An unique bonebed of enantiornithine birds from the Late Cretaceous of southeast

Abstract

Despite abundant discoveries of Mesozoic birds in the last few decades, knowledge of their evolution during the last 20 million years of the Cretaceous remains scant. However, this time interval is vital for understanding the rise of modern birds as well as the pattern of avifaunal turnover during the Cretaceous-Paleogene transition. We report on a remarkably rich site, called William?s Quarry, contained in the Upper Cretaceous Adamantina Formation (Bauru Group) of Presidente Prudente, western São Paulo State. Several excavations Sessão temática  Forma: Morfologia e Descrições 233 at this site have produced hundreds of partially articulated and isolated remains of small to medium-sized enantiornithine birds concentrated in a very small area (approximately 6 m2) of red-pink fluvial sandstones and claystones. The remains include numerous postcranial elements as well as many skull portions (isolated rostra, mandibles, and crania) preserved in three-dimensions. The William´s Quarry constitutes the most abundant avian Mesozoic locality in the Americas and the richest avian site of Late Cretaceous age in the world. As such, this site provides key information for contrasting hypotheses about avian diversification during the K-Pg transition and the earliest divergences of modern birds. Together with other Late Cretaceous localities from Gondwana, the information revealed at this site indicates a clear abundance of enantiornithine bird species during the ~80-70 ma interval. Such a record is difficult to reconcile with hypotheses arguing that modern (neornithine) birds originated in the southern hemisphere during the Late Cretaceous.