Systematic Palaeontology

Diapsida Osborn, Reference Osborn1903

Sauropterygia Owen, Reference Owen1860

Eosauropterygia Rieppel, Reference Rieppel1994

Family Incertae sedis

cf. Lamprosauroides goepperti von Meyer, Reference von Meyer1860 (Lamprosauroides Schmidt, Reference Schmidt1927, replaces Lamprosaurus von Meyer, Reference von Meyer1860)

Description of referred specimen RGM.1333496

The specimen comprises a partial left dentary that is broken into four pieces, which are distributed over two slabs (Fig. 1A-B). One slab preserves the anterior three pieces, which are exposed in medial view, as well as the imprints of the first two teeth and surrounding bone of the fourth slab (Fig. 1A). The fourth piece, preserved on the second slab, is exposed in lateral view (Fig. 1B). The anteriormost portion of the dentary is poorly preserved and is considerably dorsoventrally taller than the pieces directly posterior to it, but it has a similar labiolingual width. On its anterior portion, the Meckelian groove appears to reach its terminus (Fig. 1A), which might indicate that it represents the posteriormost region of the mandibular symphyseal surface. The Meckelian groove is narrow and distinctly present on the three anterior pieces. In the anteriormost part, it is positioned at mid-height approximately, whereas in the other two pieces it is positioned close to the ventral margin of the dentary. On its posteriormost part the portion of the dentary below the alveolar margin is remarkably thin (Fig. 3B), but it is dorsoventrally slightly taller than the preceding section. Here, the dentary would likely have articulated with the surangular and angular, and a distinct Meckelian groove cannot be distinguished in the µCT and histological data in this area. The lateral surface of all four pieces of the dentary is slightly convex.

In total, 11 functional teeth are visible, most of which are equally spaced relative to each other, except for a relatively larger space between T4 and T5 and a closer proximity between T7 and T8 (Fig. 1A-B). Based on the µCT data, the position of seven additional empty alveoli (i.e. not containing teeth) could be identified (Fig. 4A; indicated with an asterisk in Fig. 1). The teeth are numbered from anterior to posterior (Fig. 1A, right to left; Fig. 1B, left to right), with each empty alveolus being counted as an additional tooth position (i.e. positions 3, 6, 9, 11, 13, 15 and 17). Two clear imprints of teeth are preserved on the posterior end of the slab containing RGM.1333496.a (Fig. 1A), which represent the imprints of T12 and T14 (i.e. the two anteriormost teeth on RGM.1333496.b; Fig. 1B). All teeth are preserved within an alveolus, except for T4 and T5 (Fig. 1A). T4 is likely preserved in situ, but the corresponding section of the lower jaw, including its alveolus, has been lost. Anterior to the disarticulated T5, the cross-section of a broken off tooth has been preserved. Its relatively large cross-section indicates that it is part of an enlarged fang, and it most likely represents the base of T5. T1-T4 are comparatively small and are slightly curved posteriorly (Fig. 1C-E). The three subsequent teeth are fangs (T5-T8; Fig. 1F-G). They are long, slender and more strongly recurved than T1-T4. T5 and T7 are the longest teeth in the entire sequence, whereas T8 is smaller and less recurved. The subsequent tooth (T10) is similar in size and shape to the four teeth preserved on the second slab (T12, T14, T16 and T18). T10, T12, T14, T16 and T18 are not recurved but have straight anterior and posterior margins. They are also considerably mesiodistally wider at their base than the preceding teeth, together resulting in a triangular shape in lateral view (Fig. 1H-J). Thus, the overall dentition is heterodont. The anterior teeth (T1, T2, T4, T5, T7 and T8) show a large size disparity, whereas the posterior teeth (T10, T12, T14, T16 and T18) are more similar to each other in size and shape. The fangs are very slightly lingually directed apically, whereas the other teeth are virtually straight. Except for T1, T2 and T4, all visible teeth are labiolingually constricted towards the centre at their base, with T7, T10, T14 and T18 showing this constriction the clearest (Figs. 1 and 2F-G). All teeth are single cusped and bear clear apico-basal striations on their apical halves (Fig. 1C-J).

Fig. 4.

