Enantiornithes

The Enantiornithes (literally, "opposite birds", by which name they are popularly known), also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense). They were the most abundant and diverse avialan group known from the Mesozoic era and the first to spread across the globe. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Internally, they notably differed in the arrangement of shoulder bones, with a reversed or "opposite" ball-and-socket joint. In Enantiornithes, the scapula functions as a socket and the coracoid as a ball, whereas in modern birds, these bones function respectively as a ball and a socket.

They were mostly small, about the size of sparrows. Their fossils are found only in the Cretaceous, and they diverged from the ancestors of modern birds early in that period, more than 130 million years ago. Over seventy species of Enantiornithes have been named, but some names represent only single bones, so it is likely that not all are valid. The Enantiornithes became extinct at the Cretaceous–Paleogene boundary, along with Hesperornithes and all other non-avian dinosaurs.