Gastornis gigantea and its prey, illustration. Gastornis was a large flightless bird that lived between the late Paleocene (55 million years ago) and the middle Eocene (45 million years ago). Also known as Diatryma, it grew up to 2 metres tall and possessed a huge sharp beak and large powerful legs with sharp talons. This suggests it was a predator and probably hunted using ambush methods or pack hunting. Here, it is preying on a small mammal. This species, G. gigantea, dates from the middle Eocene of western North America.