EVIDENCE FOR FIRES, ARIDITY, DINOSAUR BABIES, AND EGGS IN LATE CRETACEOUS OF BIG BEND NATIONAL PARK, TEXAS

Dinosaurs from Big Bend National Park, Texas are some the southernmost Late Cretaceous records from the Western Interior of North America. Much less is known about Big Bend's dinosaurs compared to northern areas. New microvertebrate sites from inland floodplain deposits of the upper Aguja Formation (late Campanian-early Maastrichtian) have yielded numerous (300+) dinosaur eggshell fragments and many juvenile dinosaur teeth (hadrosaur, ankylosaur, tyrannosaurid, Saurornitholestes, and other small theropods). Abundant juvenile tyrannosaurids, small theropods, juvenile hadrosaurs, and a diverse assemblage of dinosaur eggshell types suggest that fossils from more than one nesting site were transported. Other fossils include burned wood, seeds, gastropods, Ophiomorpha, fish, salamanders, lizards, turtles, and crocodilians. The Big Bend theropod assemblage in the late Campanian-early Maastrichtian was less diverse than contemporaneous northern areas, and was more similar to the late Maastrichtian of southern Alberta (Scollard Formation) in the following: Richardoestesia isosceles and pachycephalosaurs were present and Troodon and Dromaeosaurus were absent. Among the non-dinosaur vertebrates, many that are common in Big Bend are rare or absent from contemporaneous northern assemblages. These differences are due to different paleoclimatic conditions in Big Bend. For example, evidence that Big Bend was periodically hot and arid comes from abundant horizons with paleocaliche nodules (paleocaliche forms in soils under hot and dry conditions). Additionally, evidence for occasional fires in Big Bend comes from burned wood. Future research will track changes in vertebrate paleocommunities during the paleoclimatic changes of the last ten million years of the Cretaceous.