Introduction

A complete Lower Cretaceous turtle from the Iberian Peninsula has been found in the Barremian fossil site of Las Hoyas (Cuenca). This finding is noteworthy because complete turtle skeletons from the Lower Cretaceous are rare. Sanz et al. (1988) briefly described two specimens from Las Hoyas (MCCM-LH 84 and MCCM-LH 254) and noted the cruciform aspect of the plastron, the presence of hyo-hypoplastral fontanelles and their stylized limbs. These features are only available on MCCM-LH 84 because the other cited specimen is a fragment of carapace. Although Sanz et al. (1988) noted that these characters are present in several testudine families, such as Toxochelyidae, Macrobaenidae, and Chelydridae, they nevertheless referred the Las Hoyas turtles to Toxochelyidae indet. The monophyly of Toxochelyidae has subsequently been rejected (Gaffney and Meylan 1988; Meylan et al. 2000; Averianov 2002). Jiménez-Fuentes (1995) also mentioned these specimens and considered them to be a “toxocheloid” related to Chelonioidea. He recognized the particular elongation of the digits of the manus of MCCMLH 84 as a shared feature between Las Hoyas specimens and chelonioids. Nevertheless, this is a common feature among aquatic turtles (Joyce and Gauthier 2004) and the Las Hoyas specimens do not present any synapomorphies diagnostic of the clade Chelonioidea (sensu Joyce 2007). Thereafter, the Las Hoyas specimen was identified as an undetermined basal representative of Centrocryptodira (Ortega et al. 1999) or as an undefined new form with “chelydroid” aspects, but without the autapomorphies of Chelydridae (Lapparent de Broin 2001). Some aspects of the specimens were not available for Sanz et al. (1988) and they suggested that it would be necessary to prepare the skull and the outer surface of the carapace and plastron of the specimen MCCM-LH 84 to analyze it, because most of the characters used for the identification of turtle taxa could not be observed. The specimen has fortunately since been prepared for study. MCCM-LH 84 preserves a large percentage of the elements of the cranial and, especially, postcranial skeleton. The exquisite preservation of this specimen, in a lithographic limestone, therefore allows the description here of a new basal freshwater eucryptodiran genus and species.

Institutional abbreviation.—MCCM, Museo de las Ciencias de Castilla-La Mancha, Cuenca, Spain.

Eucryptodiran turtles in the Upper Jurassic and Lower Cretaceous of Europe

Basal eucryptodiran turtles (sensu Joyce 2007) are well represented in the European Jurassic by the endemic littoral Thalassemydidae, Plesiochelyidae, and Eurysternidae (Danilov 2008). Thalassemydidae are present in the Kimmeridgian of Switzerland (Lapparent de Broin et al. 1996) and probably in France (Lapparent de Broin 2001). Eurysternidae range from the upper Kimmeridgian to the lower Tithonian of France, Germany and Switzerland, and are represented by Achelonia formosa, Euryaspis radians, Eurysternum wagleri, Hydropelta meyeri, Idiochelys fitzingeri, and Solnhofia parsonsi (Lapparent de Broin 2001). Several genera of Plesiochelyidae have been recognized in the Upper Jurassic of England, France, Germany, Switzerland, and Portugal: Plesiochelys, Craspedochelys, Tropidemys, and Tholemys (Lapparent de Broin 2001). Although there are tentative reports of plesiochelyids from the Valanginian (Lower Cretaceous) of Switzerland (Lapparent de Broin 2001), these turtles are typically thought to be inhabitants of coastlines, disappearing during the Jurassic—Cretaceous transition, when a regression restricted the European epicontinental seas and drastically reduced the habitats for these aquatic reptiles (Bardet 1994). The English taxon Hylaeochelys may be close to Plesiochelyidae (Lapparent de Broin 2001) or even a member of this group (Hirayama et al. 2000; Milner 2004). The phylogenetic position of the English Portlandemys mcdowelli is difficult to determine (Joyce 2007), although it might to be a plesiochelyid (Milner 2004). Other European Jurassic taxa that could belong to these families or be closely related to them are Anaphotidemys robusta, “Acochelys” approximata or “Thalassemys” marina (Lapparent de Broin 2001; Joyce 2003), a species perhaps closer to Cryptodira than Plesiochelys solodurensis (Joyce 2007).

