Growing more than 10 feet (3 meters) long, Protostega was among the largest turtles to ever live. Unlike most turtles, whose shells are made of expanded and fused bones that form a relatively solid dome, widely spaced bones that looked more like the rafters of a roof held up Protostega's leathery shell. Though the shell design provided less protection, the lighter load combined with powerful, flipper-like front legs made Protostega a strong, inexhaustible swimmer. Females likely migrated hundreds of miles to lay eggs on sandy beaches, much like sea turtles do today.
Movement onshore, however, was difficult. An adult female Protostega may have weighed a ton or more, a hefty load to drag out of the ocean to lay eggs. But lay eggs on the beach they did by the dozens, a reproductive strategy of safety in numbers that helped at least a few survive to adulthood. In fact, marine turtles were the only seagoing reptiles to escape extinction at the end of the Cretaceous, 65 million years ago.
A large and pointed head with a sharp beak and strong jaws probably helped the ancient turtles feed on slow-moving marine creatures such as jellyfish and shellfish as well as seaweed and floating carcasses—much as their descendents do today. Shark teeth embedded in Protostega bones housed at a museum in Chicago suggest the turtles were sometimes a meal themselves.