NEW MIXED MARINE-NONMARINE MICROVERTEBRATE FAUNA FROM THE UPPER CRETACEOUS (LATE SANTONIAN-EARLY CAMPANIAN) MENEFEE FORMATION, NORTHWESTERN NEW MEXICO

We report here a new microvertebrate fauna from the Allison Member of the Upper Cretaceous (late Santonian-early Campanian) Menefee Formation at the eastern edge of the San Juan Basin, northwestern New Mexico. This fauna is significant because: (1) it quadruples the known vertebrate diversity of the unit; (2) it yields numerous marine taxa, especially sharks, from a formation generally considered to be nonmarine; (3) therian mammal teeth recovered from the site are generally rare in the Cretaceous; and (4) the mammals from this site are the oldest mammal fossils known from New Mexico at this time. The fauna was collected from an intraformational conglomeratic lag in a sandstone complex at NMMNH locality 5636. New records from the Menefee Formation from this site include the rays Ptychotrygon, Dasyatis, Myledaphus, Ischyrhiza, "Protoplatyrhina," and "Pseudohypolophus," the sharks Lonchidion, Squatina, Squatirhina, Cretodus, Scapanorhynchus, Serratolamna, Cretolamna, Carcharias, Chiloscyllium, and Cretorectolobus, indeterminate amid, pycnodontid, and phyllodontid osteichthyans, the theropod dinosaur Richardoestesia, squamates, and both multituberculate and therian mammals. Several of the selachian records may represent new taxa and many, including Ischyrhiza, Ptychotrygon, Cretodus, Scapanorhynchus, and Cretolamna are known primarily to exclusively from marine deposits. The high quality of preservation, with numerous well-preserved specimens including delicate features, precludes extensive reworking and instead suggests that locality 5636 records significant marine influence on this Menefee Formation sand body. Mammal fossils recovered from NMMNH locality 5636 are particularly significant because they are the first records of mammals from the formation, include rare records of Cretaceous therians, and may facilitate correlation with broadly coeval mammal-bearing strata in Utah. The fauna and other lines of evidence support an early Campanian age for the Allison Member of the Menefee Formation and the site holds the potential to better tie terrestrial biochronology (e.g., the Aquilan land-mammal "age") to marine biostratigraphy and biochronology.