In the Early Cretaceous of Laurasia, turtles are distributed in three distinct areas: North America, Asia, and Europe (Hirayama et al. 2000). In North America, paracryptodires dominate, in Asia eucryptodires, and in Europe both groups (Hirayama et al. 2000). European and North American faunas comprise the paracryptodiran clades Pleurosternidae and Baenidae, and Solemydidae, another basal pancryptodiran group of uncertain systematic position (Lapparent de Broin 2001; Danilov 2008; Joyce et al. 2011; Lyson and Joyce 2011). The record of basal eucryptodires in the European Cretaceous is limited to Brodiechelys brodiei, known from shells from the Barremian of England (Lydekker 1889; Nopcsa 1928). Brodiechelys may be closely related to, or a member of, Plesiochelyidae (Lapparent de Broin 2001) or Xinjiangchelyidae, a group from the Middle Jurassic—Early Cretaceous of Central Asia (Hirayama et al. 2000; Danilov 2005, 2008; Danilov and Parham 2007). Cryptodires have also been identified in the European Cretaceous, the dermochelyoid Protostegidae and the putative trionychoid Peltochelys duchastelii, from Belgium (Meylan 1988; Meylan and Gaffney 1989; Tong et al. 2006; Joyce 2007; Pérez-García 2011). There are others eucryptodiran taxa of uncertain affinity, including Chitracephalus dumonii from Belgium (Dollo 1884), Salasemys pulcherrima from Spain (Fuentes Vidarte et al. 2003), and Sandownia harrisi, from the Isle of Wight (UK), a taxon that has been assigned to Trionychoidea (Meylan et al. 2000; Lapparent de Broin 2001) or to basal Eucryptodira (Joyce 2007).

Fig. 1.

Geographical and geological location of the fossil site of Las Hoyas (Cuenca, Spain), in the Mesozoic context of the Iberian Ranges.

A relatively abundant record of basal eucryptodires is known from the Upper Jurassic of the Iberian Peninsula (Sauvage 1898; Antunes et al. 1988; Lapparent de Broin et al. 1996; Pérez-García et al. 2008b). Non-marine basal eucryptodires are reported from North and South America, central Asia, and Australia, but not yet from Europe (Gaffney et al. 2007). Thus, the new Las Hoyas turtle Hoyasemys jimenezi gen. et sp. nov. confirms the presence of this group of turtles in the Early Cretaceous of Europe. Other specimens from the Cretaceous of Spain could belong to this node, such as the turtle specimens from the Maestrazgo Basin or from the Cameras Basin (Pérez-García et al. 2008a; Pérez-García and Ortega 2009).

Geological setting

The Las Hoyas fossil site is a well-known Lower Cretaceous Konservatlagerstätte located in the southern part of the Serranía de Cuenca (Southwestern Iberian Ranges, Eastern Spain) (Fig. 1). The fossiliferous deposits, the La Huérguina Formation (upper Barremian), are composed of laminated limestone and rare marlstone beds. The depositional environment was a freshwater subtropical carbonate wetland that overlay a lowrelief karstic terrain (Buscalioni and Fregenal-Martínez 2010). The Las Hoyas biota consists mainly of obligate aquatic organisms, such as osteichthyan fishes, decapod crustaceans, belostomid insects, charophytes, the aquatic plant Montsechia (Buscalioni et al. 2008). Amphibious forms (crocodiles, turtles, and lissamphibians) are much less abundant, and terrestrial/arboreal forms, such as insects, lizards, and basal birds, are rare as well. Large animals including dinosaurs are exceptional finds (Buscalioni and Fregenal 2006). The terrestrial macroflora is dominated by conifers (Cheirolepidiaceae) and ferns (Mantoniacea and Schizaeaceae) (Escaso et al. 2005).

Systematic palaeontology

Fig. 2.

Skull and cervical vertebrae of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM-LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).

Fig. 3.

Ventral view of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM-LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).

Fig. 4.

Dorsal view of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM-LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).

