Some 400 million years ago, animals emerged from the ocean for the first time and began to thrive on land. But after eons of evolutionary experimentation, some lineages peaced out of the terrestrial world and embraced a full-time life at sea again. While whales and dolphins are among the best known marine creatures to follow this path, it turns out some ancient relatives of modern crocodiles, called thalattosuchians, took the same aquatic turn while dinosaurs roamed the planet.

According to a study published Monday in the Proceedings of the National Academy of Sciences, these predatory crocodilians are the only archosaurs — the group that also includes birds and dinosaurs — known to have shifted from land to a lifestyle spent always in the ocean.

Thalattosuchian fossils are scattered across the world, and the researchers believe their transition from land to sea began about 182 million years ago. While they likely breathed through their noses rather than blowholes, these Jurassic animals eventually evolved to have smooth skin and body shapes reminiscent of whales and dolphins today.