Pseudopus pannonicus (Kormos, 1911)

Loreal, Erwan, Syromyatnikova, Elena V., Danilov, Igor G. & Cernansky, Andrej, 2023, The easternmost record of the largest anguine lizard that has ever lived - Pseudopus pannonicus (Squamata, Anguidae): new fossils from the late Neogene of Eastern Europe, Fossil Record 26 (1), pp. 51-84 : 51

Pseudopus pannonicus (Kormos, 1911)

Figs 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15

Locality, horizon and material.

1. Petroverovka, early-middle Turolian (MN 11 - MN 12): one trunk vertebra (ZIN PH 7/281); four caudal vertebrae (ZIN PH 8/281, 9/281, 10/281, 11/281); and 49 osteoderms (ZIN PH 1-6/281; 12-54/281).

2. Kalfa, Middle Sarmatian age (=Vallesian; MN 10a): five dentaries (ZIN PH 1/277; 2/277; 3/277; 4/277; 5/277); three maxillae (ZIN PH 6/277; 7/277; 8/277); two parietals (ZIN PH 17/277, 18/277); and 57 osteoderms (ZIN PH 9-15/277; 19-68/277).

3. Lucheshty, late Ruscinian (MN 15): one frontal (ZIN PH 2/278); one parietal (ZIN PH 6/278); one fragment of the braincase (ZIN PH 1/278); one presacral vertebra (ZIN PH 7/278); and four osteoderms (ZIN PH 3-5/278; 8/278).

4. Etulia (Ovrag Nadezhda, Tretiy Ovrag, Strausovyj Ovrag local points), late Ruscinian (MN 15): five parietals (ZIN PH 2/279, 3/279, 4/279, 5/279, 6/279), one dentary (ZIN PH 1/279), one presacral vertebra (ZIN PH 7/279), and 11 osteoderms (ZIN PH 8-18/279).

5. Khadzhi-Abdul, late Ruscinian (MN 15): one maxilla (ZIN PH 1/282), two frontals (ZIN PH 2/282, 3/282), two parietals (ZIN PH 4/282, 5/282).

6. Tatareshty, late Ruscinian (MN 15,?MN 16a): one dentary (ZIN PH 1/280); and one lacrimal (ZIN PH 2/280).

7. Gaverdovsky, Turolian (MN 11): one dentary (GIN 1144/230); one maxilla (GIN 1144/231); one parietal (GIN 1144/232); three trunk vertebrae (GIN 1144/233; 1144/234; 1144/235).

8. Volchaya Balka, Turolian (MN 11): one frontal (GIN 1143/600); one portion of a braincase (GIN 1143/605); five trunk vertebrae (GIN 1143/602; 1143/603; 1143/606; 1143/607; 1143/608); three caudal vertebrae (GIN 1143/601; 1143/604; 1143/609); and 12 osteoderms (GIN 1143/610-621).

Description.

Maxilla. Most maxillae available in the material are incomplete except for the right maxilla ZIN PH 1/282 (Figs 2 - 3 ). This specimen is fairly complete and very large with a length of 36.93 mm and a height of 11.05 mm, thus making it the largest Pseudopus , or anguine even, maxilla ever found. To our knowledge, this is the largest maxilla found among anguids as a whole; see Fig. 2 ). In any case, all maxillae are robust and relatively large. The exception is GIN 1144/231 which is small and fragile.

The maxilla is anteroposteriorly long and relatively robust with a slight medial curvature at its anterior end. It consists of two major portions: the dental portion bearing the marginal dentition and the dorsally extending nasal process. The nasal process is roughly trapezoidal in shape. It is tall, forming an almost perpendicular wall, although it curves slightly medially at its dorsal extremity. The anterolateral wall of the lacrimal bone was abutting the nasal process of the maxilla where it begins to rise dorsally (i.e., at the level of the lacrimal recess). The anterodorsal margin of this process joins the nasal, whereas its posterodorsal portion broadly overlaps the prefrontal; the latter facet is clearly visible. The dental portion is almost complete, bearing teeth, although a few are missing anteriorly. The tooth row possesses 19 tooth positions (16 teeth are still attached). A short area posterior to the last tooth lacks dentition. In the anterior portion, the maxilla is forked, being divided into two rami. The septomaxillary ramus is thinner and taller than the external ramus. It is also anteromedially flexed and bordered by a small shallow fossa. In medial view, a prominent, concave, and relatively thick, horizontal supradental shelf is present. In the anterior region, a well-developed lip of bone is located on the dorsal surface of the dental shelf. This lamina can be referred to as the septomaxillary lamina (sensu Klembara et al. 2017). The superior alveolar foramen is deep and wide, located at the level of the posterior margin of the nasal process - at the level between the 9th and 10th tooth positions (counted from posterior, see Fig. 2C, D ). The adjacent supradental shelf is expanded medially in this area, forming a prominent palatine articulation. The jugal articulation is located in the posterodorsal portion of the posteroventral process, it is wide and deep. The ectopterygoid articulation is present on the posterior end of the maxilla, reaching the level of the 4th tooth position (counted from posterior). In lateral view, the otherwise more-or-less smooth dental portion is pierced by labial foramina in the area ventral to the nasal process. In ZIN PH 1/282, eight foramina are present, being located in a row. However, fewer are present in other maxillae, possibly related to differing preservation states (Fig. 3B, E ).

The area above the foramina, which also reaches the ventral region of the nasal process, is covered with se-veral fused, ornamented osteoderms. This ornamentation consists of several irregular ridges and grooves (Figs 2B , 3B, E ). The dorsal portion of the nasal process is smooth.