Phylogenetic analysis

In order to determine the systematic position of Hoyasemys jimenezi, we used the data matrix from Joyce (2007), consisting of 56 taxa and 136 characters. Because of the variation shown by characters 73 (vertebral B: shape of the vertebrals) and 76 (plastron B: central plastral fontanelle) during ontogenetic development, we chose not to code these for Hoyasemys. Since the state of character 99 (anal A: plastral scale set 7, anals) in MCCM-LH 84 is intermediate between the conditions coded as 0 and 1, it also was not coded. Finally, it should be noted that for character 132 (manus A: phalangeal formula of manus and pes), the scoring was the reverse of that in Joyce (2007). All in all, Hoyasemys jimenezi was scored for 51 of the characters from Joyce (2007) (see Appendix 1).

The phylogenetic analysis was conducted using the TNT v. 1.0 program (Goloboff et al. 2008), with Proganochelys quenstedti as outgroup. Considering that Hoyasemys is an unequivocal pancryptodire, we used a reduced version of the data matrix in which all representatives of Panpleurodira and three of Joyce's (2007) rogue taxa (Portlandemys mcdowelli, Sandownia harrisi, Mongolemys elegans) were excluded. The most parsimonious trees were found using the heuristic tree search, performing 1000 replicates of Wagner trees (using random addition sequences) followed by TBR branch swapping (holding 10 trees per replicate). All characters were considered unordered and equally weighted.

The phylogenetic analysis resulted in 20 most parsimonious trees (MPTs) of 297 steps (CI = 0.569, RI = 0.863). The majority rule tree (Fig. 5) shows the same topology as found by Joyce (2007). Hoyasemys jimenezi gen. et sp. nov. is placed as a basal member of Eucryptodira, forming a monophyletic group with the “Macrobaenidae” and “Sinemydidae”. Here we found, as did Joyce (2007), that the “Macrobaenidae” and “Sinemydidae” are non-monophyletic groups more derived than Xinjiangchelys latimarginalis, but not including Cryptodira. The node “Macrobaenidae” + “Sinemydidae” + Cryptodira (Fig. 5: node D) is diagnosed by the following synapomorphies: epiplastra elongate in shape, with long postero-medial contact with the hyoplastra (Joyce 2007: character 83); extragular scutes absent (character 92); posterior cervicals with strongly developed ventral keels (character 103); and presence of cervical central articulations (character 105). The node consisting of all these taxa except Kirgizemys hoburensis (Fig. 5: node E) is diagnosed by the absence of parietal—squamosal contact because the upper temporal emargination is well developed (character 11).

Fig. 5.

Majority rule tree from the 87 maximum parsimonious trees produced by the cladistic analysis of Hoyasemys jimenezi in the modified data set of Joyce (2007). Retention index (RI) = 0.863 and consistency index (CI) = 0.569. Values refer percentages under 100% obtained in the majority rule analysis. Branchs with percentage under 50% are collapsed. Letters refer to the nodes mentioned in the text.

Hoyasemys jimenezi forms an unnamed clade with Judithemys sukhanovi, Dracochelys bicuspis, Sinemys lens, and Ordosemy leios. The members of this clade (Fig. 5: node F) share the absence or underdevelopment of chevrons (character 117). This clade is the sister group of crown-group Cryptodira (Joyce 2007).

Hoyasemys jimenezi has three autapomorphies in this cladistic analysis: articulation between fourth and fifth cervicals through a cotyle in the fourth and a condyle in the fifth (character 108); all caudal centra amphicoelous (character 119); most digits of manus and pes with three elongate phalanges (character 132).

Comparisons

Because of its systematic position in the cladistic analysis (Fig. 5), Hoyasemys jimenezi is compared with some species traditionally attributed to “Macrobaenidae” and “Sinemydidae”, primitive eucryptodires from the Cretaceous to the Paleocene of Asia and North America (Parham and Hutchinson 2003; Lee et al. 2009): Kirgizemys hoburensis (considered a subjective senior synonym of Hangaiemys hoburensis sensu Danilov et al. 2006), Judithemys sukhanovi, Dracochelys bicuspis, Ordosemys leios, Sinemys lens, Anatolemys oxensis, Macrobaena mongolica, and Wuguia efremovi. “Macrobaenidae” or “Sinemydidae” may both be paraphyletic (Parham and Hutchinson 2003; Gaffney et al. 2007), a view supported by our study. Other basal eucryptodires are also compared: Plesiochelys solodurensis, Plesiochelys etalloni, Solnhofia parsonsi, Thalassemys moseri, Brodiechelys brodiei, and Xinjiangchelys latimarginalis.

