After the dinosaurs vanished, evolution ran a strange experiment in South America. For tens of millions of years the continent drifted as an island, cut off from the rest of the world, and into the gap where large predatory mammals might have evolved stepped the birds. The phorusrhacids, nicknamed terror birds, became some of the apex hunters of their age: tall, flightless, fast, with stubby useless wings and skulls built for killing. They first appear in the fossil record around 60 million years ago and survived in some form until the Pleistocene. Kelenken lived in the middle Miocene, roughly 15 million years ago, when this corner of Patagonia was warmer and richer than the windswept steppe it is today.

Before Kelenken, scientists pictured the giant terror birds simply as scaled-up versions of their smaller relatives, with tall rounded beaks and high round eye sockets, a guess immortalized in a famous 1895 sketch. The Comallo skull demolished that assumption. Its enormous beak, hooked like an eagle's and stretching some 46 centimeters, sat above eye sockets that were low and almost rectangular, the whole architecture far stranger and more specialized than anyone had drawn. The animal it belonged to was a giant even by terror-bird standards. Kelenken stood an estimated three meters tall, weighed well over a hundred kilograms, and outsized any phorusrhacid known before it by roughly a tenth. Here at last was direct evidence of how the largest members of the group were actually built, and reconstructing a giant from the bones of dwarfs, it turned out, had led paleontologists badly astray.

What did a bird like this do with such a head? Researchers studying its relatives found the skull had become unusually rigid, sacrificing the flexibility most birds keep between their skull bones in exchange for strength against up-and-down forces. The likely tactic was brutal and precise: rather than wrestle struggling prey, the terror bird probably struck downward with its great hooked beak in repeated, well-aimed blows, an attack-and-retreat strategy that minimized risk to itself. Its long slender legs tell another part of the story. Where earlier scientists assumed the biggest terror birds were lumbering and slow, Kelenken's gracile build suggests it may have been a genuine runner, swift across the open Miocene plains where its meals grazed.

Kelenken did not hunt alone. The rocks that yielded its skull, the Collon Cura Formation, preserve an entire vanished ecosystem, a roster of at least two dozen mammal species. There were ground sloths and armored glyptodont relatives, hoofed grazers found nowhere on Earth today, marsupial predators, even early monkeys and a host of rodents. Into this teeming community walked the terror bird, very likely near the top of the food chain. Today the same ground is dry Patagonian scrub, and the only hint of that warmer past is the bones eroding from the cliffs. The legend the Tehuelche carried turned out to be, in its way, a memory the land itself had kept.