Fossil of Calciavis grandei, an ancient flightless bird from the Eocene epoch, 50 million years ago, Green River formation, Wyoming. The well-preserved fossil shows imprints of feathers protruding from the back end of the bird (on the right). Calciavis belongs to the extinct group of birds known as the Palaeognathae. They are closely related to ostriches, kiwis and tinamous.