µCT images of RGM.1333496.b. A) Digital sagittal cross-section of the dentary, highlighting the presence of empty alveoli in between alveoli with functional teeth. B) Digital transverse cross-section and C) interpretative drawing of the root of T16 exhibiting plicidentine folds. Abbreviations: eal, empty alveolus; ft, functional tooth; jb, jaw bone.

Histological description

Near the apex, the teeth are composed of dentine and an external layer of enamel that is between 40 to 55 µm thick in T7 (Fig. 2E-F) and 33 to 65 µm thick in T14 (Fig. 2H). The clear, regular striation pattern seen on the external surface of the enamel is also clearly visible in the sections of the distal halves of the teeth. In the apical sections, the enamel/dentine junction (EDJ) is straight and not parallel to the striations as in Nothosaurus and ichthyosaurs (Fig. 2I; Sander, Reference Sander1999; Maxwell et al., Reference Maxwell, Caldwell and Lamoureux2012). At the sections taken at about mid-height of the tooth exposed above the alveolus (sections II for T7 and V for T14, respectively; Fig. 2B, G), the enamel is considerably thinner, between 15 and 34 µm thick for both T7 and T14. The most basal transverse sections do not exhibit enamel, and instead, the outer margin of the teeth is formed by cementum (Fig. 2A, D, F). The cementum is composed of a thicker outer layer of cellular cementum (between 71 and 81 µm thick in T5 and between 50 and 60 µm thick in T9), characterised by the presence of cell bodies (cementocytes) within the tissue, and a thinner inner layer of acellular cementum (approximately 9 µm thick) that lacks cementocytes (Fig. 2D; Bertin et al., Reference Bertin, Thivichon-Prince, LeBlanc, Caldwell and Viriot2018). The cementum does not exhibit the striation pattern of the enamel clearly seen in the more apical sections.

The type of dentine present throughout the various sections of RGM.1333496 is orthodentine, characterised by an extensive pattern of parallel tubules oriented between the external surface of the tooth and its centre (Fig. 2; Sire et al., Reference Sire, Donoghue and Vickaryous2009). In the lowest transverse cross-section of T7, the outer margin of the dentine is gently folded, forming a ‘cloud-like’ pattern (Fig. 2A, D). This represents a minor expression of loose plicidentine, an infolding of the dentine towards the pulp around the tooth base (Maxwell et al. Reference Maxwell, Caldwell and Lamoureux2011a). Externally, minor striation indicative of plicidentine can only be observed at the base of the fang-like T5 (Fig. 1). Further evidence for plicidentine is also visible in the µCT data (Fig. 4B-C). The pulp is only visible at the base to mid-tooth area. The size of the inner pulp is relatively small in T8, T12 and T14 (Figs. 2A, F-G, 3A) but very large in the fang-like T7 (Fig. 2A), which is attributable to resorption processes in preparation of tooth replacement.

The teeth are set in deep sockets (thecodonty sensu Bertin et al., Reference Bertin, Thivichon-Prince, LeBlanc, Caldwell and Viriot2018, i.e. thecodont implantation irrespective of tooth attachment) to which they are ankylosed at their base (Figs. 3A, 4A). Based on the longitudinal section of T12, the alveolar bone lining the alveolus is composed of woven bone containing large amounts of osteocytes and is very distinct from the tissue that forms the jaw bone (Fig. 3D). The alveolar bone is perforated by large irregularly formed vascular spaces (Fig. 3D), as seen for instance in the extant crocodylian Caiman sclerops (Berkovitz & Sloan, Reference Berkovitz and Sloan1979). The lingual margin between the alveolar bone and the general jaw bone tissue constituting the dentary is partially bordered by a thin layer of short fibres (Fig. 3C, E). The identity of these fibres is unclear, but their position between the alveolar bone and general jaw bone, rather than between the alveolar bone and cementum of the tooth root, precludes the possibility that they represent ossified remains of the periodontal ligament or associated Sharpey’s fibres (Bertin et al., Reference Bertin, Thivichon-Prince, LeBlanc, Caldwell and Viriot2018). There is no other evidence of mineralised periodontal ligament or associated Sharpey’s fibres, nor is there an open space left by degraded soft ligament. The general bone tissue of the dentary consists of highly organised, avascular lamellar tissue labial to the alveolus, whereas lingual to it the matrix is composed of coarse parallel-fibred tissue scattered with some large primary osteons (Fig. 3A, C-E).

