A U-PB AGE FOR THE LATE CRETACEOUS ALAMOSAURUS VERTEBRATE FAUNA OF WEST TEXAS

Latest Cretaceous (Maastrichtian) terrestrial vertebrates from western North America occur in two faunal provinces. The Triceratops fauna is found in Canada, Montana, and Wyoming; and the Alamosaurus fauna occurs in Utah, New Mexico, and Texas. Although the two faunas are thought to have been contemporaneous, only the northern province contains intercalated volcanic units that have been isotopically dated. Here, we present the first isotopic age from within the southern province. A single outcrop of distal tuff within the Late Cretaceous Javelina Formation in northern Big Bend National Park, Texas, contains monazite with a U-Pb age of 69 ± 1 Ma. The age is from a ²⁰⁷Pb/²⁰⁴Pb vs. ²³⁵U/²⁰⁴Pb isochron, which has been chosen to avoid the effects of 230Th-derived excess ²⁰⁶Pb. The age falls within the boundary interval between the poorly calibrated Edmontonian and Lancian North American Land Mammal Ages. The tuff bed occurs approximately in the middle of the fluvio-lacustrine Javelina Formation, about 90 m stratigraphically below the position of the K-T boundary. This position is within the local range of the sauropod Alamosaurus, below two sites that have yielded remains of the pterosaur Quetzalcoatlus, and above a site with petrified logs of the dicot tree Javelinoxylon. The range zones of all three taxa span the full thickness of the Javelina Formation elsewhere in the Big Bend region. The Alamosaurus Fauna is therefore Lancian to late Edmontonian in age. This isotopic age provides the first precise correlation between northern and southern Lancian vertebrate faunal provinces.