First isotopic (U-Pb) age for the Late Cretaceous Alamosaurus vertebrate fauna of west Texas, and its significance as a link between two faunal provinces
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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. The first isotopic age from within the southern province is presented. A single outcrop of distal tuff within the Upper Cretaceous Javelina Formation in northern Big Bend National Park, Texas, contains monazite with a U-Pb age of 69.0 ± 0.9 Ma (2 sigma). The age is from a 207Pb/204Pb vs. 235U/204Pb isochron, an approach 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 Cretaceous-Tertiary 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 dicotyledonous 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.
|Journal||Journal of Vertebrate Paleontology|
|Number of pages||7|
|Publication status||Published - 2006|
We thank D. Corrick and V. Davila of the Office of Science and Resource Management in Big Bend National Park for their assistance in conducting this research, and W. Straight and J. Browning for their help in the field. D. Barker provided helpful comments on the nature of the pyroclastic deposit. The efforts of K. Manser and S. Loewy in obtaining the U-Pb analyses are much appreciated. W. Langston is thanked for discussions throughout the course of the study. D. Eberth and J. Fassett provided reviews of an earlier version of the manuscript. We also thank two anonymous reviewers for comments that helped us to sharpen our discussion and conclusions. This study was supported by the Geology Foundation and the Jackson School of Geosciences at the University of Texas at Austin.