Ross MacPhee, the curator and paleomammalogist at the American Museum of Natural History in New York City, joins Cool Science Radio. Thousands of years ago—up until around the time that the first humans began to populate North America—the earth was populated by fantastical creatures. Mastodons, giant Irish deer, gorgon hippopotami, short-faced kangaroos, giant sloths: massive creatures lumbering across the primordial landscape. And then, for some reason, these animals died off. The explanation