Welcome to the first lesson of Palaeontology: Ancient Marine Reptiles. In this lesson we will explore the main theme of the course: the aquatic problem. In other words, what happens when a terrestrial animal returns to the water permanently? How do air-breathing, land-lubbing creatures once again adapt to life in the sea? Life in water is very different than life on land. Water is much denser than air, which affects all aspects of an animal's life including movement, sight, and hearing. In addition, animals that return to the water cannot breathe water, and so must return to the surface for air. Water also conducts heat much better than air, making staying warm and active a challenge. Despite all these obstacles, many land animals have returned to the water throughout the course of evolutionary history. In fact, many examples of them are living today including whales, seals, crocodiles, sea turtles and penguins. Each of these animals had ancestors that returned to the water. The process of overcoming the challenges associated with this transition is what we refer to as the aquatic problem. This lesson will explore many different types of adaptations that modern and extinct animals have evolved to meet these challenges. You will be introduced to some extinct groups of reptiles you have probably never heard of, and will gain a new appreciation for how well suited modern marine animals are to their environment. Just a quick note before you get started: 'Palaios' is the Greek word for 'ancient', so palaeontology or paleontology is the study of ancient life. Both spellings are correct, with palaeontology used in Britain, and paleontology more common in the US.