A recently discovered species, "Kelenken guillermoi" from Middle Miocene some 15 million years ago, discovered in Patagonia in 2006, had the largest bird skull yet found. The fossil has been described as being a , nearly intact skull. The beak is roughly long and curves in a hook shape that resembles an eagle's beak. Most species described as phorusrhacid were smaller, tall, but the new fossil belongs to a bird that probably stood about tall. The large terror birds were nimble and quick runners able to reach speeds of .