AMMONITE BIOSTRATIGRAPHY OF THE PEN AND AGUJA FORMATIONS OF THE BIG BEND NATIONAL PARK AREA

Biostratigraphy provides the backbone for most geologic and paleontologic studies. Ammonite occurrences have been documented within the Pen and Aguja Formations in the Big Bend National Park area. Biostratigraphic control within the intertonguing shallow marine shelf, deltaic, and coastal plain facies of these important invertebrate and vertebrate fossil bearing formations is mostly lacking. Specimens belonging to the genera, Placenticeras, Baculites, and Texanites are among the more common taxa collected from these deposits. However, this study also yielded rarer specimens of Scaphites hippocrepis III, previously found only in the northern part of the Western Interior, as well as numerous occurrences of Baculites haresi. The occurrence of Scaphites hippocrepis III in the uppermost Pen Formation and Baculites haresi in the both the Pen and Aguja formations aid in the correlation of these strata with northern Western Interior and Texas Gulf Coast strata as well as providing tighter biostratigraphic constraints for these strata.