Scincoidea, Oppel, 1811

Augé, Marc Louis, Dion, Michaël & Phélizon, Alain, 2021, The lizard (Reptilia, Squamata) assemblage from the Paleocene of Montchenot (Paris Basin, MP 6), Geodiversitas 43 (17), pp. 645-661 : 647-649

Scincoidea indet.

MATERIAL EXAMINED. — Two incomplete dentaries: Dentary, MNHN. F.MTC245 ( Fig. 2 ), bears four tooth loci (the most posterior teeth); Dentary, MNHN. F.MTC244 ( Fig. 3 ), bears four tooth loci.

DESCRIPTION

Dentaries, MNHN.F.MTC245 ( Fig. 2 )

This robust, right dentary (L = 3.7mm) is strongly arched (concave), its posterior part is deep and it distinctly narrows towards the anterior margin. This specimen preserves three incomplete teeth. On the lingual side the subdental shelf (sensu Rage & Augé 2010) is eroded except in the posterior part of the dentary, where it is sub-vertical and meets the dental table nearly at a right angle. It is difficult to assess the presence of a medial sulcus dentalis along the tooth bases, due to the poor preservation of the subdental shelf. A large posterodorsal process extends behind the level of the last tooth and presents a deep, inclined surface which certainly received the anteromedial dentary process of the coronoide. On that surface, a circular cavity located behind the level of the last tooth may mark the position of the anterior end of this process. Posteriorly, the sulcus Meckeli is widely open medially and it gradually narrows towards the anterior end of the specimen. Under the dental table, there is an intramandibular septum, between the level of the last-penultimate tooth positions. It shows no vertical projection.

In lateral view, no labial foramens are exposed. Posteriorly, there is a well-developed labial coronoid process, which bears no trace of contact with an anterolabial dentary process of the coronoid.

Dentition: The three incompletely preserved teeth are pleurodont and robust (amblyodont). They gradually taper towards the apex, the tooth base is enlarged and medially expanded. Two tooth bases are pierced by a large, central replacement cavity and cement deposits around the tooth bases are poorly developed. Apices are broken in all teeth, except perhaps for the last one which seems to bear a rounded crown.

Specimen MNHN. F.MTC244 ( Fig. 3 )

Incomplete left dentary (L = 5.1mm) that bears five tooth positions, and two teeth partially preserved. The morphology of dentary MNHN.F.MTC244 is nearly identical to that of

specimen MTC245 (described above). The subdental shelf is arched, vertical and moderately deep and it meets the dental table nearly at a right angle.A sulcus dentalis is present.The teeth are better preserved than those of specimen MTC245 and their apex is obtusely pointed and bears neither cusps nor striations.

The labial surface of the two dentaries bears important marks of alteration, certainly caused by digestive process ( Andrews 1990). It is worth noting that these processes could be at least partially behind the lack of striations on the teeth (see above).

COMPARISONS AND DISCUSSION

On account of the fragmentary nature of these specimens their taxonomic position is difficult to evaluate. In addition, no clear diagnostic features or synapomorphies are present in those dentaries. However, there are some features which, in combination may suggest scincoid relationships:pleurodont dentition, subdental shelf well developed, sub-vertical and forming a nearly right angle with the surface of the dental table, presence of a sulcus dentalis.At this point, it may be objected that those characters are also present in Lacertoidea (and particularly in lacertid lizards). However the first dentary (specimen MNHN.F.MTC245) differs significantly from that of most fossil and extant Lacertoidea in two characters: 1) in lingual view, the presence of a large posterodorsal extension behind the subdental shelf that received the anteromedial dentary process of the coronoid; and 2) in labial view, the dentary has an upwardly directed coronoid process that prevents the extension of the coronoid onto the labial surface of the dentary ( Daza et al. 2015). In lacertid and teiid lizards this process is reduced and the anterolateral dentary process of the coronoid is expanded anteriorly and covers the posterodorsal part of the dentary, generally leaving a clear mark on it ( Augé 2005).

Presence of a mesial and distal crest or of a single longitudinal crest on the tooth crown are certainly primitive within Scincoidea ( Kosma 2004; Richter 1994, 1995; Georgalis et al. 2017). Accessory cusps on the mesial and distal crests are virtually unknown in Scincoidea and skinks tend to have blunt, chisel-shaped tooth crowns ( Smith & Gauthier 2013; Daza et al. 2015) while most lacertids have acute cusps; absence of cusps in the dentaries from Montchenot tend to exclude them from lacertoid lizards.

Absence of striae on the tooth crowns and obtusely pointed teeth without longitudinal crest (in dentary MNHN.F.MTC244) are clearly characters that come into conflict with the generalized scincoid dentition ( Richter 1994; Böhme 2010) or scincid grade sensu Kosma (2004:5). Once again, absence of striae may be du to the poor preservation of these teeth. However, a great number of forms, notably among scincids, depart from this tooth pattern and bear specialized dentitions ( Estes & Williams 1984; Kosma 2004; Augé 2005; Nydam et al. 2013; Bolet & Augé 2014).In addition, this scincid-grade of tooth morphology is also present in some non-scincid taxa, such as some species of Gymnophthalmidae and Gekkonidae ( Nydam et al. 2013; Sumida & Murphy 1987). In addition, the lack of striae may be due to taphonomic pocesses (e.g. digestive process) that affected the enamel.

Recently, several scincoid taxa have been described in the European Paleogene, some forms being referred to scincids (e.g. Bolet et al. 2015; Augé 2005) others to cordylids s.l. ( Bolet & Evans 2013) or to Scincoidea ( Weber 2004; Folie et al. 2005; Bolet & Augé 2014; Augé & Smith 2009; Čerňanský et al. 2016). Most generally, these authors recognized that the distinction between the two families ( Scincidae and Cordylidae ) is difficult. The tooth shape and the arched dentary suggest that the fossils from Montchenot belong more likely to scincids than to cordylids. Unfortunately, these fossils are too incomplete and thus unsuitable for a more specific assignment.

As a last point, some characters of these dentaries, in combination, may suggest amphisbaenian relationships (e.g. well-developed coronoid process, robust teeth and a probable low tooth count). However, their general shape, in particular their strongly arched (concave) ventral border and subdental shelf clearly exclude this possibility. This morphology sharply contrast with the straight ventral border of the dentary that is shown by nearly all amphisbaenian members (see Gans & Montero 2008). Gans (1974) clearly demonstrates that this morphology is tied to the fossorial habits of these lizards.

Figures

FIG. 2. — Indeterminate Scincoidea, incomplete right dentary, MNHN.F.MTC245: A, lingual view; B, labial view. Scale bar: 1 mm.

FIG. 3. — Indeterminate Scincoidea, incomplete left dentary, MNHN.F.MTC244 A, labial view; B, lingual view. Scale bar: 1 mm.