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.
LAY SUMMARY
Even though various fossil ostriches (Struthioniformes) have been described from late Cenozoic deposits, the early evolutionary history of these birds has remained elusive.
Ostriches are the only extant birds with only 2 toes, but a didactyl condition also occurs in the Ergilornithidae from the Cenozoic of Asia.
These fossil birds have so far mainly been known from hindlimb bones and are currently assigned to the Gruiformes (cranes and allies).
Here we describe new fossils from the late Eocene of Mongolia, which corroborate earlier assumptions that the Ergilornithidae as well as the closely related Eogruidae are struthioniform birds.
The fossils suggest that true ostriches originated in Asia and their unusual character mosaic opens a new perspective on the evolution of skeletal characteristics of palaeognathous birds.
