Eoraptor
One of the earliest known dinosaurs, Eoraptor lived 231 million years ago when dinosaurs were just getting started. Found in Argentina in 1991, this dog-sized omnivore showed both meat and plant-eating teeth in the same jaw. Eoraptor helps scientists understand how later dinosaurs developed into specialized carnivores and herbivores. Its primitive features offer a glimpse of dinosaurs in their infancy.