Mediolacerta sp.

Georgalis, Georgios L., Čerňanský, Andrej & Klembara, Jozef, 2021, Osteological atlas of new lizards from the Phosphorites du Quercy (France), based on historical, forgotten, fossil material, Geodiversitas 43 (9), pp. 219-293 : 240-241

Mediolacerta sp.

( Fig. 18 )

REFERRED SPECIMEN. — A left dentary (NHMW 2019/0050/0001) .

DESCRIPTION

The only available specimen, the left dentary NHMW 2019/0050/0001, is almost completely preserved, with only the half posterior ventral portion being broken off and missing ( Fig. 18 ). It is an anteroposteriorly long and massive element, with a slight medial curvature at its anterior end. The tooth row is completely preserved and the alveolar crest supports 23 tooth positions (17 teeth still attached). Meckel’s groove is fully open along its entire length, although it is narrow in the anterior region ( Fig. 18B ). The alveolar foramen is located at the level of the 6th tooth position (counted from posterior). The intramandibular septum is fused to the bone, being almost horizontal in this section. Meckel’s groove is roofed by a more or less straight subdental shelf (only its posterior portion is arched). The shelf is somewhat broad only in the anterior section, but it narrows posteriorly and thus is rather thin. This is mainly caused by the presence of the facet for the splenial, situated on its ventral margin. This facet reaches the level of the 8th tooth position (counted from anterior). Anteriorly, the shelf continues to the small rectangular symphysis. Posteriorly, the dentary protrudes into a short and low coronoid process, which bears a facet for the coronoid.

The otherwise smooth lateral surface of the bone is pierced by six labial foramina ( Fig. 18A ). They are arranged in a single row, located in the dorsal half of the dentary. The posteriormost foramen is located at the level of the 6th tooth position (counted from posterior). A facet for the coronoid is present on the dorsolateral surface of the bone.

Dentition

The dentition is pleurodont and heterodont ( Fig. 18 ). The tooth size gradually increases posteriorly (although it should be noted that the last posterior tooth is slightly smaller than the penultimate one). The teeth in the anterior section of the tooth row are small, slender, and somewhat pointed, with the tooth crowns slightly curved posterolingually. Posteriorly located teeth (from the 13th one if counted from the anterior) are markedly robust and blunt in comparison to those from

the anterior region. The tooth apices of several teeth are worn (or weathered), but those which are complete show bicuspidity, with an incipient small mesial cusp being present.

REMARKS

The dentary NHMW 2019/0050/0001 described herein represents the largest lacertid from this collection. The anterior teeth are pointed, whereas those located further posteriorly are robust and blunt, but with some of them bearing mesial cusp - this character fits to the diagnosis of Mediolacerta and its so far sole named species, Mediolacerta roceki , also from the Phosphorites du Quercy (stratigraphic occurrence MP23- MP30; see Augé 2005). As such, in respect of these features, NHMW 2019/0050/0001 can be differentiated from both Pseudeumeces and Dracaenosaurus . Besides the dentition, NHMW 2019/0050/0001 also shares several other features with Mediolacerta : 1) rather thin subdental shelf; 2) small rectangular symphysis; and 3) the position of the alveolar foramen at the level of the 6th tooth position, counted from posterior. Nevertheless, there appear also to be differences among NHMW 2019/0050/0001 and other known specimens of Mediolacerta . Most principally, the tooth number of the holo- type dentary (MNHN.F.PFR11006) of Mediolacerta roceki is 19, whereas this number is 23 in NHMW 2019/0050/0001. Dentaries of the same taxon have been also described from the late Oligocene of Germany ( Mediolacerta cf. roceki of Čerňanský et al. 2016a); the complete tooth row of the German material bears 18 tooth positions. However, it should be noted that tooth number in lacertids, like virtually most lizards, might be variable and also likely to be size related (the anteroposterior length of the dentary NHMW 2019/0050/0001 is 23.7 mm, thus this dentary is larger than the holotype of Mediolacerta roceki and the German material. These numbers should not be interpreted as absolutes. However, because the differences in the overall size and the dentary tooth count objectively exist, we decided to allocate NHMW 2019/0050/0001 only to the genus level, as Mediolacerta sp.

Figures

FIG. 18. — Mediolacerta sp. Photographs of left dentary NHMW 2019/0050/0001 in lateral (A), medial (B), and dorsal (C) views. Scale bar: 2 mm.