The alveolar margin is formed by thick bone in the anterior section of the jaw. In contrast, the alveoli of the posterior section are only separated by a thin bony wall (Fig. 4A), suggesting that these teeth are set in a tall groove with relatively minor separation between them and neighbouring alveoli. This configuration is similar to that seen in immature specimens of extant Crocodylia (Miller, Reference Miller1968).

A replacement tooth is present in T7. It is clearly visible externally in lateral view at the base of the functional tooth, which has been resorbed in its centre, revealing the striated enamel of the crown of the replacement tooth (Fig. 1G). The basalmost section of T7 clearly shows the replacement tooth, which has a distinct enamel layer (∼ 40 µm) and a comparatively large pulp cavity (Fig. 2A, D). In the longitudinal thin section, directly lingual to T12, a clear and deep excavation on the alveolar margin of the dentary can be discerned, which represents a resorption pit, indicating an early stage of the development of another replacement tooth (Fig. 3A, E).

Comparison to known vertebrates from the Lower Muschelkalk

The morphology and dentition of RGM.1333496 cannot readily be assigned to any well-known Triassic vertebrate. Its morphology is not compatible with that generally seen in the dentaries of non-tetrapodomorph osteichthyans. For example, Saurichthys spp. (Rieppel, Reference Rieppel1985) and Birgeria spp. (Schwarz, Reference Schwarz1970), both large Triassic fishes, usually show ornamented jaw bones and large teeth regularly alternating with smaller teeth. General tooth morphology, as well as microstructure, also differs considerably from that observed in RGM.1333496 (Rieppel, Reference Rieppel1985: plate VII, Fig. 4; Schwarz Reference Schwarz1970: fig. 32-38). More generally, the lack of an enameloid or acrodin cap on the apical ends of the teeth further indicates that RGM.1333496 cannot be referred to an actinopterygian fish (Sasagawa et al., Reference Sasagawa, Ishiyama, Yokosuka, Mikami and Uchida2009). Furthermore, the tooth replacement process discerned from RGM.1333496 (see below), which includes the replacement tooth migrating from the alveolar margin ventrally within the jaw during early development, before moving labially into the alveolus and subsequently replacing the previous tooth, corresponds with that seen in reptiles (Edmund, Reference Edmund1960). Thus, sarcopterygian affinities can also be excluded for RGM.1333496, since replacement teeth in sarcopterygians develop on the alveolar margin of the jaw (Doeland et al., Reference Doeland, Couzens, Donoghue and Rücklin2019). Temnospondyl, and other lissamphibian affinities, can be excluded because RGM.1333496 lacks an articulation surface for the splenial on the ventrolabial side of the dentary (Jupp & Warren, Reference Jupp and Warren1986) and does not possess a pleurodont tooth implantation nor the typically pedicellate teeth seen in this group (Davit-Béal et al., Reference Davit-Béal, Chisaka, Delgado and Sire2007). It can also be excluded that RGM.1333496 represents an ichthyosaur, since none of the known ichthyosaur taxa show a similar discrepancy in tooth size as seen in this specimen and because the plicidentine exhibited on the roots of ichthyosaur teeth is generally much more extensive than that seen in RGM.1333496 (Sander, Reference Sander2000; Maxwell et al., Reference Maxwell, Caldwell and Lamoureux2012).

