A MONSTERSAURIAN LIZARD FRONTAL FROM THE CAMPANIAN WAPITI FORMATION OF ALBERTA, CANADA.

The DC Bonebed is a newly reported channel deposit from the Wapiti Formation of centralwestern Alberta, Canada. It lies in a stratigraphic interval that is likely equivalent in age to the Bearpaw Formation of southern Alberta and contains a diverse assemblage of isolated skeletal elements from many small to medium-sized vertebrates such as fish, turtles and lizards. Lizard material previously collected from the Wapiti Formation have included very well preserved cranial elements from several representatives of the infraorder Scincomorpha (Squamata), including Chamops segnis, Kleskunsaurus grandeprairiensis, and Socognathus unicuspis. Among recently recovered material from the DC bonebed is an isolated right frontal of a lizard referable to the anguimorph clade Monstersauria. Monstersaurians are known from a diversity of Mesozoic and Cenozoic fossil taxa and are represented today by the extant Gila monster and beaded lizards (Heloderma spp.). The frontal is extremely well preserved, and its finished margins clearly indicate it was not fused to its left counterpart. This is the most complete monstersaur frontal to have been reported from the Cretaceous of Alberta, and it represents one of the largest monstersaurs known from the Upper Cretaceous of North America, being of comparable size (estimated skull length about 130 mm) to the Late Cretaceous monstersaur Estesia mongoliensis from Mongolia (skull length = 150 mm in the holotype). Synapomorphies of Monstersauria that are present in the Wapiti specimen include lack of fusion between the frontals, and large, mound-like, pitted osteoderms on the dorsal surface of the frontal. Some other Cretaceous North American monstersaurs, such as Paraderma, Labrodioctes and Primaderma, possess similar pitted, polygonal osteoderms. The Wapiti specimen remains taxonomically indeterminate at the species level due to lack of adequate comparative material and could belong to any of the previously mentioned North American Cretaceous genera or to Palaeosaniwa, for which sufficient frontal material has not yet been recovered. A micro- CT scan of the specimen reveals no clear planes of separation between the osteoderms and the underlying bone forming the dorsal surface of the frontal proper, and such fusion suggests that the individual was not a developing juvenile. This specimen significantly increases the known latitudinal range of monstersaurian lizards during the Late Cretaceous, and increases the known diversity of the Campanian fauna of the Wapiti Formation.

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