Remarks. The rather small size of maxilla GIN 1144/231, very light lateral dermal ornamentation, and its overall fragility are indications of an early ontogenetic stage, likely of a juvenile and not fully developed individual. In ZIN PH 1/282 with its length of 36.93 mm, the ectopterygoid articulation reaches the level of the 4th tooth position (counted from posterior). This seems to be different from Pseudopus apodus , where it reaches the penultimate tooth position ( Klembara et al. 2017: fig. 14B). However, this may be subject not only to individual variation but especially to ontogenetic variation, because the ectopterygoid articulation reaches the level of the last tooth in the specimen of P. pannonicus from Hambach (see Čerňanský et al. 2017: fig. 7C; its length is 17 mm) and the level of the third position (counted from posterior) in the specimen from Gritsev ( Roček 2019: 15C; its length is 24 mm).

Lacrimal. This is a paired bone. The specimen ZIN PH 2/280 is the only lacrimal available in the material (Fig. 4 ). It is a left element. In lateral view, two distinguishable portions can be recognized. The first, dorsal portion of the lacrimal is roughly rectangular. It is slightly ornamented by a few ridges and grooves. The anterior margin of the lacrimal forms the maxilla articulation. The second, ventral portion of the lacrimal is elongated, extending posteroventrally (Fig. 4A ). In medial view, the posteroventrally extending ventral portion of the lacrimal bears a prefrontal articulation.

Remarks. The lacrimal from Tatareshty is the only lacrimal bone reported for P. pannonicus (in the fossil members of the genus Pseudopus , the partly preserved remains of this element were reported only in P. laurillardi , see Klembara et al. 2010). As emphasized by Villa and Delfino (2019a), lacrimal bones are generally quite rare in the fossil record of European lizards, thus making our find noteworthy. However, because of the scarcity of this element in fossil representation and the lack of knowledge about morphological variation, lacrimal bones are of rather marginal interest for fossil identification purposes. This lacrimal generally matches the morphology of the lacrimal known for P. apodus ( Klembara et al. 2017). The lacrimal from Tatareshty is ornamented, which is a feature that is known in P. apodus . However, in the latter species, it is linked to size parameters as this condition is known only for large specimens ( Klembara et al. 2017). Yet, and interestingly, this lacrimal is relatively small in comparison to that of P. apodus . If dermal ornamentation of the lacrimal in P. pannonicus is related to size as in P. apodus , then the lacrimal of P. pannonicus would be among the rare bones to be smaller in P. pannonicus than in P. apodus . Nevertheless, this lacrimal is the first and only known for P. pannonicus .

Frontal. When complete, frontals are large, anteroposteriorly long bones (Fig. 5 ). In our available material, these paired-bones are all disarticulated and none are found coalesced. The anteriormost portion of the frontal is markedly narrow, extending into a pointed nasal (=anteromedial) process. The anterolateral portion of the process bears the narrow facet for the nasal bone. The facet is sculptured by longitudinal grooves and ridges, indicating strong contact between the frontal and the nasal. The posterior portion of the frontal is the widest. Here, the posterolateral corner of the frontal is distinct and extends into the posterolateral process. In dorsal view, almost the whole dorsal surface is covered by ornamented osteodermal crust, which is fused to the bone. The ornamented surface is even extending far onto the nasal process of the frontal (Fig. 5A, C ). The only exception is present in the nasal facet and posteroventral process bearing postfrontal articulation. The ornamentation consists of short grooves, ridges and pits in the central portion of the ornamented surface. The bottoms of some pits are pierced by small foramina. The grooves and ridges diverge from the center to the periphery of the bone, becoming longer (note that this is especially prominent, in the anterior portion of the ornamented surface). The sulci, which separate epidermal osteoscutes (i.e., “shield(s)” of other authors, e.g., Klembara et al. 2010, 2017; Klembara 2012; Čerňanský et al. 2020; Georgalis and Scheyer 2021; Vasilyan et al. 2022, or "ossicula dermalia", e.g., Fejérváry-Lángh 1923; Roček 2019), are less well defined, almost difficult to recognize in some specimens. The incomparably largest osteoscute is the frontal one. It is separated from the frontoparietal and small, posteromedially located interparietal osteoscute (its triangular anterior region overlaps the posterior region of the frontal on the midline; note, however, that it is not preserved in some specimens) by the lateral frontal sulcus and the medial frontal sulcus. The first one is slightly longer than the latter. The lateral frontal sulcus runs in the anterolateral-posteromedial direction, whereas the medial, shorter one has a mediolateral course. The postfrontal articulation extends more posteriorly than laterally.

In ventral view, a large and robust frontal cranial crest can be observed. In its anterior portion, it extends into a rather well-defined and rounded prefrontal (=subolfactory) process. The anteromedial margin of this crest is thin and lightly convex. Posteriorly, it widens and gradually diminishes. It fades out at the posterolateral process of the frontal. Medially, the triangular wedge-shaped parietal tab is indicated by a facet. The anterior portion of the frontal crest is less deep, forming a sharp, medially directed ridge. In the anterior region, lateral to the frontal crest (including its lateral surface), a facet for the prefrontal is located. Its surface is rough and striated. The striation is relatively light on ZIN PH 2/282 in comparison with other specimens. The prefrontal facet is large and occupies anterior two-thirds of the frontal length, and reaches about mid-orbit level. The prefrontal and postfrontal facets are, however, not in contact, so a small lateral portion of the frontal is exposed on the orbital margin.

Parietal. Parietals (Figs 6 - 7 ) are medium-sized to large. The parietal is a large azygous element consisting of the parietal table with an ornamented surface and two posterolaterally diverging supratemporal processes. The parietal tab, when preserved, is small. The anterolateral processes are well developed, and their margins are rounded. The lateral margins of the parietal table are more-or-less straight. In dorsal view, the surface of the parietal table can be divided into two areas - a typical ornamented surface made of osteodermal crusts occupying most of the dorsal surface and a smooth area located posteriorly. The ornamented surface is large and roughly rectangular, being slightly wider than long in large specimens. The interparietal, lateral, and occipital osteoscutes are more or less well-delimited by the interparietal and occipital sulci (Fig. 6G ). In the large individual ZIN PH 6/278, however, the sulci are not recognized (see Fig. 7C ).