Skull.—As in most turtles, in Hoyasemys jimenezi the quadratojugal is present. The only basal eucryptodire in which this bone is absent is Sinemys, both S. lens and S. gamera (Joyce 2007).

In Hoyasemys jimenezi the pterygoids contact the basioccipital, as in Kirgizemys hoburensis, Judithemys sukhanovi, Dracochelys bicuspis, and Ordosemys leios but also in other basal eucryptodires, such as Portlandemys mcdowelli, Plesiochelys etalloni, and Solnhofia parsonsi (Joyce 2007). However, there is no contact between these bones in other taxa, including Sinemys lens (Brinkman and Peng 1993b; Sukhanov 2000; Sukhanov and Narmandakh 2006). The sagittal contact of the pterygoids in Hoyasemys jimenezi is shared with all basal eucryptodires except Kirgizemys hoburensis, which has lost this contact (Danilov et al. 2006; Joyce 2007).

A pair of pits on the posterior region of the ventral surface of the basisphenoid is present in several primitive pancryptodires, such as Sinemys lens, Kirgizemys hoburensis, Ordosemys leios, and Judithemys sukhanovi (Brinkman and Peng 1993b; Brinkman and Wu 1999; Tong et al. 2004; Danilov et al. 2006; Gaffney et al. 2007; Joyce 2007). This pair of pits is absent in Macrobaena mongolica, and other basal eucryptodires such as Portlandemys mcdowelli, Plesiochelys etalloni, and Solnhofia parsonsi (Danilov et al. 2006), but they are present in members of the paraphyletic group that includes all pancryptodires more derived than Xinjiangchelys latimarginalis, but excluding Cryptodira (Joyce 2007). This pair of pits is likely associated with insertions of major ligaments from the neck onto the base of the skull, allowing neck retraction (Brinkman and Peng 1993b; Joyce 2007). However, the presence of two pairs of blind depressions on this bone is not shared with any other representative of this node.

The triangular shape of the basisphenoid in Hoyasemys jimenezi is shared by some basal eucryptodires such as Portlandemys mcdowelli, Plesiochelys etalloni, and Dracochelys bicuspis (Gaffney 1976; Gaffney and Ye 1992; Gaffney et al. 2007), but not by Ordosemys leios or Kirgizemys hoburensis, in which it is rectangular (Gaffney et al. 2007).

As in most other pancryptodires, the basisphenoid of Hoyasemys jimenezi is narrower than the basioccipital. However, in Plesiochelys etalloni and Portlandemys mcdowelli the width of the rear end of the basisphenoid is the same of the basioccipital (Gaffney 1976; Rieppel 1980). The long basisphenoid of Hoyasemys jimenezi, as in Plesiochelys etalloni, contrasts with the condition in other taxa such as Solnhofia parsonsi, in with is very short (Lapparent de Broin et al. 1996).

The relatively short and wide basioccipital of Hoyasemys jimenezi is shared with Ordosemys leios, but it is longer than wide in other taxa such as Kirgizemys hoburensis and Dracochelys bicuspis (Brinkman and Wu 1999; Gaffney et al. 2007).

Vertebral column.—The transverse processes on the anterior edge of the centrum of the cervical vertebrae in Hoyasemys jimenezi is shared with the basal eucryptodires Kirgizemys hoburensis, Judithemys sukhanovi, and Xinjiangchelys latimarginalis (Gaffney et al. 2007). However, there other representatives of this group show the transverse processes in the middle of the centrum, such as Dracochelys bicuspis and Solnhofia parsonsi (Gaffney et al. 2007).

The articulations of the cervical vertebrae of Hoyasemys jimenezi are formed as in Kirgizemys hoburensis, Judithemys sukhanovi, and Ordosemys leios. This condition, also present in Cryptodira, differs from that observed in other basal eucryptodires, with amphicoelous vertebrae, such as Solnhofia parsonsi, Thalassemys moseri, and Xinjiangchelys latimarginalis (Joyce 2007).

