Pliensbachian (Lower Jurassic) radiolaria from Baja California Sur, Mexico

Lower Jurassic Radiolaria have been recovered from limestone beds and concretions in the sandstone member of the San Hipolito Formation near Punta San Hipolito, Baja California Sur, Mexico. The San Hipolito Formation and underlying Upper Triassic ophiolite form part of the Vizcaino Sur terrane. The faunas are well-preserved, abundant, and diverse. Assemblages are dominated by spumellarians, but small cyrtid nassellarians are strongly varied also; multicyrtid nassellarians comprise a lesser component of the fauna. The Baja faunas are dated mainly by comparison to radiolarians from the Queen Charlotte Islands that co-occur with ammonites of Pliensbachian age. New ammonite zones for North America established by Smith and Tipper (1996) provide precise age control for equivalent radiolarian faunas that has not been possible in the past. Baja radiolarians are also compared with faunas from east-central Oregon, Turkey, and Japan. The strong similarity between Baja California radiolarians and Tethyan faunas from Turkey supports existing paleomagnetic evidence that the Vizcaino terrane may have occupied a low-latitude position in Early Jurassic time. Two radiolarian genera (Ducatus and Religa) and twenty-two species are described as new; many other forms are discussed informally. Thirty-six previously described taxa are also documented.