The teeth of RGM.1333496 are set in deep sockets, somewhat resembling the typical thecodont implantation seen in many archosauriforms (e.g. Nesbitt, Reference Nesbitt2011). However, in contrast to the general archosaur condition, the teeth are ankylosed to the jaw exclusively at the base of the alveoli (Fig. 3A). This condition has previously been described for certain eosauropterygians, in which it was referred to as ‘basally ankylosed thecodont’ (Rieppel, Reference Rieppel2001). Tanystropheids, the only archosauromorphs currently known from the Lower Muschelkalk of Winterswijk, have a subthecodont tooth implantation (i.e. the lingual margin of the alveolus is considerably lower than its labial margin; Spiekman et al., Reference Spiekman, Neenan, Fraser, Fernandez, Rieppel, Nosotti and Scheyer2020b; Spiekman et al., Reference Spiekman, Fraser and Scheyer2021). No craniodental remains are known for ‘Tanystropheus antiquus’, the only tanystropheid known from the Lower Muschelkalk (Spiekman & Scheyer, Reference Spiekman and Scheyer2019). Amotosaurus rotfeldensis, known from the Upper Buntsandstein of Germany, which slightly predates the Lower Muschelkalk, and Tanystropheus hydroides and Tanystropheus longobardicus, known from the Anisian-Ladinian of the Alps, show dentitions that are very distinct from RGM.1333496. The marginal dentition of Amotosaurus rotfeldensis is homodont and consists of small peg-like teeth (Fraser & Rieppel, Reference Fraser and Rieppel2006). Tanystropheus longobardicus possesses fang-like teeth anteriorly and wide, tricuspid teeth in the posterior part of the jaw, whereas the larger Tanystropheus hydroides similarly possesses fang-like teeth anteriorly, but conical teeth posterior to this (Spiekman et al., Reference Spiekman, Neenan, Fraser, Fernandez, Rieppel, Nosotti and Scheyer2020a). The lack of discernible carinae (Fig. 1C-J), which are widely present in archosauriforms (although absent in tanystropheids), further indicates that RGM.1333496 does not represent a member of the stem-archosaur lineage (Nesbitt, Reference Nesbitt2011).

The dentition of RGM.1333496 differs distinctly from all known thalattosauriforms by the presence of elongate fang-like teeth. Most thallatosauriforms have a largely durophagous dentition (Thallatosauria), small conical marginal teeth (Gunakadeit joseeae) or edentulous jaws (Endennasaurus acutirostris) (Rieppel, Reference Rieppel1987; Müller et al., Reference Müller, Renesto and Evans2005; Druckenmiller et al., Reference Druckenmiller, Kelley, Metz and Baichtal2020). The marginal teeth of Askeptosaurus italicus are similar to RGM.1333496 in that they are apically recurved with a distinctive striation only in the apical region of each tooth (Müller, Reference Müller2005), but this taxon lacks the enlarged fangs present in RGM.1333496 and exhibits a pleurothecodont tooth attachment. The dentition of Eusaurosphargis sp. and saurosphargids and their implantation and replacement are currently poorly known. Generally, their implantation has been interpreted as subthecodont (Nosotti & Rieppel, Reference Nosotti and Rieppel2003; Li et al., Reference Li, Rieppel, Wu, Zhao and Wang2011; Li et al., Reference Li, Jiang, Cheng, Wu and Rieppel2014), but it is unclear whether the teeth were ankylosed to the jaw. Their teeth are generally relatively small and leaf-shaped and are therefore very different from those of RGM.1333496 (Scheyer et al., Reference Scheyer, Neenan, Bodogan, Furrer, Obrist and Plamondon2017). Placodontiform affinities for RGM.1333496 can also be unambiguously excluded since its dentition differs clearly from the highly specialised durophagous dentition of Placodontia and from the small, peg-like and slightly recurved teeth of the placodontiform Palatodonta bleekeri (Neenan et al., Reference Neenan, Klein and Scheyer2013).

The only known pachypleurosaur from Winterswijk, Anarosaurus heterodontus, has a heterodont dentition as the name implies, and the teeth have a similar striation pattern as seen in RGM.1333496 (i.e. striations are only present on the apical half of the teeth). However, RGM.1333496 clearly differs from this taxon in its larger size and in its jaw and dental morphology (Klein, Reference Klein2009). The average length of the normal teeth of an adult individual of Anarosaurus heterodontus is around 2 mm and that of the largest fang is 3 mm (Klein, Reference Klein2009). Teeth of RGM.1333496 are between 3.3 and 9 mm long. In addition, the number of teeth in Anarosaurus heterodontus is greater, and they differ in shape (short and conical in Anarosaurus heterodontus versus slender elongated or triangular in RGM.1333496) and arrangement (set in groups in Anarosaurus heterodontus versus relatively regularly spaced in RGM.1333496) (Klein, Reference Klein2009). Numerous crania and mandibles of Anarosaurus heterodontus (Klein, Reference Klein2009; Heijne et al., Reference Heijne, Klein and Sander2019) and other pachypleurosaurs (Rieppel, Reference Rieppel1989; Sander, Reference Sander1989; Rieppel, Reference Rieppel2000) are known and none show any indication of a horizontal tooth replacement as is described for nothosauroids and pistosauroids. Tooth replacement might instead be vertical (Rieppel, Reference Rieppel1995b), but a detailed understanding of tooth replacement in pachypleurosaurs is currently unclear (Rieppel, Reference Rieppel2001).

