Introduction
The late Maastrichtian type area (Jagt & Jagt-Yazykova, Reference Jagt and Jagt-Yazykova2012; Vellekoop et al., Reference Vellekoop, Kaskes, Sinnesael, Huygh, Déhais, Jagt, Speijer and Claeys2022) (Late Cretaceous) is known for a high abundance of marine vertebrate fossils, especially mosasaurs, plesiosaurs, fishes and turtles (Mulder, Reference Mulder2003; Friedman, Reference Friedman2012; Schulp & Jagt, Reference Schulp and Jagt2015; Miedema et al., Reference Miedema, Schulp, Jagt and Mulder2019). This area represents a shallow (20–40 m), subtropical [19,7°C, as calculated by van Baal et al. (Reference van Baal, Janssen, van der Lubbe, Schulp, Jagt and Vonhof2013)] marine ecosystem, right before the K/Pg extinction event (Mulder, Reference Mulder2003; Vellekoop et al., Reference Vellekoop, Kaskes, Sinnesael, Huygh, Déhais, Jagt, Speijer and Claeys2022).
Currently, the turtle record from the Maastrichtian type area comprises the giant cheloniid sea turtle Allopleuron hofmanni, represented by several complete fossils (Mulder, Reference Mulder2003), and the much rarer Glyptochelone suyckerbuyki, represented only by carapacial material (Kruytzer, Reference Kruytzer1955). Allopleuron hofmanni was interpreted as a carnivore, based on δ13C values comparable to that of extant carnivorous chelonioids (van Baal et al., Reference van Baal, Janssen, van der Lubbe, Schulp, Jagt and Vonhof2013). It was a pelagic turtle, primarily represented by adult fossils with carapacial sizes between 1 and 2 m (Kruytzer, Reference Kruytzer1955; Janssen et al., Reference Janssen, van Baal and Schulp2011). Little is known about the ecology of Glyptochelone. Finally, Platychelone emarginata represents a single, large, undescribed carapace found in the Maastrichtian type area. It is considered a nomen nudum (Mulder, Reference Mulder2003, p. 160). This specimen was recently suggested to be a basal species of Dermochelyidae, but extensive formal description is still lacking (Hirayama et al., Reference Hirayama, Nakajima and Folie2017). Several undescribed, fragmentary specimens from the Maastrichtian type area are tentatively assigned to P. emarginata (Nolis et al., Reference Nolis, Hellemond and De Bock2018).
Here we describe an isolated dentary (MAB13689) from the ENCI-Heidelberg quarry of the type Maastrichtian area. The ENCI quarry is situated near Maastricht in the Netherlands (see Fig. 1). Strata exposed span the entire upper Maastrichtian and isolated fragments of the lower Maastrichtian (Jagt & Jagt-Yazykova, Reference Jagt and Jagt-Yazykova2012; Vellekoop et al., Reference Vellekoop, Kaskes, Sinnesael, Huygh, Déhais, Jagt, Speijer and Claeys2022). The fossil is identified as cf. Ctenochelys, providing the first potential documentation of Ctenochelys from the Maastrichtian type area.
Fig. 1. The ENCI quarry south of Maastricht (the Netherlands) where MAB13689 was found. Adapted from Mulder et al. (Reference Mulder, Jagt, Kuypers, Peeters and Rompen1998).
Ctenochelys (Zangerl, Reference Zangerl1953), initially placed in Toxochelyidae, is now included in the family Ctenochelyidae, alongside Asmodochelys, Prionochelys and Peritresius (Gentry et al., Reference Gentry, Ebersole and Kiernan2019; Joyce et al., Reference Joyce, Anquetin, Cadena, Claude, Danilov, Evers, Ferreira, Gentry, Georgalis, Lyson, Pérez-García, Rabi, Sterli, Vitek and Parham2021). Placement of Ctenochelys with respect to crown-chelonioids remains contentious; several studies include this genus as a pan-cheloniid (e.g. Gentry, Reference Gentry2017; Evers et al., Reference Evers, Barrett and Benson2019), whereas other analyses recover it as a stem-chelonioid instead (e.g. Scavezzoni & Fischer, Reference Scavezzoni and Fischer2018; Gentry et al., Reference Gentry, Ebersole and Kiernan2019). Hirayama (Reference Hirayama, Callaway and Nicholls1997) synonymised all species of Ctenochelys based on postcranial material. The new material reported here, however, that there might be more variation in the otherwise poorly documented mandible of the genus.
