fed on smaller prey that could be killed and consumed more safely, by for example swallowing it whole, or that when targeting large prey, it used a series of well-targeted repetitive strikes with the beak, in a "attack-and-retreat" strategy. Struggling prey could also be restrained with the feet, despite the lack of sharp talons. A 2012 follow up study by Tambussi and colleagues analyzed the flexibility of the neck of Andalgalornis, based on the morphology of its neck vertebrae, finding the neck to be divided into three sections. By manually manipulating the vertebrae, they concluded that the neck musculature and skeleton of Andalgalornis was adapted to carrying a large head, and for helping it rise from a maximum extension after a downwards strike, and the researchers assumed the same would be true for other large, big-headed phorusrhacids. A 2020 study of phorusrhacid skull morphology by Degrange found that there were two main morphotypes within the group, derived from a seriema-like ancestor. These were the "Psilopterine Skull Type", which was plesiomorphic (more similar to the ancestral type), and the "Terror Bird Skull Type", which included Kelenken and other large members, that was more specialized, with more rigid and stiff skulls. Despite the differences, studies have shown the two types handled prey similarly, while the more rigid skulls and resulting larger bite force of the "Terror Bird" type would have been an adaptation to handling larger prey. Paleoenvironment Kelenken was discovered in pyroclastic (rocks ejected by volcanic eruptions) outcrops belonging to the Collón Curá Formation in the southeastern corner of Comallo, Patagonia, an area covered in whitish tuffs. The area's stratigraphy had only been preliminarily studied at the time, and the age of the sediments had not been adequately determined, but compared with other fossil beds of the South American Land Mammal Age and radioisotopic dating from different areas of the Collón Curá Formation, it is estimated to date to the Colloncuran age of the middle Miocene, about 15 million years ago. The formation was accumulated in a broken foreland system characterized by several basins that were disconnected from each other. The formation is composed mainly of volcaniclastic limestones and sandstones that were accumulated in continental environments ranging from alluvial (deposited by running water) to lacustrine (deposited by lakes). The Collón Curá Formation and the Colloncuran age of South America represent a time when more open environments with reduced plant covering predominated,