Anguidae

Anguidae View in CoL View at ENA ( Fig. 3C–F, M, N View Figure 3 )

European anguids have a paired frontal with a posterior end that is slightly less than twice as large as the anterior end. Usually, the constriction in the middle is not present and the lateral margin is straight or convex in dorsal view. Only one small specimen of Anguis gr. An. fragilis, MDHC 49, has a poorly developed hint of constriction and a roughly sigmoid-shaped lateral margin. This could be due to ontogenetic variation, because this specimen is much smaller than the others and could, therefore, be a juvenile. A similar ontogenetic change is present in Pseudopus apodus , according to Klembara et al. (2017). The anterior end of the frontal is pointed, because the medial process is well developed and moderately wide, whereas there is no lateral process. The articulation surface with the posterior process of the nasal is barely recognizable in Anguis gr. An. fragilis ( Fig. 3C View Figure 3 ), but more clearly visible in Pseudopus apodus ( Fig. 3E View Figure 3 ). Both the articulation surfaces with the prefrontal and the postfrontal are visible: the latter develops along the posterior-third ( Anguis gr. An. fragilis ; Fig. 3M View Figure 3 ) or fifth ( Pseudopus apodus ; Fig. 3N View Figure 3 ) of the lateral margin, whereas the former covers the rest of it. In Anguis gr. An. fragilis , they usually contact each other, but in MDHC 49 they are distinctly spaced. Pseudopus apodus has a variable condition, with a contact that can be either present or absent, even though the articulation surfaces are close to each other, also when they do not touch each other. The posterolateral process is moderately short and pointed. It develops a small (poorly developed in Pseudopus apodus ) and posteriorly directed ventral lappet that underlies the anterolateral corner of the parietal ( Fig. 3D, F View Figure 3 ). The posterior margin is roughly straight, but slightly irregular. In ventral view, a small articulation surface for the ventral lappet of the parietal is recognizable medially to the lappet of the posterolateral process ( Fig. 3D, F View Figure 3 ). The crista cranii is laminar and well developed in ventral direction. An anterior process is not clearly distinguishable from the rest of the crista ( Fig. 3M, N View Figure 3 ). A moderately low ridge starts from the anterior end of the crista and runs towards the tip of the medial process on the ventral surface of the frontal, which is otherwise smooth ( Fig. 3D, F View Figure 3 ). The dorsal surface is covered by a well-developed dermal ornamentation, which is less marked in smaller specimens and, in Anguis gr. An. fragilis , also near the posterior end of larger ones ( Fig. 3C View Figure 3 ). The ornamentation reaches the lateral margin of the bone in adults of Pseudopus apodus ( Fig. 3E View Figure 3 ), but not in Anguis gr. An. fragilis ( Fig. 3C View Figure 3 ). The frontal is almost entirely covered by the frontal shield, except for its posterior end. In both Anguis gr. An. fragilis and Pseudopus apodus , the small frontoparietal shield can be recognized by the posterolateral-half of the latter end, whereas in Anguis gr. An. fragilis a small interfrontal shield contacting the frontal one is present on the medial-half ( Fig. 3C View Figure 3 ). This latter shield is absent in the frontals of all observed specimens of Pseudopus apodus , because the contact between the frontal and the interparietal scales is located on the frontoparietal suture ( Fig. 3E View Figure 3 ). Klembara et al. (2017) showed that sometimes a small interfrontal shield might also be present in Pseudopus . The maximum length of the frontal ranges from 4 mm to 6.8 mm in Anguis gr. An. fragilis and from 15.2 mm to 17 mm in Pseudopus apodus .

PARIETAL

Parietals ( Figs 5 View Figure 5 , 6 View Figure 6 ) are quadrangular bones that can be either paired (only in gekkotans; Villa et al., 2018a) or fused into a single element. They are composed of a straight parietal table that develops the anterolateral processes at the anterolateral corners and the supratemporal processes [postparietal processes in Evans (2008) and Villa et al. (2018a)] at the posterolateral corners. The former are usually anterolaterally directed, whereas the latter are posterolaterally directed and ventrally curved. The table can be pierced by the parietal foramen.