An exceptional bone bed of Enantiornithine birds in the late Cretaceous of Brazil

Abstract

Despite abundant discoveries of Mesozoic birds in the last few decades, knowledge of theirevolution during the last 20 million years of the Cretaceous remains scant. However, this time interval isvital for understanding the rise of modern birds as well as the pattern of avifaunal turnover during theCretaceous-Paleogene transition. We report on a remarkably rich site (William's Quarry; discovered in 2004by 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 ofpartially articulated and isolated remains of small to medium-sized enantiornithine birds concentrated in avery small area (approximately 6 m²) of red-pink fluvial sandstones and claystones. The remains includenumerous postcranial elements as well as many skull portions (isolated rostra, mandibles, and crania)preserved in three-dimensions and representing at least three taxa. This site constitutes the most abundantavian Mesozoic locality in the Americas and the richest site of Late Cretaceous age in the world. As such,this site provides key information for contrasting hypotheses of avian diversification during the K-Pgtransition and the earliest divergences of modern birds. Together with other Late Cretaceous localities fromGondwana, the information revealed at this site indicates a clear abundance of enantiornithine bird speciesduring 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.