A MICROVERTEBRATE FAUNA OF THE UPPER CRETACEOUS (LATE SANTONIAN – EARLY CAMPANIAN) MENEFEE FORMATION, NORTHWESTERN NEW MEXICO

Until recently the Upper Cretaceous Menefee Formation of the San Juan Basin in northwestern New Mexico has yielded only fragmentary macroscopic fossil vertebrates, principally dinosaurs, turtles, and crocodilians. We report here an extremely rich microvertebrate locality (NMMNH L-5636) in the early Campanian Allison Member that quadruples the known vertebrate diversity of the Menefee Formation. So far more than 2,600 microvertebrate fossils have been recovered from <100 kg of matrix from this site, which lies ~1.5 m above the base of an interbedded sandstone and siltstone unit approximately 9 m below the base of the upper coal member. This assemblage includes a wide array of both non-batoid sharks (Cretodus, Carcharias, Serratolamna, Lissodus, Ischyrhiza, Squatina, and Squatirhina) and rays (Myledaphus, Pseudohypolophus, Protoplatyrhina, Dasyatis, Ptychotrygon, and one new genus), three families of bony fishes (Amiidae, Lepisosteiidae, and Phyllobodontidae), Brachychampsa-like crocodilian teeth, squamate lizard scales, a possible sphenodontid jaw fragment, dromaeosaurid (Saurornitholestes) and maniraptoran (Richardoestesia) theropod dinosaur teeth, a tyrannosaurid tooth fragment, hadrosaur tooth fragments, and the earliest confirmed New Mexican mammal fossils (“Paracimexomys group” multituberculate premolars) The assemblage is comprised of two individual assemblages together, one an autochthonous or nearly autochthonous marine assemblage, and the other an allochthonous terrestrial assemblage. The terrestrial component is a significant addition to the Mesozoic vertebrate fossil record because it is one of only a few microvertebrate assemblages from the North American Santonian-Campanian (Aquilan) transition, a poorly documented interval in Cretaceous vertebrate evolution, and thus is a rare Aquilan fauna. The selachian fauna includes several species that are either ubiquitous throughout most of the Cretaceous (e.g., Pseudohypolophus mcnultyi), or that have a stratigraphic range of Campanian-Maastrichtian (e.g., Carcharias tenuiplicatus), and thus is typical of Campanian faunas throughout the Western Interior.

Key words: Late Cretaceous, Menefee Formation, Aquilan, Campanian, chondrichthyan, mammal, multituberculate