra from meeting anteriorly. More posteriorly, the xiphiplastra meet along an interdigitating contact. However, the most important characteristic revealed by specimens NMS 20, NMS 593, and NMS 22325 is the definitive presence of a lateral plastral fontanelle in Th. hugii (Fig. 9). Based mainly on NMS 1, Bräm (1965) concluded that a lateral plastral fontanelle was absent in Th. hugii, which allowed to differentiate this taxon from eurysternids like Eurysternum (see ‘Eurysternum’ ignoratum, above). However, Bräm (1965) overlooked the fact that a lateral plastral fontanelle is clearly present notably in NMS 20 and NMS 593. NMS 22325, a large right hyoplastron from St Niklaus (collected during the 1990s excavations by the Geological Institute of Bern, Switzerland) pertaining to a specimen that was only slightly smaller than the holotype, also indubitably shows a lateral plastral fontanelle. The presence of a lateral plastral fontanelle in Th. hugii calls into question the traditional diagnoses of the Thalassemydidae and Eurysternidae.

During our review of the material, we have also identified two additional characters that allow to differentiate Th. hugii from other Solothurn turtles. The first of these characters is the presence of well-developed linear striations perpendicular to sutures between most shell elements, somewhat recalling the condition known in the Early Cretaceous Pleurosternon bullockii (e.g., Milner, 2004). These striations are clearly visible notably in NMS 1 (see Rütimeyer, 1873: plate 1; Bräm, 1965: plate 7, Fig. 4; Fig. 6), NMS 9, and NMS 22326-22327 (costals associated with the hyoplastron NMS 22325). They are also present in several specimens previously referred to E. ignoratum (see above): NMS 5, NMS 124, and NMS 412. The second character is the presence of a strong axillary buttress that is articulated over a large area on the ventral surface of the distal part of the first costal, as seen in NMS 1, NMS 412, and NMS 37251. The inguinal buttress is also relatively massive, although less so than the axillary buttress.

Bräm (1965) designated NMS 5 as the holotype of Eurysternum ignoratum and further referred NMS 124 (but see Plesiochelys langii, above) and NMS 412 to this species. However, these specimens are indiscernible from other specimens referred to Th. hugii (Fig. 6): e.g., vertebral scales with similar outlines and proportions (e.g., second vertebral length/width ratio of about 69% and 72% in NMS 1 and NMS 412, respectively); presence of clearly visible linear striations perpendicular to sutures between most shell elements (present in NMS 5, NMS 124, and NMS 412); presence of a strong attachment site for a large axillary buttress on the ventral surface of the distal part of the first costal (visible only in NMS 412). As in Th. hugii (see above; not Bräm, 1965), the plastron of NMS 5 has lateral plastral fontanelles and a central plastral fontanelle closed anteriorly by a median, interdigitating contact of the hyoplastra. A preliminary comparison of the girdle elements (notably the scapula and pubis) of NMS 5 (holotype of E. ignoratum), NMS 1 (holotype of Th. hugii), and NMS 9 (a specimen referred to Th. hugii) also reveals a very close morphology. Therefore, E. ignoratum is interpreted herein as a subjective synonym of Th. hugii.

‘Thalassemys’ moseri

Bräm (1965) typified ‘Thalassemys’ moseri based on a partial carapace and plastron (NMS 618; Figs. 7A–7D). He also referred three additional specimens to this species: a partial carapace (NMS 62; Figs. 7E and 7F), an isolated right hyoplastron (NMS 64), and an isolated left hyoplastron (NMS 111). As already noted by Bräm (1965) and Lapparent de Broin, Lange-Badré & Dutrieux (1996), the carapace of ‘Th.’ moseri is superficially similar to that of Plesiochelys etalloni: large trapezoidal nuchal with a broad and shallow nuchal notch; neurals elongated; three cervical scales; vertebrals wide and hexagonal with slightly sinuous sulci. However, ‘Th.’ moseri is characterized by the retention of costo-peripheral fontanelles in adults. NMS 618 and NMS 62 would have had an approximate carapace length of 400 mm. Specimens of similar size referred to P. etalloni are common in Solothurn (e.g., NMS 78 and NMS 107), but all have a completely ossified carapace. Furthermore, NMS 606, a juvenile P. etalloni with a carapace length of about 200 mm, also have an entirely ossified carapace. The retention of costo-peripheral fontanelles in adults is therefore a diagnostic feature of ‘Th.’ moseri. Close examination of NMS 618 and NMS 62 also reveals that their costals are very thin distally. This is clearly different from the condition known in Th. hugii, in which the costals remain relatively thick distally. Hence, the costals taper progressively distally in ‘Th.’ moseri, whereas their distal end is proportionally thicker and blunt in Th. hugii.

The plastron of ‘Th.’ moseri is best known from the holotype specimen (NMS 618). It is characterized by the presence of a central plastral fontanelle that is proportionally larger than that of P. etalloni or C. jaccardi. In contrast to Th. hugii (see above; not Bräm, 1965), the central plastral fontanelle is closed anteriorly and posteriorly by tight sutural contacts of the hyo- and hypoplastra, respectively. There is no lateral plastral fontanelle. Bräm (1965) noted that the epi- and entoplastron were not suturally connected to the hyoplastron. This reminds the condition in Th. hugii, but clearly departs from the strong sutural contact observed in P. etalloni and C. jaccardi. Finally, as suggested by Bräm (1965), there may have been a small xiphiplastral notch posteriorly.

Rieppel (1980) described a skull and associated, fragmentary shell remains (PMZH A/III 514) from the early Tithonian of La Morelière (Isle of Oléron, France) that he referred to ‘Th.’ moseri. Subsequent authors disagreed with this referral, considering that the specimen from La Morelière was a different taxon (Lapparent de Broin, Lange-Badré & Dutrieux, 1996; A Pérez-García, pers. comm., 2014). However, none of these authors studied the material first-hand. According to Rieppel’s (1980) conclusions, ‘Th.’ moseri is more closely related to Plesiochelys than to Thalassemys, but many features in the skull of ‘Th.’ moseri are plesiomorphic compared to the same features in P. etalloni and Portlandemys mcdowelli Gaffney, 1975a. Consequently, a referral of ‘Th.’ moseri to Plesiochelys does not seem appropriate. In the present study, we furthermore reveal that Th. hugii has a lateral plastral fontanelle, a feature absent in ‘Th.’ moseri. It therefore seems improbable that ‘Th.’ moseri is the closest relative of Th. hugii and its referral to Thalassemys does not seem appropriate either. In the current state of knowledge, the generic assignment of ‘Th.’ moseri remains uncertain. A thorough revision of the specimen described by Rieppel (1980) would certainly be an essential step toward a better understanding of this species, but ultimately more material is needed first to confirm or refute Rieppel’s (1980) identification, and second to gain insight into the morphology and relationships of this turtle.