The genus Ctenochelys is known from the late Cretaceous (Campanian-Maastrichtian) of the USA (Zangerl, Reference Zangerl1953; Nicholls & Russell, Reference Nicholls and Russell1990; Matzke, Reference Matzke2007) and Germany (Karl & Nyhuis, Reference Karl and Nyhuis2012), and potentially from the Paleocene of Denmark (Myrvold et al., Reference Myrvold, Milàn and Rasmussen2018). Two species are currently recognised within this genus (Hirayama, Reference Hirayama, Callaway and Nicholls1997): C. acris (Zangerl, Reference Zangerl1953) and the holotype of the genus C. stenoporus (Hay, Reference Hay1905). While C. acris occurs exclusively around the Western Interior seaway (Matzke, Reference Matzke2007), specimens of C. stenoporus including a lower jaw have been found in Germany (Karl & Nyhuis, Reference Karl and Nyhuis2012) and an isolated hyoplastron with a resemblance to C. stenoporus has been reported from the Paleocene of Denmark (Myrvold et al., Reference Myrvold, Milàn and Rasmussen2018). The present specimen shares most resemblance with the dentary of C. stenoporus as described in Karl & Nyhuis (Reference Karl and Nyhuis2012).
Institutional abbreviations
MAB: Oertijdmuseum, Boxtel, the Netherlands.
Systematic palaeontology
Order Testudines (Batsch, Reference Batsch1788)
Suborder Cryptodira (Cope, Reference Cope1868)
Superfamily Pan-Chelonioidea (Joyce et al., Reference Joyce, Parham and Gauthier2004)
Family Ctenochelyidae (Karl & Nyhuis, Reference Karl and Nyhuis2012)
Genus Ctenochelys (Zangerl, Reference Zangerl1953)
Material
An incomplete and fractured dentary (MAB13689) showing evidence of abrasion. An oblique anteroposterior fracture has displaced the right ramus over the triturating surface. The specimen was collected and prepared by Arno Savelkoul, who also donated it to the MAB collection.
Locality and stratigraphy
MAB13689 was found in the Emael member (Maastricht Formation) of the ENCI quarry (Fig. 1).
Description
In our description, we follow the updated anatomical nomenclature as defined by Evers et al. (Reference Evers, Ponstein, Jansen, Gray and Fröbisch2022). The ventral side of the dentary remains mostly embedded in the sediment, so that only the dorsal, medial and part of the lateral sides are clearly exposed. This was done due to the fragility of the material. Anteriorly, there is material missing and part of the right ramus was fractured during collection (Fig. 2). Traces of abrasion are microscopically visible at several locations.
Fig. 2. (a) Dorsal view of MAB13689 (cf. Ctenochelys); (b) Left view of MAB13689; (c) Posterior view of the symphysis of MAB13689; (d) Anterior view of the symphysis of MAB13689.
The two largely complete dentaries are tightly fused at the symphysis. The width of the intact half of the dentary is 19.7 mm (measured between the apex of the symphyseal ridge and posterolateral extremity), and it has a length of 34.5 mm (measured between the most anterior and posterior parts). No remnant of the foramen dentofaciale majus is visible on either the lateral or mediolateral surface of the dentary, suggesting that it is completely reduced, as in Dermochelys (Evers et al., Reference Evers, Ponstein, Jansen, Gray and Fröbisch2022). The dentaries bear a sharp symphyseal ridge, which ends posteriorly in a triangular elevation. Most of the symphyseal ridge has been heavily abraded, suggesting it was much higher than was preserved in the present specimen. The triturating surfaces are gently concave, forming a bowl-shaped dentary. The triturating surface extends posteriorly past the triangular elevation of the symphyseal ridge to form an oval shape. The labial ridge extends dorsally in the posterior end of the dentary, but is abraded on the anterior side. As preserved, the material suggests that the dentary had a bowl-shaped symphyseal area. A lingual ridge could not be identified. The anterior part of the symphyseal area is missing, but the orientation of the mandibular rami suggests that the dentary had a triangular shape overall. In contrast to chelonioids, the surangular in MAB13689 does not bear an anterior process as no suture scar on the posterolateral surface of the dentary could be discerned.
Discussion
MAB13689 has large concave areas on the triturating surface and a tall symphyseal ridge. Figure 3 shows a comparison of MAB13689 with closely matching genera, including Argillochelys (Lydekker, Reference Lydekker1889), Allopleuron, Procolpochelys (Hay, Reference Hay1908) and Ctenochelys. MAB13689 differs from Argillochelys in the fact that Argillochelys has labial ridges that curve upwards anteriorly forming a more pinched jaw (Moody, Reference Moody1980; Tong & Hirayama, Reference Tong and Hirayama2008; Zvonok et al., Reference Zvonok, Udovichenko and Bratishko2015). Allopleuron also has a symphyseal ridge, but it is much shorter than in MAB13689 and the triangular elevation is much higher and longer (Mulder, Reference Mulder2003). It differs from Procolpochelys by having a much narrower triturating surface and in general a narrower dentary (Weems & Brown, Reference Weems and Brown2017).
