Fossils tell us a lot about how dinosaur shapes changed over time. But learning how dinosaur behavior changed over time is a lot harder to do! That’s why we were so excited to find a treasure trove of Mussaurus patagonicus fossils in Patagonia in southern Argentina. We found fossils of eggs, hatchlings, juveniles, and adults. Mussaurus lived in the early Jurassic, about 193 million years ago. It is an early cousin of the giant sauropods like Diplodocus. Clues from our fossil find tell us about Mussaurus’ social lives. They lived in herds, nested in colonies, and spent time in age-separated groups. This is the oldest fossil evidence of dinosaur social behavior ever found!