PaleoBios 29(2):48-59, November 5, 2009 © 2009 University of California Museum of Paleontology Revised large mammal biostratigraphy and biochronology of the Barstow Formation (Middle Miocene), California DARRIN PAGNAC Museum of Geology, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701; [email protected] A new biostratigraphic zonation for the middle Miocene Barstow Formation based on the mammalian megafauna is presented. Four biostratigraphic zones are outlined, two containing fossil assemblages of early Barstovian (Ba1) age, and two with late Barstovian (Ba2) assemblages. Recommendations are made for defining and characterizing the Barstovian North American Land Mammal Age (NALMA). The Ba1 biochron is defined based on the first occurrence of Plithocyon, and the base of the Ba2 biochron is revised based on the first appearance of the antilocapridRamoceros . The ursid Plithocyon remains a valid defining taxon for the base of the Barstovian NALMA. The use of both gom- photheriid and mammutid proboscideans to define the base of the Ba1 or Ba2 is abandoned due to the diachrony of first appearances across North America. The Ba1 biochron is additionally characterized based on the first appear- ance of the equid Scaphohippus. The base of the Ba2 biochron is revised based on the first appearance ofRamoceros. The Ba2 can also be characterized by the first appearance of the borophagine canidProtepicyon and the anchitherine equid Megahippus. INTRODUCTION of the Barstow Formation (Woodburne and Tedford 1982, The Barstow Formation, with significant outcrops located Tedford et al. 1987, Woodburne et al. 1990, Tedford et al. within the Mud Hills region of the Mojave Desert (Fig. 2004), the Third or “Green Hills” Division, the Second Divi- 1) approximately ten miles north of the town of Barstow, sion, and the First Division (Fig. 2). These zones are in real- San Bernardino County, California, is renowned for its ity lithologic subdivisions developed by field crews from the mammalian fossils. The Wood Committee on Stratigraphic Frick Laboratory (see abbreviations). Because these lithologic subdivisions coincidentally contain relatively discrete fossil Nomenclature (Wood et al. 1941) designated the fossil as- assemblages, subsequent researchers (Frick 1937, Schultz semblage from the “fossiliferous tuff member” of the Bar- and Falkenbach 1940, 1941, 1947, 1949) utilized these Frick stow Formation as the type assemblage for the Barstovian Lab lithologic subdivisions to denote relative stratigraphic North American Land Mammal Age. Subsequent workers relationships of fossil assemblages. As such, no formal bio- (Tedford et al. 1987, Tedford et al. 2004) defined an early stratigraphic zonation based on established protocol has (Ba1) and late (Ba2) subdivision of the Barstovian NALMA. been presented for the mammalian megafauna of the Barstow The Barstovian NALMA is recognized as encompassing a Formation, although Lindsay (1972) proposed a fourfold geochronologic time span from approximately 16.0–12.5 biostratigraphic zonation based on mammalian microfossils Ma (Tedford et al. 2004). (Fig. 2). Herein I present a revised biostratigraphy for the A revised systematic treatment of the megafaunal con- mammalian megafauna of the Barstow Formation as exposed stituents (here defined as small mustelid or larger) has been in the Mud Hills (Figs. 2–3) and a revised biochronology for accomplished by Pagnac (2005a, 2005b, 2006). Prior to the Barstovian NALMA. The paper concludes with a brief these works, the mammalian megafauna from the Barstow consideration of the boundary between the Barstovian and Formation had not been subjected to a comprehensive de- Clarendonian NALMAs. scription and review since that presented by Merriam (1919). In this study, revisions to the biostratigraphy of the Barstow History of Study Formation, based on these systematic reviews, are presented. The development of the modern lithostratigraphic sub- These revisions include only the Barstovian taxa from the Bar- divisions of the Barstow Formation is a complicated story stow Formation, excluding the earlier Hemingfordian taxa. that involves both lithostratigraphic and biostratigraphic In the type area (Mud Hills), the Barstovian fauna occurs interpretations. For a comprehensive and detailed review of from the stratigraphic level of Steepside Quarry to several interpretations prior to 1980, see Woodburne and Tedford localities (e.g., RV 6126, UCMP V6447) which occur near (1982), Woodburne et al. (1990) and Pagnac (2005a). the stratigraphic level of the Lapilli Tuff (approximately 120 The modern lithostratigraphic subdivisions of the Bar- meters above the Hemicyon Tuff), or within the upper half stow Formation (Fig. 2) were developed by Woodburne and