Phorusrhacos longissimus

The Santa Cruz formation in Argentina contains one of the most complete South American fossil faunas of the Miocene period.

Among many bizarre species of mammals, their is a species of giant flightless bird standing six feet tall, with a horse-sized 13 inch skull. The largest avian skull yet known. The beak is tipped with a wicked hook identifying this bird as a flesh eater.

South America was isolated during most of the tertiary period so these giant birds were able to evolve apace alongside the marsupial carnivores that were the native mammalian flesh eaters. But in the Pliocene epoch, about 3 million years ago, sea levels dropped and the isthmus of Panama was revealed to form a land bridge to south America. Down came the extremely successful placental mammals such as saber-toothed cats from north America. It is believed that competition from the invading carnivores  helped to push these giant flightless birds towards more of a marginal existance and ultimately to extinction, because their fossil record in South America ends soon after the Pleistocene starts. It appears however that the Panama land bridge also allowed Phorusrhacids to move north and find a successful niche in North America well into the Peistocene, as their fossil remains are known in Texas and Florida.  

This drawing shows a Phorusrhacus that has stumbled upon a Theosodon calf (an early relative of Machrauchenia) curled up in the tall grass.