Fig. 3. Interpretative drawings of MAB13689 (cf. Ctenochelys) alongside dentaries of the genera that most closely resemble it (Mulder, Reference Mulder2003; Tong & Hirayama, Reference Tong and Hirayama2008; Karl & Nyhuis, Reference Karl and Nyhuis2012; Weems & Brown, Reference Weems and Brown2017). Dashed lines indicate missing material. Concavities and ridges on the triturating surfaces are marked with light grey and dark grey coloring respectively.
Two other turtle genera from the Maastrichtian deserve mentioning. The first is Euclastes (Cope, Reference Cope1867), which is known from the Maastrichtian of North America, South America and Africa (Foster, Reference Foster1980; Mateus et al., Reference Mateus, Polcyn, Jacobs, Araújo, Schulp, Marinheiro, Pereira and Vineyard2012; Parham et al., Reference Parham, Otero and Suárez2014). MAB13689 is easily differentiated from Euclastes by having a generally much narrower dentary and more prominent symphyseal ridge (Parham et al., Reference Parham, Otero and Suárez2014; Ullmann et al., Reference Ullmann, Boles and Knell2018). The second genus is Toxochelys (Cope, Reference Cope1873), where T. latiremis is described as having a narrow dentary, with a short symphysis and triturating surface of constant width across the dentary (Weems, Reference Weems1988; Matzke, Reference Matzke2008). This also differs from MAB13689, which has a longer symphysis and triturating surface that narrows slightly across its length.
The fossil shares the most similarities with Ctenochelys (Matzke, Reference Matzke2007; Gentry, Reference Gentry2017), in having a well-developed symphyseal ridge and large concave areas on the triturating surface, which extend posteriorly. The dentary of Ctenochelys typically has a modest hook anteriorly, this cannot be detected in the present specimen, however, as most of the anterior material of the dentary is missing. Specifically, MAB13689 most resembles the dentary of C. stenoporus as described by Karl & Nyhuis (Reference Karl and Nyhuis2012). An interesting note is that no remnants of the foramen dentofaciale could be identified in the present fossil, in contrast to the specimen described by Karl & Nyhuis (Reference Karl and Nyhuis2012).
The size of the dentary is quite small when compared to described adult individuals of Ctenochelys, with a length of 34.5 mm and some material missing anteriorly. Matzke (Reference Matzke2007) noted that the posteroventral part of the symphysis is visible in dorsal view in juvenile individuals of C. stenoporus. MAB13689 does not show this trait and is therefore not identified as a juvenile individual. There are no notable traits known in the dentary to identify subadult individuals of Ctenochelys.
Mandibular characters have received relatively little attention in most previous analyses of chelonioid phylogeny, and the placement of many basal taxa remains a topic of ongoing debate (e.g. Evers et al., Reference Evers, Barrett and Benson2019). The recent compendium of extant turtle mandibular anatomy by Evers et al. (Reference Evers, Ponstein, Jansen, Gray and Fröbisch2022) highlighted several synapomorphies for both Cheloniidae and Dermochelyidae, as well as Chelonioidea as a whole, which may prove useful for distinguishing members of respective stem lineages. However, the mandibular characters present in Ctenochelys do not unambiguously favour any placement. First, the absence of an anterior process of the surangular supports a position outside of crown-Chelonioidea. The reduced foramen dentofaciale majus in MAB13689 is more characteristic of Dermochelys, yet the hook development as described in other Ctenochelys specimens (Zangerl, Reference Zangerl1953; Matzke, Reference Matzke2007; Karl and Nyhuis, Reference Karl and Nyhuis2012; Gentry, Reference Gentry2017) is modest compared to the prominent spike-like symphyseal hook in Dermochelys (Evers et al., Reference Evers, Ponstein, Jansen, Gray and Fröbisch2022).
Conclusion
The dentary (MAB13689) found in the Emael Member of the ENCI quarry is identified as cf. Ctenochelys based on the shape of the dentary and traits such as the symphyseal ridge and a concave triturating surface. This specimen adds to the variation observed in mandibular anatomy within the genus Ctenochelys. Moreover, this provides the first potential occurrence of this genus from the Maastrichtian type area.
Acknowledgements
We want to thank reviewers Dr. Hans-Volker Karl and Dr. Andrew Gentry for their comments and insights. We would also like to thank Arno Savelkoul for making the specimen available for study. Finally, we would like to thank Lisa Egger and Thomas Wiese of the Bundesanstalt für Geowissenschaften und Rohstoffe for supplying extra figures of the mandible of Ctenochelys stenoporus described by Karl & Nyhuis (Reference Karl and Nyhuis2012). Jasper Ponstein was funded by an Elsa-Neumann scholarship.
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