SCREENWASHING UNVEILS JUVENILE DINOSAUR AND BIRD TEETH IN THE HELL CREEK FORMATION, SOUTHWESTERN NORTH DAKOTA

The Hell Creek Formation, in Montana, North Dakota, Wyoming, and South Dakota, contains vertebrate faunal assemblages preceding the Cretaceous-Paleogene (K-P) boundary. Previous studies of the Hell Creek in North Dakota focused on surface collection and excavation of vertebrate fossils. This creates a bias towards larger specimens. We screenwashed at 11 microvertebrate sites in the Hell Creek of North Dakota. This yielded higher concentrations of smaller fossils than prior studies. We identified the fossils based on the literature and identified collections at the University of California Museum of Paleontology, Berkeley (UCMP). We identified 202 dinosaur and bird specimens (mostly teeth/tooth fragments). There are 18 ceratopsian (cf. Triceratops) and 124 hadrosaurid teeth. The ceratopsian teeth have a height of 1-2 cm; the hadrosaurid teeth are ˂1 cm in height. When compared to teeth of mature ceratopsians and hadrosaurs at the UCMP, the teeth in our collection are noticeably smaller. The size difference indicates nesting sites in the area, making this collection a unique record for the North Dakota section of the Hell Creek Formation. Beside herbivorous dinosaurs, 55 theropod and bird teeth were identified, including 14 tyrannosaurids, 20 Richardoestesia (including R. isosceles), 6 dromaeosaurids (cf. Sauronitholestes), one “Paronychodon”, and 14 birds (cf. Hesperornithiformes). The tyrannosaurid teeth and tooth fragments are small, indicating juveniles. All the dinosaur and avian taxa reported here have been documented in past studies of this region. However, our screenwashing yielded more R. isosceles and bird teeth than reported from this area (based on surface collection). This highlights the importance of screenwashing. The presence of R. isosceles (piscivorous) and other taxa, supports the geologic interpretation that the Western Interior Seaway was very close to this area in the latest Cretaceous.