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 Lagerstätten3,4,5 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.