in South America, or arrived from elsewhere when southern continents were closer together or when sea levels were lower. Kelenken itself lived during the middle Miocene, about 15 million years ago. Since phorusrhacids survived until the Pleistocene, they appear to have been more successful than for example the South American metatherian thylacosmilid predators (which disappeared in the Pliocene), and it is possible that they competed ecologically with placental predators that entered from North America in the Pleistocene. Description Phorusrhacids were large, flightless birds with long hind limbs, narrow pelvises, proportionally small wings, and huge skulls, with a tall, long, sideways compressed hooked beak. Kelenken is the largest known phorusrhacid, about 10% larger than the largest phorusrhacids previously known, such as Phorusrhacos. The holotype skull is about long from the tip of the beak to the center of the sagittal nuchal crest at the upper back of the head (a size likened to the size of a horse's skull), making it the largest skull of any known bird. The hind end of the skull is wide. The tarsometatarsus leg bone is long. The head height was up to , while modern seriemas reach in height. While the weight of Kelenken has not been specifically estimated, it is thought to have exceeded . Skull Prior to the discovery of Kelenken, the skulls of incompletely known large phorusrhacids were reconstructed as scaled up versions of those of smaller, more complete relatives like Psilopterus and Patagornis, as exemplified by a frequently reproduced 1895 sketch of the destroyed skull of the large Phorusrhacos, which was itself based on that of Patagornis. These reconstructions highlighted their assumed very tall beaks, round, high eye sockets, and vaulted braincases, but Kelenken demonstrated the significant difference between the skulls of large and small members of the group. The holotype skull is very massive, and triangular when viewed from above, with the hind portion compressed from top to bottom. The upper beak is very long, exceeding half the total length of the skull, unlike in Mesembriornis and Patagornis, and is longer than that of Phorusrhacos. The ratio between the upper beak and the skull of Kelenken is 0.56, based on the distance between the bony nostril and the front tip. In spite of the crushing from top to bottom, the upper beak is high and very robust, though apparently not as high as in patagornithines, such as Patagornis, Andrewsornis, and