A remarkable assemblage of enantiornithine birds from the Late Cretaceous of southeastern Brazil

Abstract

Despite abundant discoveries of Mesozoic birds in recent decades, knowledge of their evolution during the last 20 million years of the Cretaceous remains scant. However, this time interval is essential to better understand the rise of modern birds and the pattern of avifaunal turnover during the Cretaceous-Paleogene transition. We report on a remarkably rich site (William’s Quarry; discovered in 2004 by WN) contained in the Upper Cretaceous Adamantina Formation (Bauru Group) of southeastern Brazil (Presidente Prudente, western São Paulo State). Excavations at this site have produced hundreds of isolated and partially articulated remains of small to medium-sized enantiornithine birds concentrated in a very small area (approximately 6 m2 ) of red-pink fluvial sandstones and claystones. Representing at least three taxa, the remains include numerous postcranial elements as well as many skull portions (isolated rostra, mandibles, and crania) preserved in three dimensions. As the most abundant avian Mesozoic locality in the Americas and the richest site of Late Cretaceous age in the world, this site provides key information for contrasting hypotheses of avian diversification during the K-Pg transition and the earliest divergences of modern birds. Along 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 mya interval. Such a record is difficult to reconcile with hypotheses arguing that modern (neornithine) birds originated in the southern hemisphere during the Late Cretaceous.