Between sauropod toes: a microvertebrate survey of fine-grained deposits from the Upper Jurassic Morrison Formation (northern Wyoming, USA)

First discovered during dinosaur excavations in the late 19th century, the mammal fossils of the Morrison Formation have since contributed to our understanding of Mesozoic ecosystems. Once thought to be merely nocturnal insectivores in the shadow of dinosaurs, early mammals are now recognized as ecologically diverse and much more abundant than previously assumed.
One particularly promising site is the Jurassic Mile in far northern Wyoming, a locality that has proven a true treasure trove of Mesozoic fossils. The site preserves material from both the terrestrial Morrison Formation and the underlying marine Sundance Formation, including dinosaur remains, fossilized plants, ichnofossils, fish, marine reptiles, and insects. The deposits are considered to be Late Jurassic in age, which is particularly valuable given the limited global record from the poorly understood Jurassic–Cretaceous boundary.
In 2024, a pilot study was initiated to investigate the site’s smallest vertebrate fossils, with a focus on capturing microvertebrate diversity often overlooked in large-scale excavations. Roughly 1,000 kg of fine-grained clay, interpreted as low-energy depositional sediment, was screen-washed from three stratigraphic layers using stacked sieves. The resulting fractions (~5–2.5 mm and ~2.5–0.5 mm) were dried in the field and subsequently screened under a stereomicroscope at Naturalis Biodiversity Center, Leiden.
Preliminary results highlight one particularly fossiliferous layer, informally dubbed ‘The River Channel’, which produced a rich assemblage of microvertebrates. These include numerous premolars and other molariform teeth tentatively attributed to multituberculates and/or haramiyids. Additional finds include small theropod teeth, fish remains, and various indeterminate tetrapod bone fragments. The high number of mammalian elements recovered from a relatively small amount of sediment, along with dental variation suggesting diverse feeding strategies, indicates that mammals were already an established part of a rich and complex ecosystem.
Future efforts will expand the volume of processed sediment from The River Channel and conduct detailed taxonomic analysis of the recovered material. Through these efforts, we aim to better characterize the small-bodied vertebrate fauna of the Morrison Formation and enhance our understanding of Mesozoic mammalian diversity and ecological dynamics near the Jurassic–Cretaceous transition.