Gekkomimus rugosus ( Augé, 2005 ), Auge, 2005

Bolet, Arnau, Daza, Juan D., Augé, Marc & Bauer, Aaron M., 2015, New genus and species names for the Eocene lizard Cadurcogekko rugosus Augé, 2005, Zootaxa 3985 (2), pp. 265-274 : 268-271

Gekkomimus rugosus ( Augé, 2005)

( Figs. 3 , 4 B)

Cadurcogecko rugosus (in part; Augé, 2005, p. 98) (original description). Gekkomimus rugosus ( Augé, 2005) (new combination).

Holotype. MNHN, SND 622, almost complete left dentary.

Type locality. Sindou D (MP18-19), Phosphorites du Quercy, France.

Occurrence. Only known from the type locality, Sindou D (MP18-19).

Emended Diagnosis. Scincid lizard differing from the rest of Paleogene European members of the group (Berruva, Scincoideus, Orthoscincus, Ayalasaurus, Axonoscincus, Eocordyla and Ornatocephalus ; see Weber 2004 and Augé 2005) by its fused Meckelian groove (one character that determined its original placement in the Gekkota), although at the splenial facet the upper and lower borders remain separated ( Figs. 3 , 4 B). A fused Meckelian groove is also found in the Paleogene fossil and possible scincoid Pyrenasaurus evansae ( Fig. 4 C) from Escamps (MP19, France) and Sossís (MP17a, Catalonia), from which it differs in its much larger size, the simple morphology of the crowns (highly specialized in Pyrenasaurus) and a much higher tooth count (26 vs. 11 in Pyrenasaurus), as well as a different dentary morpology (see Bolet & Augé 2014). Another fossil with fused Meckelian groove is the Upper Eocene Paracontogenys estesis from the Mission Valley Formation in California ( Golz & Lillegraven 1977; Estes 1983), but in this form dental morphology is very distinct, having tooth crowns that are more chisel-shaped, with well defined lingual striations, cristae labialis and lingualis, and a deep antrum intercristatum. The dentary tooth count is higher than in Orthoscincus (14–15), Axonoscincus (20), Berruva (20), Eocordyla (19–22) and Scincoideus (20–21) from the European Paleogene ( Augé 2005; Folie et al. 2005). Only Ornatocephalus (c. 23) and Ayalasaurus (26–27), both differing from Gekkomimus in having an open Meckelian groove (with ventral and dorsal margins approaching each other in the case of Ayalasaurus), a larger size, and an arched ventral margin of the dentary, approach it in number of dentary tooth loci.

Among living scincids, the Meckelian groove is open anterior to the splenial in most of the scincines, including Feylinia , whereas the fused condition appears in acontine skinks (e.g., Acontias ), and some lygosomine skinks (e.g., Ablepharus , Trachylepis , Sphenomorphus , Eugongylus , Tiliqua , Leiolopisma ) ( Greer 1970; Rieppel 1981; Estes 1983; Evans 2008; Gauthier et al. 2012).

Description. The holotype, which is also the only known specimen, is almost complete and measures 10 mm in length. The dentary seems to have the characteristic strong posterior bifurcation of skinks ( Evans 2008), although the posterior breakage of the bone leaves some doubts regarding this features. Assuming that two processes were present, the lower one must have been longer than it appears. The dentary also has a large dorsolateral coronoid process. The dentary bone bears 26 tooth positions, all preserving more or less complete tooth crowns except for those of the anteriormost and posteriormost regions, which are apically corroded, so observations on the detailed morphology of the crowns are based on the penultimate tooth, which is the best preserved. The dentition is homodont, with high, columnar and simple teeth. No additional cusps are present (teeth are uniscuspid), but the lack of an evident cuspis labialis may be related to a poor preservation of the crowns. Interdental spaces are narrow (about half of the tooth width) with no contact between teeth (with a few exceptions where one of the teeth seems to have adopted an abnormal implantation). The anterior half of the dentary has a fused Meckelian groove (below tooth loci 4–12), remaining open for about three to four tooth positions (at the anterior part of the tooth row), and by the 15th tooth locus the ventral margin of the subdental shelf and the ventral margin of the dentary diverge from each other at an acute angle creating a widely open posterior half of the bone. A ventrally directed opening is present along the length of the first five tooth positions. The intramandibular septum is not observable, but the open posterior region of the Meckelian groove is obscured by the presence of infilling matrix. A deep sulcus dentalis is present, and the lingual surface of the subdental platform is high for its anterior half, but strongly reduced in height posteriorly. The ventral margin of the bone is only slightly bowed, and in this regard differs from many extant skinks with dentaries with strongly curved ventral margins (e.g., Acontias , Scincus , Feylinia , Nessia , Tropidophorus , Eumeces , Typhlosaurus ; Rieppel 1981; Conrad 2008; Evans 2008).

The lingual side of the dorsolateral coronoid process of the dentary has a small facet to receive the anterior process of the coronoid, but this facet is not broad and long as in members of the Gekkota. The labial surface of the dentary presents at least four labial foramina, situated at the level of the third, fifth, tenth, and twelfth tooth positions. There are no traces of bone facets in the posterior labial margin of the dentary, but the bone is broken at this position.