The presence of at least the three opisthocoelous cervicals seen in Hoyasemys jimenezi (from the second to the fourth) is shared with Sinemys lens and Dracochelys bicuspis, with opisthocoelous cervicals 2–7 and a biconvex 8 (Brinkman and Wu 1999; Brinkman 2001). This differs from the character in most Pancryptodira, with the fourth cervical biconvex, separating the opisthocoelous anterior vertebrae from the procoelous posterior cervicals (Parham and Hutchinson 2003). The fourth cervical vertebra is biconvex in taxa such as Judithemys sukhanovi, Ordosemys leios, and Kirgizemys hoburensis (Brinkman and Wu 1999; Joyce 2007; Gaffney et al. 2007).

As in Solnhofia parsonsi, all caudal centra in MCCM-LH 84 are amphicoelous (Gaffney et al. 2007; Joyce 2007). Dracochelys bicuspis, Kirgizemys hoburensis, Ordosemys leios, and Judithemys sukhanovi have a biconcave third caudal vertebra, with procoelous anterior and opisthocoelous posterior caudals (Brinkman and Peng 1993a; Brinkman and Wu 1999; Hirayama et al. 2000; Parham and Hutchinson 2003; Gaffney et al. 2007; Joyce 2007).

The small, poorly developed chevrons in the last preserved vertebrae of Hoyasemys jimenezi are like those of Judithemys sukhanovi and Ordosemys leios (Parham and Hutchinson 2003; Joyce 2007). However, many basal eucryptodires have well-developed chevrons in nearly all caudals, such as Solnhofia parsonsi (Brinkman and Wu 1999; Hirayama et al. 2000; Gaffney et al. 2007; Joyce 2007).

Carapace.—The longer than wide carapace of Hoyasemys jimenezi is seen in almost all basal eucryptodes, but not in Ordosemys leios, in which it is as wide as long, or in some specimens of Sinemys, such as Sinemys lens or Sinemys gamera, in which the width of the carapace can be greater than or equal to its length (Brinkman 2001; Parham and Hutchinson 2003; Danilov and Sukhanov 2006; Danilov et al. 2006).

The absence of emargination in the anterior margin the nuchal plate of Hoyasemys jimenezi is shared with Wuguia efremovi or Anatolemys oxensis, taxa in which this emargination is weak or absent. Judithemys sukhanovi, Kirgizemys hoburensis, Ordosemys leios, and Sinemys lens have a small emargination. On the contrary, in Dracochelys bicuspis the emargination is well developed (Sukhanov 2000; Parham and Hutchinson 2003; Matzke et al. 2004a; Danilov et al. 2006; Danilov and Sukhanov 2006; Sukhanov and Narmandakh 2006; Danilov and Parham 2008).

Hoyasemys jimenezi shares with other primitive eucryptodires, such Kirgizemys hoburensis, Ordosemys leios, and Wuguia efremovi, the dorsal thickening of the lateral edges in the anterior peripherals (Brinkman and Peng 1993a, b; Peng and Brinkman 1993; Parham and Hutchinson 2003; Matzke et al. 2004b; Danilov and Sukhanov 2006; Danilov and Parham 2008). Guttered peripherals are absent in other taxa such as Dracochelys bicuspis, Judithemys sukhanovi, or Sinemys lens (Peng and Brinkman 1993a, b; Matzke et al. 2004b; Danilov and Sukhanov 2006).

The angled lateral edges of the vertebral scutes of Hoyasemys jimenezi are more like those of Dracochelys bicuspis than Kirgizemys hoburensis, where the vertebral scutes have nearly parallel sides (Brinkman 2001).

Plastron.—In Hoyasemys jimenezi the plastral buttresses reach only the peripherals, as in Sinemys lens, Kirgizemys hoburensis, Dracochelys bicuspis, Ordosemys leios, Judithemys sukhanovi, Xinjiangchelys latimarginalis, Solnhofia parsonsi, and Brodiechelys brodiei (Hirayama et al. 2000; Gaffney et al. 2007; Joyce 2007). In Plesiochelys solodurensis and Thalassemys moseri the plastral buttresses reach the costals (Lapparent de Broin et al. 1996; Hirayama et al. 2000; Gaffney et al. 2007; Joyce 2007).

