Troodon sp. dinosaur skeleton, computer illustration. This small, fast, agile carnivore with sharp, serrated teeth is thought to have been one of the most intelligent dinosaurs, having the largest brain to body weight ratio. Troodons existed during the late Cretaceous, around 100 to 65 million years ago. They probably hunted smaller herbivores in packs, and scavenged from the kills of larger dinosaurs. They had opposable fingers that enabled them to grasp their prey, something few other dinosaurs could do.