Anguidae, Gray, 1825

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 : 653-655

Anguidae indet.

MATERIAL EXAMINED. — MNHN.F.MTC241 ( Fig. 8 ). The incomplete left dentary represents the middle and posterior parts of the bone and its ventral margin is broken.

DESCRIPTION

This small dentary (L: 2.9 mm) carries eight rather widely spaced tooth positions, but only one tooth is preserved. In medial view, the subdental shelf and the dental row are slightly concave. The subdental shelf forms a narrow, rounded surface overhanging the sulcus Meckeli. This fossa is rather narrow and faces mostly ventrally, mainly towards the anterior end of the bone. The dentition is subpleurodont (sensu Camp 1923; Hoffstetter 1954), that is the teeth tend to be attached to a single plane and the subdental table, as defined byRage & Augé (2010) becomes an inclined surface. Hence tooth bases are attached to a moderately inclined or concave surface. Moreover, this type of implantation is often associated with a poor development or absence of the sulcus dentalis and subdental shelf; effectively the dentary from Montchenot has no sulcus dentalis and a poorly developed, rounded subdental shelf.

The lateral surface is gently convex, smooth and preserves only two labial (mental) foramina. The base of the single preserved tooth is not covered by cementum, except for two small anterior and posterior deposits. It is somewhat expanded, so that the mesial side of the tooth base comes near the dorsal margin of the subdental shelf. The tooth shaft slightly bends posteriorly and it steadily tapers dorsally towards the crown. The apex is rather rounded, with an incipient central bulb and two slight lateral shoulders. The tooth base is excavated by a central replacement pit. The tooth projects about onehalf of its height above the parapet of the dentary. There are no striae on the apex of the tooth but they could have been obliterated by weathering.

COMPARISONS AND DISCUSSION

Dentary MNHN.F.MTC241 is referred to an anguimorph lizard because it shows the following combination of characters: 1) Sulcus Meckeli faces ventrally in anterior portion of dentary

( Estes et al. 1988); 2) narrow and rounded subdental shelf; 3) teeth subpleurodont; 4) absence of sulcus dentalis; 5) crescentis or boat shaped tooth row ( Estes 1964); and 6) teeth unicuspid, without lateral cups, not closely spaced ( Camp 1923).

Some of these characters are regarded as anguimorphan synapomorphies (e.g. character 1, Estes et al. 1988), however several of them are not clear synapomorphies as they are subject to important variability among anguimorph taxa and often they are not unique to anguimorphs ( Evans 2008). For example, the narrowness and reduction of the subdental shelf are very different among anguimorph lizards: from a well developed subdental shelf in Xenosaurus to a near absence in Varanus (see other examples in Bochaton et al. 2016).

Conflicting characters that are often cited as anguimorphan synapomorphies are clearly absent in the dentary from Montchenot: tooth apex pointed and tooth replacement alternate or distally displaced replacement pit on tooth bases ( McDowell & Bogert (1954). Pointed, canine like teeth indicative of predaceous habits are common in anguimorph lizards but rounded tooth apex occasionally provided with a cutting edge ( Estes 1964) has sometimes been regarded as an anguid synapomorphy ( Estes et al 1988). Alternate tooth replacement is a clear feature of some extant and fossil anguimorph taxa, e.g. Varanus , Lanthanotus , Heloderma , Saniwa , Palaeovaranus (Ex Necrosaurus ). Otherwise, tooth replacement shows important intra-individual and intraspecific variability among anguid taxa. In some anguids the variability appears along the tooth row: in the genera Gerrhonotus and Diploglossus , the first tooth bases have a central replacement pit while the others bear a distally displaced replacement pit. The same variability is also evident in the genus Pseudopus ( Anguinae , see figures in Klembara 2012; Klembara et al. 2010, 2014). The presence of these two characters (rounded apex and tooth replacement) may be indicative of anguid affinities but the evidence is at best feeble and further comparisons with more complete material should be carried out.

Many purported terrestrial Anguimorpha have been described from the Mesozoic fossil record in Europe, Asia and North America (e.g. Hoffstetter 1967; Alifanov 2000; Evans 1994; Evans et al. 2006; Fernandez et al. 2015). In particular, the Cretaceous record includes fossils that bear some resemblances to extant terrestrial anguimorphan families (e.g.? Xenosauridae, Pérez-García et al 2015; Anguidae, Blain et al. 2010 ; Helodermatidae, Nydam 2000, 2013 ; Platynotan, Norell et al. 2007; Mo et al. 2012; Varanoidea, Houssaye et al. 2013).

In contrast, fossils attributed to anguimorphan lizards in the early European Paleocene are far more scanty. Lizard extinctions across the K/T boundary may explain this poor record ( Longrich et al. 2012) but it may also reflect the paucity of fossil lizards described so far in the European Paleocene (table 1). Among them, the incomplete dentary referred to Anguimorpha indet. found in the late Paleocene of Rivecourt ( Smith et al. 2014, fig. 7) bears some resemblances with the dentary from Montchenot: teeth subpleurodont, rounded subdental shelf, teeth apparently with a central replacement pit but all these features are plesiomorphic within Anguimorpha . Another possibility could be an attribution to another anguimorphan lizard, a pan-shinisaur whose osteoderms are present in the locality. However, except for its anguimorphan relationships, nothing in the morphology of this dentary (particularly its size) is consistent with such an attribution.

Figures

FIG. 8. — Anguimorpha,?Anguidae, incomplete left dentary, MNHN.F.MTC241: A, lingual view; B, labial view. Scale bar: 1 mm.