The dermal sculpture is made of an irregular network of grooves, ridges, and tubercles, being densely distributed. The interparietal osteoscute, when recognized, is pierced by the rounded parietal foramen. The foramen varies in size, being small in some specimens (Fig. 6A ), but larger in others (Figs 6G , 7E ). The preserved portions of the parietal osteoscutes are the largest relative to others in regard to their size. They are butterfly-wing shaped. As well as the interparietal osteoscute, their anterior portions overlap the frontal. The occipital osteoscute is large. Although its shape slightly varies in the specimens, its posterior margin is usually more-or-less straight. Posteriorly located smooth area of the parietal table is large but shorter than the ornamented surface. The supratemporal processes are not well preserved, being either damaged or broken off in most specimens. In some specimens (e.g., ZIN PH 17/277; ZIN PH 3/279; ZIN PH 6/279; and to a lesser extent ZIN PH 4/282), parts of the supratemporal processes are still preserved. They gradually taper posterolaterally.

In ventral view, the most conspicuous structure is the parietal cranial crest. These crests form sharp walls on each side, so marking the cranial vault. They run generally anteroposteriorly but are slightly convex at mid-length. Lateral to the crest, a distinct muscular attachment surface is present. Its width varies among individuals. In large specimens, its width is equal to the distance between the parietal foramen and parietal cranial crest, whereas in smaller individuals, this distance can be smaller (in some specimens, almost about one-third of the distance between the foramen and the crest). On GIN 1144/232, a small pit is opened posteriorly to the parietal foramen (Fig. 7B, H ). The parietal fossa is large and deep, followed posteriorly by the parietal trough. The anterior margin of the fossa is slightly elevated, so this region has a slightly bulged (or swollen) appearance. The fossa is laterally bordered by a sharp juxtafoveal crest. The postfoveal crest is strong and well defined. It is present as a massive ridge continuing from the junction of the end of the juxtafoveal crest and the parietal cranial crest. The supratemporal processes bear the ventrolateral ridges. These ridges are medially bordered by the parietal arch and laterally bordered by the ventrolateral surface. The base of these ridges abuts the lateral wall of the posterior portion of the parietal cranial crest. In lateral view, parietals are slightly externally convex, and the supratemporal processes are flexed posteroventrally.

Virtual microanatomy and histology. The three micro-CT scans of the parietals revealed a robust bone structure with a very similar internal microanatomy in terms of a vascular network (Fig. 6J, M, O ). It revealed relatively large, but not complex meshwork of channels and small cavities. The larger interconnecting vascular cavities of irregular shape are located only in the ventral section of the parietal table of ZIN PH 17/277 (Fig. 6K, L ). At this level, they are arranged around the central region, laterally and posteriorly from the parietal foramen (the bony area surrounding the foramen itself is, however, without cavities or channels). From here, a network of thinner interconnected channels extends anterolaterally and mainly posterolaterally (beneath the smooth area). The ZIN PH 18/277 and smaller specimen ZIN PH 6/279 are slightly different - large cavities are absent (Fig. 6M, N, O, P ). Here, the extensive, but very fine interconnecting vascular spaces form a dense network. Overall, the condition in Pseudopus pannonicus resembles the one in the extant Pseudopus apodus (Fig. 6R-T ). However, parietals ZIN PH 17/277 and ZIN PH 18/277 of P. pannonicus are distinctly thicker than the one in the latter taxon (see Fig. 6L, N vs. T).

In P. apodus - and to a certain degree in the parietals of P. pannonicus here - the dorsal portion of the parietal shows numerous foramina which lie inside of pits (for P. apodus , see Klembara et al. 2017: fig. 12A). CT coronal section (Fig. 6L, N, P, T ) revealed that the foramina represent openings of canals which continue inside to the bone and are connected to meshwork. Note that the finer histological details such as growth marks and cell lacunae of the bone are not visible.

Remarks. Although the dorsal opening of the parietal foramen is varying in diameter in our specimens, when closely observed in ventral view, it seems that the foramina piercing the parietals are of similar size across specimens. Thus, it is possible that this apparent variability in the size of the parietal foramen mentioned here above may likely be explained by differing degrees of osteodermal crust development rather than representing an actual range of individual variation. To support this idea, when observing specimen ZIN PH 6/279 (Fig. 6G, H ) in which the osteodermal crust surrounding the parietal foramen is damaged, one can see that both the internal and external openings of the parietal foramen are of the same size. When observing specimen ZIN PH 17/277 in ventral view, a part of the osteodermal crust overlapping the dorsal opening of the parietal foramen can be seen through its ventral opening (Fig. 6B ). The small secondary pit described on the parietal GIN 1144/232 (Fig. 7H ) is related to individual variation. A similar condition has also been described and discussed for Pseudopus pannonicus specimens from Gritsev (Ukraine) by Roček (2019). This can be seen in other taxa such as Ophisaurus as well (see Čerňanský and Klembara 2017). As pointed out in more detail by Smith et al. (2018), the apparition of this secondary pit is caused by the "pineal-related cartilage" immediately underlying the bone.

Braincase. The following description is mostly based on specimen GIN 1143/605 as the other specimen from Lucheshty is rather poorly preserved, displaying few features. The specimen from Volchaya Balka is large (Fig. 8 ). The sphenoid, otic and occipital bones are completely fused in this specimen, and the sutures between the individual bones are hardly distinguishable. Despite its rather damaged right paroccipital process and both alar processes, it is in an overall good shape. In dorsal view, the unpaired supraoccipital lies at the midline and forms the dorsal portion of the posterior braincase, the dorsal part of the inner ear capsule, and the dorsal margin of the foramen magnum. It has a high ascending process with which the ventral crest of the parietal articulates. In lateral view, the process gradually rises dorsally. In dorsal view, the posterior margin of the supraoccipital possesses a wide, V-shaped notch located over the foramen magnum.

