Late Cretaceous woody dicots from the Aguja and Javelina Formations, Big Bend National Park, Texas, USA

Angiosperm woods occur throughout Upper Cretaceous (84-66 million years old) continental strata of Big Bend National Park, Texas, USA. Vertebrate remains occur along the same stratigraphic levels, providing a rare opportunity to reconstruct associations of sedimentary facies, wood remains, and vertebrate remains. The wood collection sites span a vertical stratigraphic succession that corresponds to an environmental transect from poorly-drained coastal salt- or brackish water swamps to progressively better drained freshwater flood-plains lying at increasingly greater distance from the shoreline of the inland Cretaceous sea and at higher elevations. The eight dicot wood types of the Aguja Formation differ from the five types of the Javelina Formation, paralleling a change from a fauna dominated by duckbill and horned dinosaurs to a fauna dominated by the large sauropod, Alamosaurus. These woods increase the known diversity of Cretaceous woods, and include the earliest example of wood with characteristics of the Malvales. The lower part of the upper shale member of the Aguja contains numerous narrow axes, some seemingly in growth position, of the platanoid/icacinoid type, and of another wood that has a suite of features considered primitive in the Baileyan sense. Duckbill dinosaur remains are common in the facies with these woods. In contrast to other Cretaceous localities with dicot wood, Paraphyllanthaxylon is not common. Dicotyledonous trees are most abundant at the top of the Aguja and the lower part of the Javelina Formations in sediments indicating well-drained inland fluvial flood-plain environments. One locality has logs and insitu stumps, with an average spacing of 12-13 metres between each tree, and trees nearly 1 metre in diameter. To our knowledge this is the first report of anatomically preserved in situ Cretaceous dicot trees. Javelinoxylon wood occurs at all levels where remains of the giant sauropod Alamosaurus occur. The vertebrate faunas of the late Cretaceous of New Mexico and Texas are said to comprise a 'southern' fauna distinct from the 'northern fauna' of Alberta and Montana. The wood remains are consistent with such provincialism. It has been suggested that dicots were not commonly trees in the late Cretaceous of the northern part of the western interior of North America. The Big Bend woods provide direct evidence for dicot trees having more than a subordinate role in Cretaceous vegetation at lower latitudes. Most of the dicot wood types of Big Bend are characterized by high proportions of parenchyma, over 50% in one type. Whether these high proportions of parenchyma are correlated with the higher CO2 levels of the Cretaceous and/or the pressures exerted by aggressive browsing by large dinosaur herbivores is unknown.