The upper and lower jaws and dental morphology of Nothosaurus spp. are also well-known (Rieppel & Wild, Reference Rieppel and Wild1996; Rieppel, Reference Rieppel2000; Rieppel, Reference Rieppel2001; Shang, Reference Shang2007). Nothosaurus spp. always bear two maxillary fangs, which are equally long, set close to each other, and curve posteriorly. The other teeth are also slender and conical but comparatively much smaller than the fangs, with all of them being similar in size (Rieppel, Reference Rieppel2000; Rieppel, Reference Rieppel2001; Shang, Reference Shang2007). Teeth of Nothosaurus spp. are striated down to their root (Fig. 5A). In these features, the teeth of Nothosaurus spp. differ clearly from RGM.1333496. Tooth replacement has been studied for several sauropterygian taxa, including Nothosaurus spp., Pistosaurus longaevus, several species of the genus Cymatosaurus, several placodonts and several pliosaurid plesiosaurs (Burckhardt, Reference Burckhardt1896; Edinger, Reference Edinger1921; Edmund, Reference Edmund1960; Rieppel, Reference Rieppel1997; Rieppel, Reference Rieppel2001; Shang, Reference Shang2007; Neenan et al., Reference Neenan, Li, Rieppel, Bernardini, Tuniz, Muscio and Scheyer2014; Sassoon et al., Reference Sassoon, Foffa and Marek2015). In these taxa replacement teeth are developed in large, separate, ‘alveolarised’ cavities or crypts directly lingual or ventral to the corresponding alveolus. The presence of these crypts can be readily recognised superficially by the presence of a dental lamina foramen (DLF) on the alveolar margin of the jaw lingual or ventrolingual to the corresponding alveolus (Rieppel, Reference Rieppel2001). However, the occurrence of this configuration was likely variable among sauropterygians, as it has so far not been established for pachypleurosaurus and is only known for a limited sample of placodonts, nothosauroids, pistosauroids, cymatosaurs and pliosaurids (Rieppel, Reference Rieppel2001; Sassoon et al., Reference Sassoon, Foffa and Marek2015). Furthermore, crypts vary in size and position along the dental margin, being small and more closely positioned to, or even confluent with, the corresponding alveolus throughout the replacement cycle for posterior maxillary teeth in Nothosaurus sp. (Shang, Reference Shang2007; type B crypts therein). No DLFs are externally visible in RGM.1333496 (personal observation, N. Klein), and there appears to be no large crypt for the development of replacement teeth based on the µCT data (personal observation, S.N.F. Spiekman) and the thin section of T12 (Fig. 3).

Fig. 5.

Several lower jaws and maxillae from the Lower Muschelkalk of Poland and Winterswijk. A) A left mandibular ramus of Nothosaurus sp. from Winterswijk (TWE 480000391) in lateral view. B) The holotype of Lamprosauroides goepperti, a right maxilla (MGU Wr 3871s) from Krappitz in Upper Silesia, in lateral view. C) Close-up of the two posteriormost preserved teeth (6^th and 7^th counted from anterior) of MGU Wr 3871s. D) Close-up of the enlarged fang (3^rd counted from anterior) of MGU Wr 3871s. E) A left maxilla (GPIT-PV-31630) in lateral view referable to Lamprosauroides goepperti from Gogolin in Upper Silesia. The image of GPIT-PV-31630 was kindly provided by Agnes Fatz (GPIT) and has been mirrored for direct comparison with the holotype. F) A left mandibular ramus of unknown affinities (BGR, uncatalogued) from Zakrzów (formerly Sacrau), Poland, in lateral view. Note the triangular shape of teeth and the roughly striated base of the two well-exposed teeth. The arrows are pointed anteriorly.