In posterior view, the foramen magnum is large, roughly hexagonal. The occipital condyle, which is formed by otooccipital and basioccipital, is heavily damaged. Only its base is present. The latter is flat and trapezoidal (Fig. 8D ). Its dorsal margin is much longer than its ventral one. Its lateral margins are rather straight and diverging dorsally. Dorsolaterally from the base of the condyle, there is set of foramina (preserved on the left side): two foramina for hypoglossal nerve (XII) - one is located closer to the occipital condyle, whereas the second more anterolaterally - both located in the depression; here, dorsal to the second one, a foramen for vagus nerve (X) is present and dorsal to it, there is a fourth foramen - potentially for accessory nerve (XI). The otooccipitals are strongly posterolaterally expanded to form the paroccipital processes - only the left one is preserved. It is well-developed, being robust and laterally expanded. A well-developed dorsal ridge runs posterolaterally along the entire dorsal surface of the paroccipital process. This dorsal ridge is slightly curved, mildly convex. The dorsal margin of the spheno-occipital tubercle is well-developed. The spheno-occipital tubercles are rectangular and slightly depressed between their dorsal and ventral margins. These tubercles are expanded ventrolaterally, well below the level of the ventral margin of the occipital condyle. There is a small and medially-pointed extension on the ventral margin of the spheno-occipital tubercles (Fig. 8D ). A swelling of the internal surface of the braincase ("bulla tympani" sensu Klembara et al. 2010) is present, well visible inside through the foramen magnum (Fig. 8C, D ).

In ventral view, there is a saddle between the basal (spheno-occipital) tubercles, immediately anterior to the area of the occipital condyle. Anterior to the saddle, there is a longitudinal depression running far on the sphenoid. The depression diminishes anteriorly and disappears at the level of the 1/3 of the anterior length of the braincase. The ventrolateral margins of the basal tubercles are prominent, stout.

In lateral view, beginning on the anterior margin of the paroccipital process and continuing strongly onto the prootic is a groove for the vena capitis lateralis (sensu Klembara et al. 2010), which is dorsally bounded by the prootic crest. The prootic crest is well-developed, being sharp. The area ventral to the prootic crest is quite significantly depressed. It is pierced by the fenestra ovalis: the oval window of the braincase in which the footplate of the stapes fits. Ventral to the fenestra is the large occipital recess. These two are separated by a fine, sharp interfenestral crest (Fig. 8H ). Anteriorly, immediately below the prootic crest, there is a small foramen for the facial nerve (V). It lies inside the groove for the vena capitis lateralis and is covered by the crest in lateral view. Slightly dorsal to the prootic crest and below the alar process, an additional crest is found. It is running on both sides of the braincase, its course being parallel to the prootic crest. It is not as sharp as the prootic crest but it is sufficiently well defined to be easily observed. More anteriorly, more-or less at the level of the ventral portion of the occipital recess, the bone is pierced by the large posterior opening of the Vidian canal (Fig. 8G, H ).

Anteriorly, the Vidian canal opens lateral to the base of the parasphenoid process (Fig. 8C ). The anterior foramen for the abducens nerve (VI) is located directly above the anterior opening of the Vidian canal. A pair of the internal carotid foramina are located centrally, dorsal to the base of parasphenoid process. A sharp crista sellaris is found dorsal to these foramina and runs between the well-developed alar processes. A foramen for a facial nerve opens ventromedially (inside of the braincase) from the prootic crest, inside of the oval depression. In the internal side of this depression, a foramen for the vestibulocochlear nerve (VIII) is also present. The posterior internal area of the braincase is pierced by the perilymphatic foramen. Further posteriorly, the foramen for the vagus nerve and for the hypoglossal nerve are located, piercing the internal posterior area.

Dentary. Several dentaries are available in the material. Because the preservation quality varies among these specimens (Figs 9 - 11 ), the following description is mainly based on the best-preserved mature individuals, i.e., specimens ZIN PH 1/277 (Fig. 11A, B ), ZIN PH 3/277 (Fig. 9 ), and to an extent ZIN PH 1/280 (Fig. 10 ; this specimen represents, however, an immature individual). Tooth rows are mostly incomplete in all specimens, except for ZIN PH 1/280; thus, the total number of tooth positions is difficult to observe.

Among the best-preserved specimens, ZIN PH 1/280 displays 16 tooth positions (all teeth are still attached except for the small last one; Fig. 10B ). The other preserved specimens are large-sized. In medial view, both ZIN PH 1/277 and ZIN PH 1/280 possess their symphyseal region being preserved at the anterior end of the bone (Figs 10B-D , 11A ). The symphysis is provided with a kidney-shaped symphyseal facet. The Meckelian canal is fully opened. Anteriorly, it reaches the symphysis in these two above-mentioned specimens, notching it ventrally. Further, the canal continues as a narrow groove, but it widens in the posterior portion of the bone due to the rising of the dental crest in this section. The Meckelian canal is dorsomedially roofed by an almost concave dental crest, whereas it is ventrally bordered by the ventral crest. The ventral crest is more-or-less straight. The ventral and dental crests bear facets for the splenial articulation. In the complete, but immature specimen ZIN PH 1/280, the facets reach anteriorly the level of the 7th tooth position (counted from anterior).

