EVIDENCE OF DINOSAUR HERBIVORY IN THE UPPER CRETACEOUS AGUJA FORMATION, BIG BEND NATIONAL PARK, TEXAS

A locality in the Late Cretaceous (Campanian) Aguja Formation near Gano Spring in Big Bend National Park preserves scattered remains of hadrosaurian dinosaurs in close association with woody scrambling vines. The vines are referable to Baileyan wood types I and II and platanoid/icacanoid wood types I and II. The close association between the vines and herbivorous dinosaurs suggests a possible foraging relationship. The fossiliferous interval is comprised of olive-gray to dark gray mudstone, lenticular discontinuous channel sandstone, sideritic ironstone nodules, and lignite, all of which indicate a stratigraphic position low in the upper shale member of the Aguja Formation. Detailed stratigraphy of the vine-bearing interval, and the internal ring structure of the vines, indicate gentle, regular pulses of sediment input consistent with low-velocity flood events, insufficient to cause mechanical damage to the vines preserved at the site. The external branch stumps observed on the vines are consistent with shear removal as determined by experimental branch removal results using extant tree and vine branches. The stumps indicate that the branches of the vines were dominantly removed by shearing consistent with herbivory. The internal anatomy of the vines shows tracheid-filled false rings that are generally caused by mechanical removal of a portion of the plant during life. These lines of evidence taken together indicate that the vines were foraged upon by herbivorous dinosaurs, most likely hadrosaurs and ceratopsians preserved in the same deposits.