Chometokadmon is a basal gekko

A few years ago,
Evans et al. 2006 re-introduced us to the Lower Cretaceous lizard, Chometokadmon fitzingeri (MPN 539) from Italy. That genus was originally described by Costa 1864. The Evans team nested Chometokadmon between Dorsetisaurus and Xenousauridae at the base of the Anguimorpha (varanids + helodermatids). Note that Xenosaurus and Heloderma have laterally facing nares, not dorsal nares.

Figure 1. Chometokadmon in situ. Known for over 100 years, this flat skulled gekko had longer toes than typical.

Figure 1. Chometokadmon in situ. Known for over 100 years, this flat skulled gekko had longer toes than typical.

The large reptile tree (LRT) nests Chometokadmon at the base of the geckos, between Tchingisaurus and Gekko smithii. Like geckos, Chometokadmon lacks a postorbital and thus has a confluent orbit + both temporal fenestra. Helodermatids have a similar temporal architecture lacking temporal bars, but do not have a triangular rostrum in dorsal view.

Perhaps
the Evans team made a mistake in identifying a quadrate alone as a quadrate + squamosal (Fig. 2). In most geckos, the quadrate is a tall slender bone, but in the basalmost gecko in the LRT, Tchingisaurus (Fig. 2), the lateral quadrate has an anterior rim that dorsally bends posteriorly, like the purported squamosal in Chometokadmon. No close relatives have a squamosal with the shape proposed by Evans et al. The triangular outline of the skull in dorsal view along with the short teeth are also gekko traits not found in candidates proposed by the Evans team.

FIgue 2. Skull and reconstruction of Chometokdamon by Evans et al. 2006.

FIgue 2. Skull and reconstruction of Chometokdamon by Evans et al. 2006. Note the loss of the postorbital and jugal bars.

A comparison to other geckos
(Fig. 3) makes the case rather clear to Chometokdamon may be one of them. A skull twice as wide as tall plus the confluence of the orbit with the both the upper and lower temporal fenestrae are gecko traits.

Figure 1. Click to enlarge. Tchingisaurus, a basal Gekkotan, according to the large reptile tree.

Figure 3. Tchingisaurus, a basal Gekkotan, according to the large reptile tree.

Figure 3. Gekko smithii is an extant member of a genus that extends to the Early Cretaceous. Note the lack of temporal bars and the forward extension of the supratemporal along the lateral parietal.

Figure 4. Gekko smithii is an extant member of a genus that extends to the Early Cretaceous. Note the lack of temporal bars and the forward extension of the supratemporal along the lateral parietal, as in Chometokadmon.

As a basal gekko
Chometokadmon joins two rather closely related and coeval basal pro-snake genera Ardeosaurus and Eichstattisaurus that we discussed earlier here and were mistakenly  considered basal geckos by Simoes et al. 2016. Their mistake, once again, was taxon exclusion, a problem often solved by the large gamut of taxa in the LRT.

References
Costa OG 1864. Paleontologia del Regno di Napoli, III. Atti dell’Accademia Pontaniana 8, 1e198.
Evans SE, Raia P, Barbera C 2006. The Lower Cretaceous lizard genus Chometokadmon from Italy. Cretaceous Research 27:675-683.
Simões TR, Caldwell MW, Nydam RL and Jiménez-Huidobro P 2016. Osteology, phylogeny, and functional morphology of two Jurassic lizard species and the early evolution of scansoriality in geckoes. Zoological Journal of the Linnean Society (advance online publication) DOI: 10.1111/zoj.12487 http://onlinelibrary.wiley.com/doi/10.1111/zoj.12487/fullwiki/Ardeosaurus

wiki/Chometokadmon

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