The anterodorsal margin of the anterior inferior alveolar foramen forms a shallow, elongate notch (the splenial spine is not preserved) at the dental crest, being located at the level of the 5th tooth position in ZIN PH 5/277, at the level between 5th-6th tooth position in ZIN PH 3/277 and at the level of the 6th tooth position in ZIN PH 1/280. The opening of the alveolar canal, the alveolar foramen, is located at the level of the posterior portion of the anterior inferior alveolar foramen (Fig. 11C ). The intramandibular septum, which separates the alveolar canal from the Meckelian canal, is almost horizontal. In the posterior portion, the dental crest bears a large facet for an anteromedial process of the coronoid bone. This facet has a rough surface and extends anteriorly to the level of the 4th tooth position (counted from posterior) in the larger individuals and to the level of the 3rd tooth position in ZIN PH 1/280. However, there is a small area that separates the anterior end of the facet from the anterior inferior alveolar foramen, thus the coronoid facet does not reach the foramen. This separating area is difficult to interpret in larger individuals - most likely represents a facet for the splenial (see Discussion). In any case, there is a clear dorsal facet for the splenial posterior to the anterior inferior alveolar foramen in ZIN PH 1/280 - the foramen only interrupts the facet here. This posterior portion of the dorsal facet for the splenial reaches the level of the 3rd tooth position (counted from posterior). The angular process is well-preserved only on ZIN PH 1/280 whereas it is damaged - only the root portion is preserved in other specimens. In ZIN PH 1/280, the posterior tip of this process does not surpass the coronoid process but terminates just anterior to the latter process. The coronoid process is posterodorsally oriented and slightly pointed on several specimens. The surangular process is preserved in some specimens (e.g., ZIN PH 2/277, ZIN PH 1/280). It is large and surpasses both angular and coronoid processes posteriorly (Fig. 11D ). The ventral edge of the surangular process bears a distinct notch forming the anterior margin of the anterior surangular foramen. In ZIN PH 2/277 (Fig. 11C, D ), the foramen is located completely in the slit in the posteriormost portion of the surangular process, and the slit is posteriorly not completely closed. The foramen seems to be absent in ZIN PH 1/280. The ventral edge of the surangular process of this specimen is rather straight (Fig. 10B ). On this particular specimen, the surangular sinus (sensu Klembara et al. 2014; i.e., sinus supraangularis in Roček 1984, 2019), located between the surangular and angular processes, is wedge-shaped.

In lateral view, the otherwise more-or-less smooth surface of the bone is pierced by labial foramina. These form a series located in the mid-line of the bone, and they number four in the best-preserved specimens.

Remarks. The specimen ZIN PH 1/280 is very well preserved and displays a different tooth morphology in comparison to any of the other larger specimens presented here. This specimen displays typical features of Pseudopus ( Klembara et al. 2014) such as the teeth being rather conical and slenderer with tips slightly curved posteriorly in the anterior half of the dentary, and becoming gradually stouter and more robust in the posterior half. Moreover, the tooth morphology (i.e., thick and conical teeth with pointed apices and posterior teeth being larger and stouter) displayed here is reminiscent of that of juvenile Pseudopus apodus (see Klembara e al. 2014). As pointed out in P. apodus , the spacing of teeth varies during the ontogeny and as the individuals mature, the teeth of the dentary become more densely spaced ( Klembara et al. 2014). Although for a different clade, tooth spacing was also hinted at as a possible juvenile feature by Smith (2011). Henceforth, the general morphology and spacing of teeth is suggesting that ZIN PH 1/280 pertained to a juvenile individual of Pseudopus . Klembara (2012) stated that teeth from dentaries of Pseudopus ahnikoviensis from the type locality were all devoid of striations. However, as emphasized by Čerňanský et al. (2015, 2017a), this could be linked to intraspecific variability or taphonomy as the latter authors reported for Pseudopus cf. ahnikoviensis material from the Early Miocene and Middle Miocene of the German localities of Amöneburg ( Čerňanský et al. 2015) and Hambach ( Čerňanský et al. 2017a) respectively. Similar variability has been shown for P. apodus ( Klembara et al. 2014). All currently known P. ahnikoviensis are confined to the Early and Middle Miocene (MN 3-MN5; Klembara 2012; Čerňanský et al. 2015, 2017; Klembara and Rummel 2018) of Central Europe whereas ZIN PH 1/280 originates from the much younger deposits of Tatareshty (MN 15). This juvenile specimen is relatively similar in size (20.73 mm in length) to the range known among the adults of P. ahnikoviensis . On the other hand, P. ahnikoviensis is distinguished from all other Pseudopus species by the autapomorphic feature of the dentary in the presence of a surangular spine. Here, ZIN PH 1/280 does not bear a surangular spine but rather a damaged angular process on closer examination. The morphology of the dentary of Pseudopus confertus is not currently known. Moreover, this species is solely restricted to a single locality from the Early Miocene (MN3) of the Czech Republic. The dentary of Pseudopus laurillardi is known in much more detail than the aforementioned Pseudopus species. This species is slightly younger. It is known from Miocene localities ranging from MN4 to MN7/8 ( Rage and Bailon 2005; Klembara et al. 2010; Ivanov et al. 2020). One of the most conspicuous and autapomorphic features of the dentary in P. laurillardi is the distinct and medially-extending subdental shelf, thus forming a markedly deep sulcus as well as a large medial ridge ( Klembara et al. 2010). The latter authors also state that the development of the subdental shelf is affected by ontogenetic processes, being rather narrow in smaller specimens and growing throughout development. Our specimen does not show signs of any subdental shelf, alike P. ahnikoviensis , P. pannonicus , or P. apodus . The posteriormost portion of the dentary of P. laurillardi is described as being very short, reaching a length equating to roughly the length occupied by the two posteriormost teeth ( Klembara et al. 2010). Here, the posterior portion of ZIN PH 1/280 is notably longer, reaching almost twice that length. It is possible, however, that the immature state of this individual affects the expression of that character. In addition to the various differences with several Pseudopus species exposed here, the stratigraphical position of the deposits of Tatareshty is also more congruent with the temporal range of either P. pannonicus or P. apodus . Finally, ZIN PH 1/280 exhibits an interesting feature in its facet for the splenial. It is most similar to that described for other dentaries of P. pannonicus presented here, which appear to have much taxonomical interest (see Discussion), thus comforting the attribution of this juvenile dentary to P. pannonicus .

Dentition. Maxillary teeth are small, cylindrical, and slender anteriorly (Fig. 2 ). These gradually increase in size posteriorly, becoming more bulbous up to the fifth-to-last teeth. Teeth are closely packed. Apices of teeth are lightly striated, both lingually and labially, and pointed. On some teeth, a more-or-less faint anteroposterior cutting edge can be observed. In the dentary, teeth are rather closely packed as well. The most complete specimens show a variation in teeth size along the tooth row. In the anterior portion of the tooth row, teeth are generally more cylindrical and slender, gradually increasing in size and robustness. The largest tooth is located slightly posterior to the mid-length of the dentary. The last four to five teeth are smaller, gradually decreasing in size. In ZIN PH 1/280, the apices of teeth are pointed and slightly striated. A cutting edge is distinct (Fig. 10D ).