Remarks. Daza et al. (2014) noted great differences between the dentaries of C. piveteaui and C. rugosus regarding length, height, number of tooth loci, and neurovascular foramina, among other characters. Cadurcogekko piveteaui has a long and low dentary, bearing about 40–45 tooth positions, instead of 26 tooth loci in the shorter dentary of C. rugosus ( Augé 2005) . Moreover, it was stated that the presence of a partially unfused Meckelian groove, as reported by Augé (2005), only occurs among extant gekkotans through late postovopositional embryonic stages. Augé (2005) himself expressed his doubts regarding the attribution of the second species to Cadurcogekko : [En revanche, il paraît plus discutable de réunir ces deux formes dans un genre commun mais la similitude de leurs maxillaires (voir diagnose), qui les éloigne de tous les autres Gekkonidae dont l’ostéologie a été étudiée, montre que ces deux formes ont une ascendance commune, en un mot elles forment un taxon monophylétique] (pp. 99 in Augé 2005). It is clear now that some of the characters of the maxilla supported the presence of a second species of Cadurcogekko , but the unusual features of the (incorrectly) identified dentary was raising doubts about this referral.

The short and rather robust dentary, the low tooth-count, the simple but slightly heterodont (in size) dentition, the morphology and mode of closure and fusion of the Meckelian groove, and, in dorsal view, a straight instead of medially bowed dentary, are all characters against a referral to Gekkota. The presence of a well-developed sulcus dentalis, a non-elongated dentary, a large lateral coronoid process of the dentary, and a tall posterior region of dentary (in contrast to a subequal height between the anterior and posterior region in gekkotans), are all characters that fit well with scincoids. A high number of teeth (26) is not incompatible with referral to Scincoidea, as some scincids present much higher tooth-counts (e.g., 35 in Eutropis macularia , see Kosma 2004). Some gekkotans have dentaries with very low tooth counts (e.g., Aprasia ; Camp 1923; McDowell & Bogert 1954; Rieppel 1984; Evans 2008; Daza & Bauer 2015), but normally they have much higher dentary tooth counts than MNHN-SND 622.

There are no European fossil scincoids with a fused Meckelian groove except for the putative scincoid Pyrenasaurus (see above) and the Neogene scincids tentatively assigned to Mabuya ( Bailon 1991; Böhme & Ilg 2003) and Ablepharus ( Darevsky & Tshumakov 1962) . Note, however, that the genus Mabuya is now considered as restricted to the Americas (Mausfeld et al. 2002), and the only extant genera of scincids with a fused Meckelian canal recorded in Europe correspond to the lygosomines Trachylepis (formerly part of Mabuya ) and Ablepharus .

Among squamates, a fused Meckelian groove is not exclusive to gekkotans. It is also uniformly present in xantusiids and dibamids, and it is widely distributed among pleurodont iguanians, scincids, and gymnophthalmids, and seldom present in amphisbaenians and snakes (see data matrix in Gauthier et al. 2012). It never occurs in acrodontan iguanians, lacertids (perhaps in Omanosaura , see Kosma 2004), or anguimorphs ( Gauthier et al. 2012). An intermediate state, where closure occurs but a suture remains, is known in some cordylids, scincids, teiids, and iguanids. A fused Meckelian canal is rare among cordyliforms (but see Cordylus cordylus and Platysaurus capensis in Kosma 2004 as examples of exceptions). Among scincids, most scincines present an open Meckelian groove, it is fused in acontines, and variable among lygosomines ( Evans 2008).

Among all the lizard clades in which a fused Meckelian groove occurs, the morphology of the dentary of MNHN-SND 622 most closely resembles that of acontine and lygosomine scincids. The New Caledonian Lioscincus is a representative generalized lygosomine scincid, chosen as a representative of skinks broadly (see Fig. 4 E).

Scincoids have been recognized relatively late in the Paleogene fossil record of Europe, mainly in the last decade (note that the reported presence of Plestiodon at Quercy was wrong ( Hoffstetter 1944), and that the cordylid lizards recognized by Hoffstetter (1942) were in fact iguanid lizards [ Augé 1987]). Scincoideus is a relatively plesiomorphic form from the Paleocene of Belgium ( Folie et al. 2005, but see Smith & Gauthier 2013 for an alternative interpretation suggesting affinities to lacertiforms), and in most cases it is not easy to distinguish between fossil scincids and cordylids. There is a tendency, however, for the recognition of a greater number of scincids than cordylids for the Paleogene, probably because of the lack of diagnostic characters of cordyliforms in such fragmentary dentaries. Later European scincids, like Chalcides , have an open Meckelian canal. The genus Mabuya , with a closed Meckelian canal, was tentatively identified in the late Pliocene of Illes Medes (Iberian Peninsula) ( Bailon 1991), and has been reported from several other localities from Spain, Greece and Bulgaria ( Böhme & Ilg 2003). Note, however, that extant scincids in Eurasia and Oceania include Asymblepharus ( Fig. 4 D), Ablepharus , Trachylepis , Chalcides , Eumeces , Eurylepis , and Ophiomorus , but not Mabuya (see above). In Europe, the genus Chalcides is widespread in the south, and the genera Ablepharus and Ophiomorus are restricted to the south-east (Balkan Peninsula).

Figures

FIGURE 3. Holotype of Gekkomimus rugosus (MNHN, SND 622) in labial (A) and lingual (B) views, from Sindou D (MP 18 - 19). Scale bar in mm.

FIGURE 4. Lizard dentaries in lingual view for comparison purposes: A. Cadurcogekko piveteaui (MNHN, QU 17163), B. Gekkomimus rugosus (MNHN, SND 622), C. Pyrenasaurus evansae (UM-ECB 1703), D. Asymblepharus alaicus (modified from Darevsky & Tschumakov, 1962). E. Lioscincus nigrofasciolatus (MCZ A- 27345). In C and D the image was reversed to facilitate comparison. Scale bars in mm.