Early evolution of the biological bird: perspectives from new fossil discoveries in China

Abstract

New discoveries of fossil birds belonging to the Jehol Biota uncovered from Lower Cretaceous lacustrine deposits in northeastern China continue to greatly enrich our understanding of the first major avian radiation. The exceptional preservation of some fossils provides a rare chance to discuss many biological issues that are usually impossible to address in paleontological studies, such as: the ossification pattern of the sternum in the extinct group Enantiornithes, which is unlike that of modern birds and all other archosaurs; the discovery of preserved crop, gizzard, and intestinal contents in several clades which suggest that a near-modern digestive tract including specialized crop morphologies evolved early during avian evolution; and the rare preservation of ovarian follicles which support hypotheses that the right ovary was lost in Aves due to the limitations of powered flight. Together, these data allow a partial reconstruction of the biology of Aves very close to its origin. While no skeletal or integumentary features are recognized to define Aves, we identify two possible soft tissue features that may biologically define Aves relative to other amniotes: the presence of a crop and the loss of the right ovary.