A theropod tooth assemblage from the lower Aguja Formation (early Campanian) of West Texas, and the roles of small theropod and varanoid lizard mesopredators in a tropical predator guild

A theropod tooth assemblage from the lower shale member of the Aguja Formation in West Texas is part of a diverse microvertebrate fauna, designated the Lowerverse local fauna, of early Campanian age (c. 80-82 Ma). The fauna includes as many as nine distinct theropod taxa along with several indeterminate archosaurs and birds. Theropod tooth types (indeterminate tyrannosaurids, cf. Saurornitholestes, cf. Richardoestesia, cf. Paronychodon) are similar to those found in the upper shale member of the Aguja, as well as in other Campanian theropod assemblages from western North America. However, the most abundant tooth morphotype is unique, and attributed to a new varanoid lizard with remarkably theropod-like dentition, herein designated Dryadissector shilleri (gen. et sp. nov.). The presence of many unique theropod tooth morphotypes in the Lowerverse local fauna suggests that there remains significant undiscovered diversity among small theropods in southern latitude faunas, and accords with recognition of distinct latitudinal biomes during Campanian time in western North America. Due to their similar dentition, small theropods, along with varanoid lizards, may have served similar ecological roles as competitive mesopredators in the Campanian tropical predator guild.