A JURASSIC STEM-GEKKOTAN FROM THE MORRISON FORMATION REVEALED VIA COMPUTED TOMOGRAPHY IMAGING

Squamates, with more than 11,000 species, are a major portion of extant tetrapod biodiversity. However, their conflict remains between phylogenetic hypotheses generated from morphological versus molecular data. This may be due in part to poor sampling of stem members of disparate crown clades, particularly from the Jurassic, the estimated point of origin of most of the crown ‘backbone’ clades. Here, we identify a stem gekkotan from the Kimmeridgian Brushy Basin member of the Morrison Formation. This new species replaces the Tithonian Eichstaettisaurus as the oldest stem gekkotan currently known.

This identification is based on a reexamination of specimen DINO 19514. It consists of a disarticulated partial skull that includes the maxillae, prefrontals, frontals, parietal, left jugal, right postfrontal and squamosal, a partial braincase, both dentaries, and the fused left postdentary elements. Previous examination of DINO 19514 assigned it to the scincomorph Paramacellodus. We utilize computed tomography to re-examine the morphology of DINO 19514 in previously inaccessible detail and find it to be a new taxon that is neither Paramacellodus nor a scincomorph. This new taxon is diagnosed by an enlarged pineal foramen, a relatively wide inter-orbital portion of the frontals (more than 50% of the width of the frontoparietal suture), a postfrontal fused to the postorbital, and a wide parietal nuchal fossa.

We incorporated DINO 19514 into a large dataset of squamates (165 out of 791 characters x 169 species). Both maximum- and implied-weights (K=12) parsimony infer DINO 19514 as the earliest-diverging stem gekkotan, sister to a clade containing Eichstaettisaurus, Norellius and crown gekkotans. DINO 19514 grants insight into the condition at the base of Pan-Gekkota. As in other stem gekkotans, it has paired frontals, in contrast to the fused condition of the crown. While incomplete, it is apparent that the subolfactory frontal processes are like those of other stem gekkotans in being intermediate between the ancestral squamate condition and the crown gekkotan condition in which they meet and fuse on the ventral midline. Likewise, the elevated marginal tooth count of DINO 19514 is intermediate between the low ancestral count and the high count of crown gekkotans. The lack of palatal dentition unites DINO 19514 with stem and crown gekkotans, pushing this loss back into the Jurassic. The posterior teeth of DINO 19514 are unicuspid, a feature that it shares with other stem and basal crown-gekkotans.