Late Hemphillian (Late Miocene) vertebrate fauna from the Black Mesa Quarry, Chamita Formation, Rio Arriba County, New Mexico

Abstract

—The Black Mesa Quarry contains a newly discovered vertebrate fauna of late Miocene (late Hemphillian) age from the Chamita Formation near Lyden, Rio Arriba County, northern New Mexico. This fauna is derived from the Cuarteles Member stratigraphically high in the Chamita Formation. The site is only 10-16 m below the overlying Pliocene Servilleta Basalt and is in a relatively intact landslide block on the eastern slope of Black Mesa. Eight taxa of mammals are currently known from the Black Mesa Quarry: the puma-sized cat Pseudaelurus cf. P. hibbardi, the one-toed horse Dinohippus interpolatus, the giant camel Megatylopus cf. M. matthewi, the llama-like camel Hemiauchenia cf. H. vera, an unidentified antilocaprid, the rabbit Hypolagus cf. H. gidleyi, a smaller unidentified rabbit, and a small unidentified murid rodent. Further work at the Black Mesa Quarry, together with additional screenwashing for microvertebrates, will certainly add to the faunal list. The Black Mesa mammals, in particular D. interpolatus, M. matthewi, and H. gidleyi, are similar to the well known late Hemphillian faunas from the San Juan Quarry and Rak Camel Quarries. These other quarries also occur in the Chamita Formation about 7 km to the southwest. Several 40Ar/39Ar radioisotopic dates on tephras indicate that the San Juan Quarry and Rak Camel Quarries are between 6.8 and 7.0 Ma. The Black Mesa fauna is either time-equivalent to or somewhat younger than the San Juan Quarry fauna and Rak Camel Quarries fauna (5.9-6.8 Ma), but is older than latest Hemphillian (~5 Ma) faunas such as Walnut Canyon in southwestern New Mexico. FIGURE 1. Index map showing the location of upper Hemphillian fossil sites in the Chamita Formation, including the Black Mesa Quarry, Española basin, Rio Arriba County, northern New Mexico. 1. Chamita Formation type section; 2. San Juan Quarry and Rak Camel Quarries; 3. Black Mesa Quarry; 4. Lyden Quarry. 409 LATE HEMPHILLIAN VERTEBRATE FAUNA FROM THE BLACK MESA QUARRY species of large mammals in the fauna, a medium-sized felid, a small camelid, and an antilocaprid, are represented by only one or several fossils. Based on the discovery of several leporid teeth and postcranial elements on the surface of the quarry, we screenwashed a small sample (~100 kg) of sediment and have recovered additional specimens of rabbits, as well as small rodents. The Black Mesa Quarry fauna is certain to increase as we are planning further prospecting, excavating, and screenwashing, as well as prospecting in nearby exposures of the Chamita Formation. We follow Tedford et al. (2004) in using the biochronology of the Hemphillian (late Miocene and earliest Pliocene, 4.9-9.0 Ma) North American land-mammal “age” (NALMA). Other abbreviations used are: AMNH (American Museum of Natural History) and NMMNH (New Mexico Museum of Natural History). Tooth positions are abbreviated as follows: upper teeth are designated by upper case letters followed by a number giving the tooth position (e.g., P4 is an upper fourth premolar), and lower teeth are designated by lower case letters and number for tooth position (e.g., m1 is a lower first molar).