The broad epiplastra of Hoyasemys jimenezi are also seen in Xinjiangchelys latimarginalis, but Sinemys lens, Dracochelys bicuspis, Kirgizemys hoburensis, and Ordosemys leios have narrow epiplastra (Brinkman and Wu 1999). The elongate epiplastra of Hoyasemys jimenezi, with a long posteromedial contact with the hyoplastra, are similar to Kirgizemys hoburensis, Judithemys sukhanovi, and Dracochelys bicuspis, but different from the squarish epiplastra with a minor posterior contact with the hyoplastra in Plesiochelys solodurensis, Solnhofia parsonsi, and Xinjiangchelys latimarginalis (Joyce 2007).

Appendicular skeleton.—The hook-shaped fifth metatarsal of Hoyasemys jimenezi is the general condition in turtles (Brinkman 2001). The complete phalangeal formula of the pes of Hoyasemys jimenezi is not known, but is compatible with the primitive formula for Pancryptodira, 2-3-3-3-3, present in eucryptodiran taxa such Judithemys sukhanovi (Parham and Hutchinson 2003).

Discussion

Adult basal eucryptodes can exhibit several characters seen in juveniles of other groups of turtles. In particular, the large size of the skull in relation to the carapace, the ornamentation on the vertebral scutes, the presence of fontanelles, the presence of broad vertebral scutes, or the interdigitating interhyoplastral and interhypoplastral sutures observed in the type of Hoyasemys jimenezi are shared with other adult eucryptodires.

The ratio of the length of the skull to the carapace in Hoyasemys jimenezi is about 25%, similar to Idiochelys fitzingeri and Eurysternum wagleri (Lapparent de Broin et al. 1996). Solnhofia parsonsi is diagnosed by a skull that is 40% the length of the carapace (Joyce 2000).

The ornamentation of forward-radiating ridges on the vertebral scutes in Hoyasemys jimenezi is present in adults of some basal eucryptodires. Xinjiangchelys qiguensis, X. tianshanensis, and X. radiplicatus show this dorsal ornamentation of the carapace, but the pattern and intensity is different, being weakest in X. qiguensis and strongest in X. radiplicatus (Matzke et al. 2004b). This pattern is also present in some Eurysternidae such as Eurysternum wagleri (Lapparent de Broin 2001). In some “Macrobaenidae” such as Kirgizemys hoburensis, the carapace is also sculptured by grooves (Brinkman and Peng 1993a).

Peripheral fontanelles are present in adult specimens of Eurysternidae, Dracochelys bicuspis, Ordosemys brinkmania, undetermined representatives of Sichuanchelys, Xinjiangchelys chowi, and some species of Sinemys such as S. gamera (Brinkman and Peng 1993b; Lapparent de Broin et al. 1996; Joyce 2000; Brinkman 2001; Maisch et al. 2003; Matzke et al. 2005; Sukhanov and Narmandakh 2006; Danilov and Parham 2007, 2008), but are absent in other taxa such as Kirgizemys hoburensis, Sichuanchelys chowi, Sinemys lens, and Xinjiangchelys qiguensis (Brinkman and Peng 1993a, b; Maisch et al. 2003; Matzke et al. 2004b; Danilov and Parham 2008).

The fenestrated plastron is primitive for Eucryptodira (Brinkman and Peng 1993b). A central fontanelle in the plastron is present in Ordosemys leios, Solnhofia parsonsi, and Eurysternum wagleri. However, other basal Eucryptodira lack this fontanelle, such as Judithemys sukhanovi, Kirgizemys hoburensis, Brodiechelys brodiei, and Xinjiangchelys latimarginalis (Hirayama et al. 2000; Joyce 2000; Sukhanov 2000; Danilov and Parham 2008). In other turtles, including Plesiochelyidae and Sinemys lens, this fontanelle can be retained or absent in adults (Bräm 1965; Gaffney 1975b; Lapparent de Broin et al. 1996; Gaffney et al. 2007).

The broad vertebral scutes of Hoyasemys jimenezi represent the primitive condition for Eucryptodira, observed in taxa such as Eurysternidae, Plesiochelyidae, Xinjiangchelyidae, Ordosemys leios, Dracochelys bicuspis, and Judithemys sukhanovi (Gaffney 1975c; Brinkman and Peng 1993a; Lapparent de Broin et al. 1996; Hirayama et al. 2000; Joyce 2000; Sukhanov 2000; Matzke et al. 2004b; Danilov et al. 2006). In some of these taxa, the vertebral scutes are considerably wider than in Hoyasemys jimenezi, extending to the peripheral plates in the case of Xinjiangchelys qiguensis (Matzke et al. 2004b).

