The other half of avian evolution: Cyril Walker's contribution

Abstract

Enantiornithine birds represent a basal dichotomy in avian evolution. During the Cretaceous, they were the dominant birds in terrestrial habitats and were nearly as diverse in this habitat as were the Cenozoic modern birds before the development of their most diverse group, the Passeriformes. They achieved this in part through the evolution of a flight mechanism that duplicated the backstroke evolved by ornithurines, but with many different solutions including those in the shoulder girdle emphasized by Walker. Among these inventions is a unique caudal structure analogous to the ornithurine pygostyle that supported their characteristic elongated tail feathers, and a furcular keel that resembles that of hoatzins, but is differently organized from the hypocleidium of ornithurine birds. They share with Archaeopteryx similarities in the formation of the quadratojugal, a unique process on the ischium and the detailed formation of the ankle joint. The Early Cretaceous record of China shows that ornithurine birds had already perfected a modern flight stroke at a time when enantiornithine birds were still making that transition. This vast radiation of Mesozoic enantiornithine birds was unknown until discovered by Cyril Walker, an event that must rank as one of the turning points in ornithological history.

Keywords:

• Aves
• Enantiornithine
• Ornithurine
• Archaeopteryx

Acknowledgements

Cyril Walker generously gave access to his materials and helped me in ways almost beyond counting. I am grateful to him and to Gareth Dyke, who has taken up Cyril's legacy with the enantiornithines. Angela Milner and the late Alan Charig helped me in many ways during my stay at the Natural History Museum in London. The IVPP (Beijing) and Z. Zhou provided me with access to most of the newer pertinent material on enantiornithines and early ornithurines. My work there was supported in part by the Center for East Asian studies and a General Research Grant at the University of Kansas. The figures are by M. Tanner and E. Ebert. D. Burnham and A. Falk read the manuscript and made helpful comments.