An angiosperm woodland in the Javelina Formation (Upper Cretaceous), Big Bend National Park, Texas, U.S.A.

Abstract

Part of a paleo-forest comprised of eight in situ permineralized tree stumps is preserved in the Javelina Formation (middle Maastrichtian, c. 69 Ma) near Alamo Creek in western Big Bend National Park. The stumps are rooted in an immature organic-rich paleosol resembling a modernday Aquept, developed in a landscape depression subject to gleization with a shallow fluctuating groundwater table. Seven of the stumps pertain to the dicotyledonous angiosperm Metcalfeoxylon, and one is a cuppressoid/podocarpoid conifer, indicating that is part of a mixed conifer-dicot forest. The site exhibits a woodland architecture, with stands of closely spaced trees separated on average by 10 m, and open gaps over 100 m in between. Trunk diameters vary from 33 to 97 cm, with estimated crown heights from 23 to 48 m, and represent trees of varied ages in a mature forest. Irregular growth rings are developed in the conifer wood, but absent in the Metcalfeoxylon woods. The local climate may have had one or more annual dry seasons, and the conifers were either more susceptible to moisture stress, or genetically predisposed to express growth rings more markedly than the contemporaneous dicots. Metcalfeoxylon is also the dominant tree type at another paleo-forest preserved near McKinney Spring in the underlying Aguja Formation (late Campanian, c. 75 Ma). Metcalfeoxylon may have grown preferentially in low-lying riparian environments subject to flooding and burial by alluvial overbank deposits, thus favorable to preservation of the stumps in situ. A deficiency of precipitation, along perhaps with browsing pressure exerted by dinosaur herbivores, may have been responsible for maintaining the open woodland physiognomy evident at both sites.