MIOCENE AND PLIOCENE VERTEBRATES FROM ARIZONA

We present an overview of the Miocene and Pliocene vertebrates of Arizona, spanning the time period from about 2 to 20 Ma. The best known Miocene faunas are Wellton and Anderson Mine from the late Arikareean or Hemingfordian North American land-mammal "age" (NALMA); Milk Creek Forma- tion sites from the Clarendonian NALMA; and Camel Canyon, Redington, White Cone, and Wikieup rep- resenting the Hemphillian NALMA. The Wellton fauna is composed of the small camelid Stenomylus and the heteromyid rodent Mookomys, and the Anderson Mine fauna contains the rhinoceros Diceratherium and the camelid Oxydactylus. The protolabidine camelids Protolabis coartatus and Michenia yavapaiensis and borophagine canids Carpocyon robustus and Epicyon haydeni indicate a Clarendonian age (9.0-12.5 Ma) for sites in the Milk Creek Formation (e.g., Milk Creek Quarry, Manzanita Quarry). White Cone in the Bida- hochi Formation and Redington in the Quiburis Formation both contain diverse small mammal faunas of late Hemphillian age (4.9-7.0 Ma). Wikieup from the Big Sandy Formation and Camel Canyon from the Quiburis Formation are also late Hemphillian in age based on biostratigraphy and magnetostratigraphy. Pliocene vertebrates (Blancan NALMA) are well known from Arizona, with more than 15 faunas spanning much of the Blancan (1.8-4.9 Ma). Many of these faunas have been dated by a combination of biostratigraphy and magnetostratigraphy. Small mammals are better known from Arizona Blancan sites than are large mammals. The best known early Blancan (2.7-4.9 Ma) faunas are (from oldest to youngest): Verde, Benson, Clarkdale, Bear Springs, Duncan, and Country Club. The best known late Blancan faunas (1.8-2.7 Ma) are (from oldest to youngest): Wolf Ranch, 111 Ranch, Pearson Mesa, San Simon, California Wash, and Curtis Ranch. The Verde and Clarkdale faunas from the Verde Formation in central Arizona are primarily composed of small mammals. Verde is the oldest Blancan fauna in the state (4.3-4.8 Ma). The Bear Springs fauna from the Safford basin and the Duncan and Country Club faunas from the Duncan basin are derived from strata of the Gila Group in southeastern Arizona, and date to the latter part of the early Blancan (~3.0-3.6 Ma). The Blancan faunas from the St. David Forma- tion in the San Pedro River Valley in southeastern Arizona have played a key role in understanding the biochronology of Blancan faunas throughout North America, comprising one of the first sequences of superposed vertebrate faunas in North America to be dated using magnetostratigraphy and mamma- lian biostratigraphy. The early Blancan Benson fauna (~3.3 Ma) is the oldest in this sequence of faunas, whereas late Blancan faunas include Wolf Ranch (~2.7 Ma), and California Wash and Curtis Ranch (2.0-2.2 Ma). The late Blancan in North America is defined by the first appearance of mammals of South American origin that participated in the Great American Biotic Interchange. Wolf Ranch occurs in the uppermost Gauss Chron (~2.7 Ma) and documents the earliest North American record of the porcupine Erethizon, a member of the Interchange fauna. The 111 Ranch fauna from the Safford Basin provides perhaps the best documented first appearance datum for the Interchange fauna in temperate North America. Two Interchange mammals, the glyptodont Glyptotherium and the capybara Neochoerus, occur in the uppermost Gauss Chron in the 111 Ranch section, just below the Gauss/Matuyama boundary at 2.58 Ma. The geochronology of the 111 Ranch and Wolf Ranch faunas is important in dating the onset of the Interchange in temperate North America at about 2.7 Ma.