Extant birds (Neornithes) are one of the most widespread and diverse vertebrate lineages, but their early evolution is poorly understood. Although molecular analyses contend that most major extant bird lineages diverged during a period of explosive radiation following the Cretaceous-Paleogene (K-Pg) extinction, several enigmatic fossils from the latest Mesozoic suggest that some clades evolved before the end of the Cretaceous. We describe a historically collected bird assemblage from the Atlantic coastline and conclusively demonstrate its latest Cretaceous age. Phylogenetic analyses show these fossils are assignable to neornithines, and many may represent early members of Galloanseres (ducks, chickens, and kin). These fossils comprise the most diverse Mesozoic neornithine fauna known. Several of these neornithines had massive body masses reaching or exceeding those of the largest known Mesozoic birds and indicate that at least one family of wading birds, the Presbyornithidae, may have persisted through the K-Pg extinction event.The pronounced size variation observed in this assemblage indicates that early neornithines inhabiting coastal ecosystems were highly diverse and displayed tropic specialization comparable to modern coastal avifaunas.
All methodological notes of relevance are in the main manuscript and supplement.