Cymatosaurus spp. are so far only known from cranial remains and one lower jaw of questionable taxonomic identification (Rieppel, Reference Rieppel2000), although postcranial material from Winterswijk might also be referable to this genus (Klein, Reference Klein2010; Sander et al., Reference Sander, Klein, Albers, Bickelmann and Winkelhorst2014; Klein et al., Reference Klein, Voeten, Lankamp, Bleeker, Sichelschmidt, Liebrand, Nieweg and Sander2015). Teeth of Cymatosaurus spp. are striated down to the base of the exposed tooth and are generally shorter and more robust compared to nothosauroids and RGM.1333496. None of the teeth known for Cymatosaurus spp. exhibit the triangular shape seen in the posterior teeth of RGM.1333496. Cymatosaurus spp. have paired maxillary fangs like all (other) non-plesiosaurian pistosauroids and nothosauroids (von Huene, Reference von Huene1944; Rieppel, Reference Rieppel2000; Maisch, Reference Maisch2014). According to Maisch (Reference Maisch2014), fangs of Cymatosaurus spp. typically have a rounded cross-section and posterior maxillary teeth are all similarly shaped, being short, pointed and straight, with low crowns. These posterior maxillary teeth are all small-sized, although they are relatively larger than those of nothosauroids. Tooth replacement is horizontal in all known skulls of Cymatosaurus spp. as in nothosauroids and pliosaurid plesiosaurs (Rieppel, Reference Rieppel2001; Sassoon et al., Reference Sassoon, Foffa and Marek2015). However, in contrast to nothosauroids, where crypts of replacement teeth are generally separated by a bony bridge from the alveoli of the functional teeth, the alveoli of the functional and replacement teeth are usually confluent (8-shaped) in Cymatosaurus spp. (von Huene, Reference von Huene1944). A lower jaw from the lower Anisian of the Austrian Alps, which was previously assigned to the genus Anarosaurus (von Huene, Reference von Huene1958), has been reassigned to Cymatosaurus multidentatus based on the absence of spatulate teeth, a broadening of the mandibular symphysis and a constriction of the snout (Rieppel, Reference Rieppel1995b). In this likely juvenile specimen, the replacement teeth of the dentary posterior to the symphysis appear to be particularly closely associated with the functional teeth, being positioned directly ventrolingually to them. The lack of distinct and fully separated crypts for the replacement teeth in Cymatosaurus multidentatus represents a configuration similar to that seen in RGM.1333496 (Fig. 3A). However, as mentioned above, the morphology of the teeth of RGM.1333496 differs from that of any known species of Cymatosaurus. The teeth of Cymatosaurus multidentatus are very tightly bunched together and as such also clearly differ from RGM.1333496.

Lamprosauroides goepperti was described by von Meyer (Reference von Meyer1860) based on a right maxilla from the Lower Muschelkalk of Krappitz in Upper Silesia (MGU Wr 3871s; Fig. 5B-D). Two additional maxillae, both from the Lower Muschelkalk of Upper Silesia, can be referred to the same taxon and are kept at the BGR (Rieppel, Reference Rieppel1995a) and at the GPIT (GPIT-PV-31630; Fig. 5E), respectively. The position of the margin of the large external naris on the maxillae is clear and is located close to the anterior end of the bone and relatively close to the lateral margin of the skull. The orbital margin cannot be identified. This might indicate that the maxilla was either very broad or that the orbit was located more medially or dorsally than in known pistosauroids, including Cymatosaurus spp., and nothosauroids. Posteroventral to the external naris, the maxilla bulges laterally in GPIT-PV-31630, followed by a fossa directly posterior to this protrusion. This might represent the same condition as described for the maxilla of Cymatosaurus fridericianus, which is somewhat broadened laterally at the level of the enlarged maxillary fangs to accommodate their roots (Rieppel, Reference Rieppel1997). The teeth are set in alveoli and are mainly curved lingually and only slightly backwards. The apical half of the crown is finely striated in all preserved teeth. In some teeth, the base also exhibits a rough plicidentine striation, whereas the middle part of the teeth is smooth.

