Lacertidae, Bonaparte, 1831

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 : 650-653

? Lacertidae indet.

MATERIAL EXAMINED. — MNHN.F.MTC246, one fragmentary jaw, possibly a dentary ( Fig. 5 ); MNHN.F.MTC239 distal end of a left humerus ( Fig. 6 ).

DESCRIPTION

Fragmentary jaw (L = 2.9 mm) with eight tooth positions that preserve two complete, pleurodont teeth. Tooth shaft sub-cylindrical, nearly columnar, with an incipient medial bulging. The shaft is slightly constricted under the apex which bears two (anterior and posterior) small cutting edges which join up on the tip of the apex. Otherwise, the apex is blunt and bears no cusps. The tooth base is slightly enlarged and is inserted into cementum as well as the labial side of the tooth shaft. Tooth bases are implanted on an horizontal dental table. A central replacement pit pierces the tooth base whose medial side comes near the dorsal margin of the subdental shelf (or supradental shelf) so that there is a reduced sulcus dentalis. The subdental shelf is shallow, rounded and there is no marked angle between it and the dental table.

DISCUSSION AND COMPARISONS

Affinities of this specimen are particularly elusive due to its poor preservation and, to our knowledge, the very unusual morphology of the tooth apex bears no resemblance to those seen in any extant lizard families, except in some lacertid lizards ( Kosma 2004). This tooth morphology is certainly not the result of wearing or preservational artifacts as several well-preserved jaws from the coeval locality of Cernay show the same tooth morphology (see below).

Several characters suggest lacertid affinities: teeth strictly pleurodont, horizontal dental table, posteriorly reduced sulcus dentalis, central replacement pits on tooth bases.However,some of those characters are common to several lizards families, like Iguanidae , Scincidae or Cordylidae and neither is regarded as a possible apomorphy.

Presence of cutting edges on the lateral sides of the rounded apex may suggest anguimorphan (more precisely anguid) affinities, as well as the reduction of the sulcus dentalis. However, the pleurodont implantation of the teeth represents an important conflicting character (anguid lizards have a subpleurodont implantation, sensu Hoffstetter 1954). Some features suggest the possibility of referring it to the family Iguanidae (non-acrodont Iguanian, sensu Estes et al. 1988): iguanid lizards have pleurodont teeth and a reduced sulcus dentalis, mostly in the posterior part of the dental row.However, no iguanid is known to have a tooth apex reminiscent of those of MNHN.F.MTC246.

The localities of Cernay yielded many dentaries that share the same typical dentition with dentary MNHN.F.MTC246 from Montchenot (collection Phélizon, deposited in the MNHN). Cernay is coeval with Montchenot ( Paleocene , MP6) and the two localities are in close geographical proximity. These dentaries have not been described yet but they show clear affinities with lacertid lizards: teeth pleurodont, sulcus dentalis reduced posteriorly, absence of a posterodorsal extension of the subdental shelf in lingual view, and reduction of the posterodorsal process of the dentary labially that bears a shallow depression which indicates the contact with the anterior (dentary) process of the coronoid. Taken at face value, this posterior reduction of the dentary is characteristic of lacertoid lizards ( Teiidae and Lacertidae ).In contrast, the posterior part of the dentary of iguanid lizards (mainly the posterodorsal pocess) extends widely onto the coronoid.Otherwise, these dentaries lack teiid features (e.g. clearly heterodont dentition, presence of important cement deposit on tooth base) and the tooth morphology of MTC246 (mainly its apex) is reminiscent of those of a Miocene Lacertidae from Sansan, MN6 ( Edlartetia sansaniensis Augé & Rage, 2000 ) and even of extant lacertids like Lacerta agilis grusinica Peters, 1960 ( Kosma 2004: fig. 34) and Zootoca vivipara (Lichtenstein, 1823) ( Kosma 2004:fig.51). This attribution is only tentative due to the poor preservation of the teeth and depends on the description of the material of Cernay.

Humerus ( Fig. 6 )

As the humerus of all full leged lizards, this specimen has an elongated, sub-cylindrical shaft (diaphysis) that forms the middle part of the bone. In lizards, the proximal and distal ends are expanded but the proximal end of specimen MNHN.F.MTC239 is broken off. Ventrally, the distal end presents two obliquely trending radial (lateral, or capitellum sensu Romer 1956) and ulnar (mesial) condyles separated by a notch (condylar gutter). A deep radioulnar fossa (sensu Tschopp et al 2018) is located proximal to the ulnar condyle.

Mesially, the entepicondyle is moderatelly developed into a knob-like expansion. On the lateral side of the distal extremity, the ectepicondyle is weakly developed, as in most limbed lizards

except varanoids that have a marked crest on the distal end of the humerus. The ectepicondylar foramen is certainly visible in ventral view but several cavities due to weathering are also present on the lateral side of the ectepicondyle.

A large triangular depression is present just above the condyles and a rather large foramen (here called supra-condylar foramen) opens on the proximal rim of this depression.This foramen and its associated depression are clearly observed in almost all limbed squamates except varanoids ( Smith2009). It conveys the radial and ulnar nerves branching from the brachial plexus ( Lécuru 1968).

What is clear from this description is that the incomplete humerus from Montchenot presents a generalised morphology observed in most limbed lizards, except varanoids( Lécuru 1969; Smith 2009).

Entepycondyles of gekkonid lizards are far more developed (projected mesially) than that of humerus MNHN.F.MTC239 (e.g., Lécuru 1969: fig. 12). Generally, in gekkonids and Scincoidea the supra-condylar foramen opens just above the condyles, while in Lacertoidea , Iguania and Anguimorpha it opens well above the condyles. Hence, humerus MTC239 could belong to a member of those taxa (i.e. Lacertoidea , limbed Anguimorpha different from Varanidae or Iguanidae ). The presence of a deep radioulnar fossa and of the large obliquely trending radial condyle suggest the possibility of referring this humerus to the family Lacertidae . Furthermore, observation of the humerus of the extant genus Timon ( Lacertidae , Lacertinae, Tschopp et al. 2018 : fig. 20) shows a number of interesting resemblances with the humerus of Montchenot. However, it must be stressed that these observations are limited to only a few specimens and species in each family which casts doubts on their taxonomic value.

Modifications by digestion are evident on the surface of this fossil: in digestion, corrosion is often localized in articular ends, with the epiphyses severely pitted,while the diaphysis is less severely damaged as in the humerus from Montchenot (e.g. Andrews 1990; Fernandez-Jalvo & Andrews 1992; Denys et al. 1995).

Figures

FIG. 5. —?Lacertidae, fragmentary jaw, possibly dentary?, MNHN.F.MTC246: A, labial view; B, lingual view. Scale bar: 1 mm.

FIG. 6. —?Lacertidae, distal end of a left humerus, MNHN.F.MTC239, ventral view. Scale bar: 1 mm.