Extinct crane-like birds (Eogruidae and Ergilornithidae) from the Cenozoic of Central Asia are indeed ostrich precursors

We describe new fossils from the late Eocene of Mongolia, which show that the crane-like Eogruidae and Ergilornithidae are stem group representatives of the Struthioniformes (ostriches). Currently, both taxa are unanimously assigned to the neognathous Gruiformes (cranes and allies). However, ergilornithids show a progressive reduction of the second toe, and a few earlier authors likened these birds to ostriches, which are the only extant birds with just 2 toes. So far, eogruids and ergilornithids were mainly known from hindlimb bones from the Cenozoic of Asia, and here we provide important new data on the skeletal anatomy of these birds. A partial skull exhibits characteristic features of palaeognathous birds, and ostriches in particular. In its distinctive shape, it furthermore closely resembles the skull of the Eocene palaeognathous Palaeotididae, which are here also considered to be stem group representatives of the Struthioniformes. A femur from the late Eocene of Mongolia likewise corresponds to that of ostriches in derived traits, whereas cervical vertebrae exhibit features of neognathous birds. The fossils suggest that true ostriches (crown group Struthionidae) originated in Asia, and the Neognathae-like morphology of some bones opens a new perspective on the evolution of skeletal characteristics of palaeognathous birds.