of the middle member and the upper member of the Barstow Tedford (1982), Tedford et al. (1987) and Woodburne et al. Formation (Figs. 1–3). (1990). The basal member, the Owl Conglomerate (Fig. 2), To date, three informal “faunal zones” have been recog- consists of at least 200 m of granitic conglomerates. These nized within the “Barstovian portion” (as outlined above) basal conglomerate layers grade laterally into the granitic 49 PALEOBIOS, VOL. 29, NUMBER 2, NOVEMBER 2009 breccia of the Pickhandle Formation (Ingersoll et al. 1996). approximately 570 m of fluvial conglomerates, sandstones and The base of the Owl Conglomerate is demarcated by the clays. The top of the middle member is marked by the promi- Red Tuff. The top of the Owl Conglomerate is marked by nent Skyline Tuff, a 1–2 m thick detrital, ash-fall tuff bed. the Rak Tuff. The unnamed upper member (which begins with the Atop the Owl Conglomerate is the unnamed middle Skyline Tuff) consists of about 270 m of lacustrine clays and member (which begins with the Rak Tuff) which comprises mudstones, with limited occurrence of sandstone facies. The HEMICYON RODENT CARNIVORE QUARRY HILL HELLGATE COMPOSITE RV 6126, UCMP V6447 CANYON BASIN BASIN BASIN SECTION UCMP V65140 m ft UCMP V6448, RV 6127, UCMP V6447, UCMP V6448, LAPILLI TUFF RV 6126 6128, 6129 RV 6127, 6128, RV 5201 LINDSAY 5 6129 Robbins Q, Hidden Hollow Q, HEMICYON Robbins, Hidden Hollow Leader Q UCMP V2301, TUFF Easter Leader UCMP V2301, V3849, RV 5201 UCMP V3849 3000 New Hope 900 4 Easter Q, UCMP V65147, V1399, V1400, RV 5101 RV 5101, Dip Slip HEMICYONTUFF Hemicyon Stratum, UCMP V65147, Hemicyon Stratum, RV 6607 UCMP V1399, HIATUS RV 6607 UCMP V1400 RAINBOW LOOP Mayday, New Hope Q DATED New Year 800 3 Dip Slip Q UCMP Vz65134 SKYLINE TUFF Mayday Q, New Year Q, UCMP V65134 TUFF 2500 2 Skyline, Starlight, Starlight Q, RV 6131 Sunder Ridge, RV 6131 Hailstone Skyline Q, Sunder Ridge Q, 700 Hailstone Valley View Valley View Q OREODONT 2000 STEEPSIDE Saucer Butte 600 Saucer Butte Q TUFF QUARRY BASIN Turbin Q Turbin Sandstone, RV 4701 500 Sandstone Q, RV 4701 Oreodont Q Rak Deep Deep Q, Rak Q 1500 1 Steepside Steepside Q 400 M U D H I L L S Barstow Syncline 300 1000 Fossil Bed Road Road RAK Irwin TUFF 200 Fort 500 100 N RED TUFF Old CA-58 0 0 Barstow 15 58 15 40 0 3 miles Scale Figure 1. Stratigraphic sections from the Barstow Formation in the Mud Hills, CA, region (for specific geographic locations of sec- tions refer to Woodburne et al. 1990 and MacFadden et al. 1990). Stratigraphic locations of vertebrate localities are also indicated. Numbers in composite section correspond to the following tuff beds: 1—Oreodont Tuff, 2—Skyline Tuff, 3—Dated Tuff, 4—Hemi- cyon Tuff, 5—Lapilli Tuff. Insert illustrates geographic location of Mud Hills region and Barstow Syncline area of the Mojave Desert where significant deposits of the Barstow Formation crop out. PAGNAC—BIOSTRATIGRAPHY, BIOCHRONOLOGY REVISIONS IN THE BARSTOW FM. 50 upper member is capped unconformably by Quaternary al- Wood et al. (1941) designated the fauna from the “fos- luvium. Each of the three members shares an interfingering siliferous tuff member” of the Barstow Formation (equivalent relationship with a western fanglomerate facies of distinct to the unnamed upper member of Woodburne et al. 1990) lithology (Steinen 1966, Woodburne et al. 1990) that has as the type assemblage for the Barstovian North American never been recognized as a formal lithostratigraphic unit. Land Mammal Age. The fossils from the “fossiliferous tuff Frick field crews recognized five distinct lithostratigraphic member” of Merriam (1919) were considered equivalent to intervals within the Barstow Formation (Fig. 2): the Fifth the assemblage from the First Division of the Frick Labora- or Red Division, the Fourth or Rak Division, the Third tory, thus limiting the original definition of the Barstovian to or Green Hills Division, the Second Division and the First that zone. At this point, lithostratigraphy and biochronology Division (Barstow Fauna of Woodburne et al.1990). Frick became inter-mixed, resulting in a confusing characterization field crews recognized relatively distinct fossil assemblages of the type Barstovian assemblage. The Frick zones were es- associated with each of these divisions, although these data sentially lithostratigraphic units that coincidentally contained were never formally published. Subsequent researchers (Frick distinct fossil assemblages. Thus, the original definition of 1937, Schultz and Falkenbach 1940, 1941, 1947, 1949) the Barstovian NALMA was based on fossil occurrences utilized these lithostratigraphic divisions to indicate the restricted to lithostratigraphic zones rather than a desired relative stratigraphic relationships