The strongly interdigitating interhyoplastral and partial interhypoplastral sutures of Hoyasemys jimenezi are similar to those observed in adults of other taxa such as Xinjiangchelys qiguensis, Dracochelys bicuspis, or Macrobaena mongolica (Brinkman 2001; Maisch et al. 2003; Matzke et al. 2004b; Tong et al. 2004).

These comparisons mean it is impossible to determine the ontogenetic age of Hoyasemys jimenezi. For this reason, some characters were not coded for Hoyasemys jimenezi in the cladistic analysis, nor taken into account in the diagnosis of this new taxon. In this analysis, Hoyasemys jimenezi is considered to be a member of Eucryptodira more derived than Xinjiangchelys latimarginalis, but primitive relative to Cryptodira (Fig. 5).

The Uña locality (Cuenca, Spain) is the closest fossil site both spatially and temporally, and it has yielded a rich vertebrate fauna. Turtles are represented by shell fragments, including a “chelydroid-like” turtle (after Broin in Krebs 1995), which might be Hoyasemys jimenezi, although diagnostic characters are lacking.

Although some basal eucryptodires more primitive than Hoyasemys jimenezi, such as Thalassemydidae, Plesiochelyidae and Eurysternidae, are interpreted as coastal chelonians (Danilov 2008), closer relatives of Hoyasemys jimenezi, such as “Macrobaenidae” and “Sinemydidae”, may have been freshwater forms. In fact, Kirgizemys hoburensis, Dracochelys bicuspis, Sinemys lens, and Ordosemys leios are interpreted as adapted to freshwater environments, and described as “chelydroid” in appearance (Renous et al. 2008). Judithemys sukhanovi was a more specialized swimmer (Parham and Hutchinson 2003). The Las Hoyas deposits were formed in a shallow and permanent lacustrine system (Fregenal-Martínez and Meléndez 1995), and the fauna is largely freshwater (Buscalioni et al. 2008).

The adaptation of turtles to an aquatic life involves modifications both in shell shape and in the limbs (Renous et al. 2008). An incomplete and reduced shell, like that in this group of Eucryptodira, seems to be adequate in aquatic environments (Renous et al. 2008). The low shell is hydrodynamically efficient (Claude et al. 2003; Parham and Hutchinson 2003). The reduced fenestrated and cruciform plastron of the aquatic “Macrobaenidae” and “Sinemydidae” (Brinkman and Peng 1993a) allows good leg mobility. However, a reduced plastron could also facilitate other functions such as cutaneous respiration over a greater surface area, so it does not always imply an adaptation to swimming (Parham and Hutchinson 2003).

Adaptation to different modes of aquatic life has led to significant changes in the limbs of turtles. Hoyasemys jimenezi lacks the adaptations of marine forms, in which the forelimbs are much longer than the hindlimbs and are modified into rigid flippers and the hindlimbs into semi-rigid paddles. This is achieved by an increase in the length of the metacarpals and phalanges, the loss of articulations between these, similar length of the femur and humerus, a peculiar morphology of the pisiform, and flattened limb bones (Pace et al. 2001; Renous et al. 2008). Hoyasemys jimenezi, as well as the rest of basal Eucryptodira, clearly lacks these characters. However, the limbs of Hoyasemys jimenezi are similar to those of semiaquatic and highly aquatic freshwater forms (Pace et al. 2001; Renous et al. 2008), in that the femur is a little longer than the humerus, the paddles are small and consist of mobile digits, long claws on all digits, and a typical phalangeal formula of 2-3-3-3-3. In contrast, in highly aquatic freshwater forms such as Trionychidae and Carettochelyidae the paddles are more developed, with long mobile digits, hyperphalangy in phalanges IV and V, and claws only in the first three digits (Joyce 2007; Renous et al. 2008). Although the exact number of phalanges of the last two digits of the hands or feet are not known in Hoyasemys jimenezi, the fifth digit of the hand has no more than three phalanges, the last being a claw, as in semiaquatic turtles. Therefore, the carapace and limbs suggest Hoyasemys jimenezi was an inhabitant of freshwater, compatible with the environment where it was found.