Trunk vertebra. These presacral vertebrae are relatively well preserved. They are medium-sized to large. These vertebrae are overall rather robust, more compressed rather than elongated (Fig. 12 ).

They are procelous with the centrum in the shape of a conical frustum. In dorsal view, the vertebrae are constricted at mid-length. Prezygapophyses and postzygapophyses are laterally expanded. The articular facets of the prezygapophyses are oval, slightly elongated in the anterolateral-posteromedial direction, and dorsomedially inclined. The prezygapophyses slightly surpass the level of the anterior margin of the cotyle (e.g., GIN 1143/602), whereas the postzygapophyses do not reach the level of the posterior end of the condyle. The articulation facets of the postzygapophyses are oval, laterally expanded, and ventrolaterally inclined. In lateral view, the prezygapophyses and postzygapophyses are connected by the well-developed interzygapophyseal ridge. The neural arch is lightly depressed medial to the prezygapophyses. The dorsal region of the neural arch slightly rises in height posteriorly. The posteriorly developed neural spine starts to rise at about vertebral mid-length. Unfortunately, it is not complete in any of these specimens, the tip of the neural spine being broken in most specimens (Fig. 12C, H, M, R, W ). It thickens posteriorly, thus displaying a hatchet-like morphology in a cross-section. The neural canal is sub-circular, tunnel-like. Its dorsal height is smaller than the dorsal height of the cotyle (for this character, see Čerňanský et al. 2019). In lateral view, the synapophyses are broad, more or less kidney-shaped, and laterally directed. Some specimens display small tubercles located medial to each synapophysis. In ventral view, the centrum is wide but gradually narrows posteriorly - the subcentral ridges are straight and gradually converge posteriorly. The ventral surface is flat. A faint ridge runs anteromedially in some specimens (e.g., GIN 1144/234). Both condyle and cotyle are markedly depressed. The condyle is separated from the body of the centrum by a distinct narrowing, although note that a distinct precondylar constriction (as seen in varanids; e.g., Čerňanský et al. 2022: fig. 5) is absent (Fig. 12B ).

Remarks. The two features indubitably allow the allocation of these vertebrae to Pseudopus (see Čerňanský et al. 2019): (1) the straight course of the lateral margins of the centrum convergent posteriorly; and (2) the dorsoventral height of the cotyle is higher than the height of the neural canal.

These dorsal vertebrae are robust and large, having short and compressed appearance in comparison to the more elongated vertebrae of P. apodus .

Caudal vertebra. The caudal vertebrae are rather poorly preserved. These are medium-sized and rather narrow and anteroposteriorly elongated (Fig. 13 ).

They are procelous. In dorsal view, the vertebrae are constricted at mid-length. Prezygapophyses slightly surpass the anterior margin of the cotyle. The articular facets of the prezygapophyses are oval, laterally expanded, and dorsomedially inclined. There is a faint ridge deriving from the posterolateral corner of the prezygapophyseal facet. These ridges meet medially. In some specimens from Volchaya Balka (e.g., GIN 1143/601; GIN 1143/604; GIN 1143/609), a small apophysis (i.e., the dorsal paraseptal apophysis; Hoffstetter and Gasc 1969, 271) sits atop the level where these ridges meet (Fig. 13P ). The articular facets of the postzygapophyses are oval, laterally expanded, and ventrolaterally inclined. The neural spine on these vertebrae is only ever partly preserved. It is slender and pointed. The cross-section of the neural spine is sub-circular. The neural canal is oval, clearly smaller than the cotyle. The subcentral ridges are more-or-less straight. The centrum possesses pleurapophyses. However, only their bases are preserved. These are broad and laterally directed, and gradually narrow distally. The prezygapophyses and pleurapophyses are connected by a vertical wall. The posteromedial margins of the pleurapophyses do not overlap the anterior margins of the postzygapophyses. Some vertebrae (e.g., GIN 1143/604) possess an autotomy foramen at the base of each pleurapophysis, but no transverse autotomic split is present in any of these caudal vertebrae. The centrum is also pierced by several foramina. Only the bases of the haemapophyses are partly preserved on some vertebrae (Fig. 13B, G, L, Q ). These bases are posteroventrally oriented. The cotyle and condyle are both depressed. The condyle is slightly smaller than the cotyle.

