A new assemblage of fossil leaves and wood from The Aguja Formation (Upper Campanian) of West Texas: An interfluvial community in a volcanic landscape

Abstract

A new assemblage of fossil leaves and wood is described from the uppermost Aguja Formation (upper Campanian) of Big Bend National Park, Texas, USA. While diverse leaf assemblages have been reported from Campanian through Maastrichtian strata in more northerly areas of North America, few have been so far described from southern localities. The leaf assemblage was found preserved within a lenticular tuff bed. The number of species (7) of leaves in the assemblage is low and includes one fern and six dicots. Most specimens represent taxa which have small leaves with entire margins, a morphology consistent with a dry, warm climate. Thin sections of several fossil wood samples found nearby at the same stratigraphic interval as the tuff reveals that the fossil woods represent conifers with irregularly-spaced growth rings. The assemblage may represent shrubby vegetation within an open, conifer-dominated woodland which developed within a periodically dry environment. However, the assemblage may also represent early successional species that grew in areas that had been previously disturbed by deposition of pyroclastic material. A comparison of two localities in the uppermost Aguja Formation reveal the occurrence of two types of vegetational communities – one exemplified by presumably riparian, evergreen angiosperm woodlands and another exemplified by interfluvial conifer forest with an associated herbaceous community. Given that both sites preserve very similar depositional facies, it seems that observed differences involving fossil wood taxonomy and structure are likely related to the local availability of water.