The answer is a sea turtle.
The creature had a backbone, so it must be vertebrate. Among the vertebrates, there are no infinitive number of animals with shells. It could be only turtles. The characteristic such as a beaklike mouth without teeth supports a surmise. Limbs in the form of flippers suggest that animal must live in the water, and with such a dimensions it could live only in seas. All of these speak for the sea turtle fossil.