Remarks. Caudal vertebrae are usually difficult to confidently identify between anguine lizards and are commonly only attributed to indeterminate anguines (e.g., Čerňanský et al. 2017; Georgalis et al. 2017, 2018), even though more precise affinities can sometimes be supposed (e.g., Georgalis et al. 2018, 2019a). This becomes even more difficult in the case of large anguid caudal vertebrae as these are strongly similar to those of varanids ( Estes 1983; Georgalis et al. 2018). As summarized by Georgalis et al. (2018), some features can nonetheless be used to distinguish caudal vertebrae between large anguids and varanids. Essentially, haemapophyses in the former clade are directly fused to the centrum whereas these are sitting on articulation facets in the latter clade (i.e., pedicles in Georgalis et al. 2018). Additionally, as pointed out by these authors as well, the neural spine also proves useful in distinguishing between anguid and varanid caudal vertebrae, although the distinction is a more delicate matter for the anteriormost caudal vertebrae of anguids which are closer to the morphology exhibited in varanids than the more posterior vertebrae ( Georgalis et al. 2018). Among anguids, the neural spine is tube-like and posteriorly inclined whereas, among varanids, the neural spine is rather laterally-compressed and sub-vertical to wholly vertical ( Georgalis et al. 2018). In the case of the material presented here, the structures present ventrally in the posterior portion of the centrum are unlikely to be facets for articulation and are, on the contrary, fused directly to the centrum. Moreover, although incomplete in most specimens, the neural spine of the vertebrae of our material are tube-like and rather posteriorly inclined. Therefore, potential varanid affinities among our material can be discarded. Then, as stated previously, identifications of caudal vertebrae among anguines are difficult and tend to be only referred to as indeterminate anguines, thus rendering our attribution less conservative than usual in comparison. A similar suggestion, although more conservative, was offered by Georgalis et al. (2018) for a limited amount of material (i.e., specimen RP1 299: a single caudal vertebra, likely among the most anterior caudals based on the morphology of its neural spine) from the Late Miocene (MN10) of Greece. These authors referred this specimen to an indeterminate anguine, discarding (although not completely) its possible varanid affinities following a reasoning similar to ours. These authors stated that this specimen could tentatively be attributed to Pseudopus pannonicus due to its large size and geographic and stratigraphic positions. Acknowledging that this matter is more sensitive in the case of such limited material, the argumentations of these authors as well as the replacing of that specimen inside a spatiotemporal rationale appear rather sensible. Thus this specimen could warrant a more precise attribution to P. pannonicus , hence documenting additional occurrences of that species in the European Neogene. Here, we attribute the caudal vertebrae from our material to the genus Pseudopus because none are showing an autotomic split, a feature that is present and well-visible in either Anguis or Ophisaurus ( Hoffstetter and Gasc 1969; Čerňanský et al. 2019). This condition is, however, currently unknown in either Ragesaurus and Smithosaurus based on the material available ( Bailon and Augé 2012; Vasilyan et al. 2022). We refer the vertebrae described here to the species P. pannonicus due to the fact that this species is the sole (with the exception of the recently described and rare occurrences of Ophisaurus from Lucheshty and Etulia; Syromyatnikova et al. 2022) and most abundant representant of anguines in the localities studied here as evidenced by the additional cranial material described here.

Interestingly and as mentioned in the description above, although some vertebrae are bearing autotomy foramina, none are displaying an autotomic split. To add to these observations, it can be noted that neither “half-vertebra” that could be attributed to autotomized Pseudopus elements, nor halves separated post-mortem were found in our material.

Rib. In the available material, a single rib is present (Fig. 14A-C ). It is medium-sized. It is lightly bent ventrally as well as lightly compressed anteroposteriorly. The head of the rib is only partly preserved. It displays a more-or-less kidney-shaped articular facet. The anterior process of the proximal end of the rib is preserved whereas its posterior process is not. The distal end of the rib is also lightly damaged. Dorsally, the rib bears a distinct ridge.

Osteoderms. Among the herein studied material, osteoderms represent the most common element. Indeed, a total of 133 single osteoderms are recorded. These elements are most abundant in the Kalfa locality and rarest from Lucheshty. In general, some osteoderms are more rectangular and slightly more elongated (e.g., Fig. 14F, R ) whereas other osteoderms are more trapezoidal, shorter, and stouter (e.g., Fig. 14D, I ). However, all these elements share the following features: rather large, flat, thick, and rather robust. The external surface of the osteoderms can be divided into two distinct areas: a short and smooth gliding surface in the anterior portion and a large, ornamented posterior portion. This ornamentation consists of tubercles mainly in the central part of the ornamented surface and a network of grooves and ridges around the periphery (Fig. 14 ). Occasionally, pits are found inside these grooves. Some osteoderms have a low, elongate ridge running almost along their middle (e.g., Fig. 14P ). In addition, some specimens possess a lateral narrow bevel (i.e., longitudinal crest in Vasile et al. 2021). This feature is, however, certainly most variable as this bevel can be seen on either side of the osteoderms, sometimes both, or rarely on neither side. These different beveling conditions are likely to reflect different placements on the body and can be indicative of left-sided or right-sided osteoderms (e.g., Gauthier 1982; Vasile et al. 2021). In ventral view, osteoderms are rather smooth. Some specimens show light grooves and/or are pierced by foramina (e.g., Fig. 14G, J ). In lateral view, these osteoderms are to be rather thick (e.g., Fig. 14H, K ).

Remarks. Osteoderms are generally difficult to identify precisely and rarely useful for alpha-taxonomic determinations. Although the morphology of anguid osteoderms tends to be rather generic (i.e., vermicular ornamentation of the external surface; Gauthier 1982; Georgalis et al. 2019b), some possibilities of differentiation between the various anguine genera exist and have been heavily used for fossil osteoderms. For instance, it has been very common to distinguish between osteoderms of Anguis , Pseudopus and Ophisaurus . Indeed, osteoderms of the former are rather small and thin, not rectangular, and devoid of a longitudinal keel ( Tesakov et al. 2017; Syromyatnikova et al. 2019; Loréal et al. 2020), whereas osteoderms of the latter two genera are rectangular, larger and both keeled ( Čerňanský and Klembara 2017). However, even the keeled feature is not entirely reliable for isolated osteoderms because it is known that some osteoderms of Pseudopus can be devoid of this keel, as shown by several authors ( Schmidt 1914; Spinner et al. 2015; Vasile et al. 2021). Initially described by Schmidt (1914), the presence of a longitudinal keel in specimens of Pseudopus apodus is indeed variable at the individual scale, following a gradient of expression both anteroposteriorly and dorsoventrally across the body. In addition, as pointed out more recently by Vasile et al. (2021), in Pseudopus , osteoderms of the dorsomedial region display a medial ridge whereas those of the laterodorsal and lateroventral regions do not. Such results were reaffirmed recently by Spinner et al. (2015). Therefore, the variability described on the presence or absence of a medial ridge is likely reflecting different body topology rather than reflecting actual taxonomical differences. As for the range of variability in the bearing of a lateral bevel, as mentioned above, it can be explained by the positions on the body on which each osteoderm was originally located rather than by taxonomical differences.

Figures

Figure 2. Pseudopus pannonicus: virtually segmented model of a right maxilla (ZIN PH 1 / 282) in medial (A), lateral (B), dorsal (C), posterodorsomedial (D), and ventral (E) views.

