The ancestors of our modern mesothelae, a sub-order of spiders, could have been the size of a human head. "If it was alive today, it would be hunting cats". In this animation video, it hunts down a Petrolacosaurus, the - soon to be deceased - reptile. A plausible scene from the Carboniferous period (about 325 million years ago). Source: "Walking with Monsters" (BBC / Discovery (2005)).