What is "ornithothoraces"

Ornithothoraces is a group of avialans that includes all enantiornithines ("opposite birds") and the euornithes ("true birds"), which contains modern birds and their ancestors. The name Ornithothoraces means "bird thoraxes". This refers to the modern, highly advanced anatomy of the thorax that gave the ornithothoracines superior flight capability compared to more primitive avialans. This anatomy includes a large, keeled breastbone, elongated coracoids and a modified glenoid joint in the shoulder, and a semi-rigid rib cage.

The earliest known members of the group are the enantiornithes Protopteryx fengningensis and Eopengornis martini, as well as the euornithine Archaeornithura meemannae, all from the Sichakou Member of the Huajiying Formation in China, which has been dated to 130.7 million years old. At least one other enantiornithine, Noguerornis gonzalezi, may be even older, at up to 145.5 million years ago, though its exact age is uncertain.