Figure 3. Pseudopus pannonicus: right maxilla (ZIN PH 8 / 277) in medial (A), lateral (B), and dorsal (C) views; left maxilla (ZIN PH 6 / 277) in medial (D), lateral (E), and dorsal (F) views. Scale bars: 2 mm.

Figure 4. Pseudopus pannonicus: lacrimal (ZIN PH 2 / 280) in lateral (A), medial (B), and ventral (C) views. Scale bar: 2 mm.

Figure 5. Pseudopus pannonicus: photographs of a left frontal (ZIN PH 2 / 278) in dorsal (A) and ventral (B) views; virtually segmented model of a left frontal (ZIN PH 2 / 282) in dorsal (C), ventral (D), and lateral (E) views. Scale bars: 2 mm.

Figure 6. Pseudopus pannonicus: virtually segmented models of the parietal ZIN PH 17 / 277 in dorsal (A), ventral (B), and lateral left (C) views; parietal ZIN PH 18 / 277 in dorsal (D), ventral, and lateral left (F) views; parietal ZIN PH 6 / 279 in dorsal (G), ventral (H), and lateral left (I) views. Virtual slices of the parietal ZIN PH 17 / 277: axial section at the mid-level of the dorsoventral thickness of the parietal table (J), at the ventral level inside of the parietal table (K) and coronal section at the level of the parietal foramen (L); ZIN PH 18 / 277: axial section at the mid-level of the parietal table (M) and coronal section (N); ZIN PH 6 / 279: axial section at the mid-level of the parietal table (O) and coronal section (P). Extant Pseudopus apodus DE 52, part of the dorsal skull roof: axial section at the mid-level of the dorsoventral thickness of the parietal table (R), at the ventral level inside of the parietal table (S) and coronal section (T).

Figure 7. Pseudopus pannonicus: photographs of the parietal ZIN PH 3 / 279 in dorsal (A) and ventral (B) views; parietal ZIN PH 6 / 278 in dorsal (C) and ventral (D) views; parietal ZIN PH 4 / 282 in dorsal (E) and ventral (F) views; parietal GIN 1144 / 232 in dorsal (G) and ventral (H) views. Scale bars: 2 mm.

Figure 8. Pseudopus pannonicus: virtually segmented model of the posterior portion of a braincase GIN 1143 / 605 in dorsal (A), ventral (B), anterior (C), posterior (D), anterolateral (E), posteroventrolateral (F), lateral left (G), and lateral right (H) views.

Figure 9. Pseudopus pannonicus: virtually segmented model of the right dentary ZIN PH 3 / 277 in lateral (A), medial (B), ventromedial (C), and dorsal (D) views.

Figure 10. Pseudopus pannonicus: virtually segmented model of the left dentary ZIN PH 1 / 280 (juvenile individual) in lateral (A), medial (B), ventromedial (C), and dorsal (D) views.

Figure 11. Pseudopus pannonicus: left dentary (ZIN PH 1 / 277) in medial (A) and lateral (B) views; left dentary (ZIN PH 2 / 277) in medial (C) and lateral (D) views; left dentary (ZIN PH 5 / 277) in medial (E) and lateral (F) views. Scale bars: 2 mm.

Figure 12. Pseudopus pannonicus: trunk vertebra (ZIN PH 7 / 281) in dorsal (A), ventral (B), anterior (C), posterior (D), and right lateral (E) views; trunk vertebra (GIN 1143 / 602) in dorsal (F), ventral (G), anterior (H), posterior (I), and right lateral (J) views; trunk vertebra (GIN 1144 / 234) in dorsal (K), ventral (L), anterior (M), posterior (N), and right lateral (O) views; trunk vertebra (GIN 1143 / 606) in dorsal (P), ventral (Q), anterior (R), posterior (S), and right lateral (T) views; trunk vertebra (GIN 1143 / 607) in dorsal (U), ventral (V), anterior (W), posterior (X), and right lateral (Y) views. Scale bars: 2 mm.

Figure 13. Pseudopus pannonicus: photographs of caudal vertebra (ZIN PH 10 / 281) in dorsal (A), ventral (B), anterior (C), posterior (D), and right lateral (E) views; caudal vertebra (ZIN PH 11 / 281) in dorsal (F), ventral (G), anterior (H), posterior (I), and right lateral (J) views; caudal vertebra (GIN 1143 / 609) in dorsal (K), ventral (L), anterior (M), posterior (N), and right lateral (O) views; virtually segmented model of the caudal vertebra GIN 1143 / 604 in dorsal (P), ventral (Q), anterior (R), posterior (S), and right lateral (T) views. Scale bars: 2 mm.

Figure 14. Pseudopus pannonicus: rib (ZIN PH 16 / 277) in anterior (A), posterior (B), and proximal (C) views; photographs of osteoderm from Lucheshty (ZIN PH 3 / 278) in external (D) and internal (E) views; virtually segmented model of osteoderms from Kalfa, specimen ZIN PH 9 / 277 in external (F), internal (G), and lateral (H) views, specimen ZIN PH 10 / 277 in external (I), internal (J), and lateral (K) views; specimen ZIN PH 11 / 277 in external (L), internal (M), and lateral (N) views; specimen ZIN PH 19 / 277 in external (O), internal (P), and lateral (Q) views; specimen ZIN PH 20 / 277 in external (R), internal (S), and lateral (T) views; specimen ZIN PH 21 / 277 in external (U), internal (V), and lateral (W) views; photographs of osteoderms from Petroverovka, specimen ZIN PH 1 / 281 in external (X) and internal (Y) views; specimen ZIN PH 2 / 281 in external (Z) and internal (AA) views; specimen ZIN PH 3 / 281 in external (AB) and internal (AC) views. Scale bars: 2 mm.

Figure 15. The lectotype, MAFI V 2023.1.14.1. (former Ob. 5058) of Pseudopus pannonicus: left dentary from the Late Miocene of Polgardi 2, Hungary, in medial (A) and lateral (B) views.