TWO NEARLY COMPLETE LIZARD SKELETONS FROM THE UPPER CRETACEOUS (CAMPANIAN) TWO MEDICINE FORMATION, MONTANA, USA, OFFER NOVEL INSIGHTS INTO THE MORPHOLOGY AND EVOLUTION OF SQUAMATA

Nearly all Mesozoic lizards of North America are known from isolated and fragmentary bones such as jaws and vertebrae from vertebrate microfossil localities. Recent field work at the Egg Mountain locality in the Upper Cretaceous (Campanian) Two Medicine Formation of Montana has yielded an exceptional collection of well-preserved fossil lizard jaws, skulls, postcrania, and nearly complete skeletons. This largely autochthonous assemblage, which includes similarly well-preserved fossil mammals, dinosaurs and dinosaur egg shells and egg clutches, and invertebrate trace fossils, offers an excellent opportunity to reconstruct a vertebrate terrestrial ecosystem from the Late Cretaceous of North America. Here we present new results of our morphological and phylogenetic studies of an undescribed lizard based on two nearly complete skeletons (MOR 6627 and 7042) from the Egg Mountain locality. High-resolution computed tomography (CT) and virtual 3D reconstructions revealed morphological details otherwise hidden by the surrounding matrix. We performed a phylogenetic analysis of 279 taxa that were scored for 807 morphological characters. The Egg Mountain lizard is recovered as the basal-most known member of Scincogekkonomorpha (i.e., stem + crown scleroglossans). Three unambiguous apomorphies support that node: 1) presence of a prearticular crest, 2) a proximally expanded clavicle with a notch or fenestra, and 3) the symphyseal portion of the pubis is more than half again as long as the tubercular portion. That placement diverges from our previous results in which the Egg Mountain lizard, represented solely by MOR 6627, consistently was nested within Iguania. We primarily attribute that topological difference to our improved morphological understanding of the Egg Mountain lizard (i.e., MOR 7042 and CT data) but caution that these results are preliminary pending additional character sampling of both specimens. The presence of a basal scincogekkonomorph during the Late Cretaceous has important implications for determining character state polarities nearest the major dichotomy of the squamate tree (i.e., Scincogekkonomorpha + Iguanomorpha) and provides important insights into the early evolution of lizards from North America and globally.