Von Meyer (Reference von Meyer1860) identified Lamprosauroides goepperti as a nothosaur, which was contested by Gürich (Reference Gürich1884). Schrammen (Reference Schrammen1899) considered Lamprosauroides goepperti a cymatosaur. However, the fragmentary holotype of Lamprosauroides goepperti lacks diagnostic features and the species is therefore considered a nomen dubium (Rieppel, Reference Rieppel1995a). Consequently, shared diagnostic characters between this taxon and Cymatosaurus spp. cannot be confidently established. Nevertheless, Rieppel (Reference Rieppel1995a: p. 391) did consider close cymatosaur affinities for Lamprosauroides goepperti based on a similarity in dentition. However, there are several clear differences between Lamprosauroides goepperti and all known cymatosaur species. In Cymatosaurus spp., all maxillary teeth are striated from the tip to the base, whereas in Lamprosauroides goepperti the teeth are not striated in the middle section of the teeth and only some teeth are striated at the base. Furthermore, in all known cymatosaur species the teeth in the posterior portion of the maxilla are considerably smaller and more closely spaced than in the rest of this element (Rieppel, Reference Rieppel2000; Maisch, Reference Maisch2014), whereas in all known specimens of Lamprosauroides goepperti these teeth are not as closely spaced and the size of the posterior maxillary teeth does not differ distinctly from those on the anterior end of the maxilla. Compared to Cymatosaurus spp., the fangs of Lamprosauroides goepperti are also more slender. Another basal pistosauroid or eosauropterygian, Corosaurus alcovensis, which is known from the latest Early Triassic of North America, lacks the fang-like teeth observed in Lamprosauroides goepperti (Storrs, Reference Storrs1991).

In general, the tooth morphology of Lamprosauroides goepperti is distinct from known eosauropterygians. The external naris is larger than that seen in Pistosaurus longaevus and Cymatosaurus spp., but its proximity to the lateral margin of the snout is similar to the condition seen in nothosauroids. The orbital margin is not visible in Lamprosauroides goepperti, which likely indicates a broad bony bridge between external naris and orbits, much broader than usually present in nothosauroids (von Meyer, Reference von Meyer1860; Gürich, Reference Gürich1884), but possibly similar to Cymatosaurus spp. In summary, Lamprosauroides goepperti shows a mixture of features that do not exclude eosauropterygian affinities, but which differentiate it from any known cymatosaur, nothosauroid, or pistosauroid taxon.

Because Lamprosauroides is only known from isolated maxillae, it has no overlapping morphology with the left dentary of RGM.1333496. However, there are several similarities that indicate that RGM.1333496 could belong to the same species as the maxillae referred to Lamprosauroides goepperti or a closely related taxon. The dentition of Lamprosauroides goepperti is very similar to that of RGM.1333496, since the anterior teeth are elongated and curved somewhat posteriorly, whereas the posterior teeth are considerably shorter and more triangular (Figs. 1A-B and 5B-E). The apical ends of the teeth are striated both in RGM.1333496 and in Lamprosauroides goepperti, but the base of the teeth is smooth in the former (Fig. 1C-J), whereas those of the latter are also striated (likely forming plicidentine), with only the middle sections of the teeth being smooth (Fig. 5B-E). Tooth implantation and replacement in the maxillae of Lamprosauroides goepperti are unclear, but as mentioned earlier, like RGM.1333496, Lamprosauroides goepperti appears to lack the DLFs typical of many Triassic sauropterygians (personal observation N. Klein).

Finally, an undescribed, partial left dentary from the Lower Muschelkalk of Zakrzów (formerly Sacrau, Poland), kept at the BGR (BGR uncatalogued, Fig. 5F), shows a remarkably similar dentition to the teeth of the posterior slab of RGM.1333496. Two adjacent teeth are complete and in situ, whereas three in situ teeth positioned further anteriorly are only partially preserved. As in RGM.1333496, the completely preserved teeth are triangular and show a fine striation in the upper half and a rough plicidentine striation at the base, whereas the middle part is smooth. The teeth also appear to be evenly spaced, with a similar distance between the teeth as observed in the posterior part of RGM.1333496. This specimen represents the closest morphology to the peculiar configuration seen in the posterior slab of RGM.1333496, and it could represent the same species, possibly Lamprosauroides goepperti or a closely related taxon.