crushed and corroded, piled on with no apparent order and forming a nearly spherical mass with the skull in the center" that resembled giant owl pellets, suggesting that phorusrhacids may have swallowed their prey whole and regurgitated the indigestible parts similar to owls. However, Ameghino never formally described these specimens and they have not yet been relocated, making it difficult to determine if they are phorusrhacid pellets. Fossilized pellets from northwestern Argentina have also been suggested to pertain to small phorusrhacids like Procariama. Extinction During the Miocene and early Pliocene epochs, there was an increase in the phorusrhacid population size in South America, suggesting that, in that time frame, the various species flourished as predators in the savanna environment. With the emergence of the Isthmus of Panama 2.7 million years ago, carnivorous dogs, bears, and cats from North America were able to cross into South America, increasing competition. (They had been preceded by procyonids as early as 7.3 million years ago.) The population of phorusrhacids declined thereafter according to older hypotheses, suggesting that competition with newly arrived predators was a major contributor to their extinction. Similar ideas have been considered for sparassodonts and for South America's terrestrial sebecid crocodilians. However, the role of competitive displacement in South American predator lineages has been questioned by some researchers. The timing of turnover events and the decline of South American predators do not correlate well with the arrival of large carnivores like canids or sabretooths (although they do correlate well with the earlier-arriving procyonids, which evolved to large body size in South America, but these were omnivorous), with native South American predator lineages (including most phorusrhacids and all sparassodonts and sebecids) dying out well before the arrival of most larger placental carnivores. Bathornithids, which were similar in ecology and are likely close relatives of phorusrhacids, existed entirely within North America during part of the Cenozoic and competed successfully for a time with large carnivorans such as nimravids, before becoming extinct in the Early Miocene, about 20 million years ago. The phorusrhacid Titanis expanded northward into a southern North America during the Interchange and coexisted for several million years with large canids and big cats like Xenosmilus, before its extinction about 1.8 million years ago. There were some suggestions that phorusrhacids, like the majority of Pleistocene megafauna, were killed off by human activity such as hunting or habitat change. This idea is no longer