Screen-washing sediment from microvertebrate sites can produce thousands of fossil teeth and bones from fish to mammals, and can provide new insights into the paleoenvironments of an area. The Hell Creek Formation is a famous late Cretaceous terrestrial deposit in the northern Great Plains. The Hell Creek in southwestern North Dakota has produced numerous dinosaurs and other vertebrates and the K/P (Cretaceous/Paleogene) boundary is well-constrained from litho-, magneto-, and chemostratigraphy, paleobotany, palynology, and vertebrate paleontology. In a previous study, the vertebrate biostratigraphy was documented based on numerous vertebrate fossils from many sites leading up to the K/P boundary. However, this work was based on surface collection and excavation; no screen-washing of microvertebrate sites was done. Were small specimens such as small fish and shark teeth missed or under-sampled? If screen-washing is done, does this change the paleoenvironmental interpretations? We addressed this by screen-washing 11 microvertebrate sites (all tied to the K/P boundary), using fine-mesh screens and microscopes to sort the matrix. This produced thousands of small specimens (teeth and bones) from numerous vertebrates, including: 1) many fish, sharks, and rays, especially Lepisosteus (gar) and Myledaphus (guitar fish); 2) amphibians, lizards, snakes, turtles, crocodylians, and champsosaurs; 3) numerous small (hatchling) hadrosaur and ceratopsian dinosaurs; 4) theropod dinosaurs including juvenile tyrannosaurids, cf. Saurornitholestes, Richardoestesia, and “Paronychodon”; 5) birds; and 6) mammals (multituberculates: Mesodma, Meniscoessus, Cimolodon; marsupials: pediomyid and alphadontid; and eutherian: cf. Gypsonictops). There are two new and important results. First, there were dinosaur nesting sites in this area (based on the numerous teeth from hatchlings). Second, fish, sharks, and rays were much more abundant than previously recognized, indicating that the Western Interior Seaway was further to the west than previously recognized. The relative abundance of a fish-eating theropod (Richardoestesia isosceles) and of a toothed, fish-eating bird (cf. Hesperornithoformes) support this new paleoenvironmental interpretation.