It’s a special moment when a baby learns how to go from crawling on all fours to walking on two legs, especially considering how rare this transition is in the animal kingdom. Only a few other species are thought to make the shift from crawling to upright walking as they age—and they’re all dinosaurs.

Now, researchers have added compelling new evidence for one member on that list: a sauropod named Mussaurus patagonicus. (Also see fossil footprints from sauropods that lived in what’s now Scotland.)

When Mussaurus hatched, it would have been small enough to fit cozily inside a human hand, earning it a genus name that means “mouse lizard.” But after just eight years of gobbling up ferns and other plants,