Bird feathers from a Late Cretaceous coprolite

Fossilized feathers are rare, yet critical for understanding the evolution of flight in pennaraptoran dinosaurs and the appearance of crown birds in the Late Cretaceous.^1,2 Feathers associated with skeletal remains are abundant in several Lower Cretaceous Lagerstätten^3,4,5 but nearly absent from Upper Cretaceous deposits.^6,7 As such, the evolution of avian feathers close to the emergence of the crown-group Neornithes remains largely unknown. Here we report on an exceptionally preserved three-dimensional pennaceous feather preserved in a medium-sized theropod dinosaur coprolite found in the uppermost Maastrichtian Hell Creek Formation in North America. Arguably the best-preserved feather ever recovered from Mesozoic lithic deposits, it demonstrates that the anatomically modern lightweight and stiff feather with a square, medulla-filled rachis was in place in Pennaraptora before the end-Cretaceous mass extinction. Skeletal remains from the same coprolite suggest the feather belongs to a hesperornithiform, a lineage of flightless aquatic birds within Ornithurae but outside Neornithes.⁸ Consistent with this identification, the feather exhibits a low structural index and high barb density, adaptations for reduced penetrability present in extant aquatic birds.^9,10,11 Tiny associated gar scales are interpreted as the bird's last meal. Microcomputed-tomography data reveal the preservation of additional associated feathers, including extinct morphotypes, within the coprolite,³ suggesting these morphotypes persisted even in crownward, non-neornithine birds until the end-Cretaceous mass extinction. Differences in plumage, especially with regard to body feathers most responsible for thermal insulation, may have played a role in the extinction of all non-neornithine pennaraptorans during the hypothesized asteroid-induced impact winter.¹².