[MUSIC] Hello, my name is Scott Persons. I'm a paleontologist at the University of Alberta. Let me welcome you to our course on marine reptiles. Life on Earth began in the oceans more than 3.5 billion years ago, meaning that you and I and every living thing is a descendant of a marine organism. Then, around 380 to 400 million years ago, a few brave fish-like creatures made the transition from sea to land, and became the first terrestrial vertebrates. These animals were the first <b>tetrapods</b>, which were named for their four feet. As tetrapods evolved adaptations for life on land, one lineage, called <b>amniotes</b>, evolved an extra protective layer around their embryos called an <b>amnion</b>, which protected the developing young from drying out. And this allowed amniotes to lay eggs on dry land and completely cut their ties to the water. Amniotes continue to develop specializations for living on land and diversify to fill in an amazing array of terrestrial habitats. Interestingly, it wasn't long before some of these terrestrial amniotes once again embraced an aquatic lifestyle. We'll never know for certain why they returned to the oceans. They may have been searching for more abundant food sources or could have been avoiding predators. The only thing the fossil evidence shows with certainty is that they did leave the land and returned to the water. So what happens when a terrestrial amniote returns to the water permanently? How do air breathing, landlubbing amniotes once again adapt to life in the sea? We can see that throughout the course of evolutionary history, many terrestrial amniotes have returned to the water. 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 and faced new challenges in their aquatic environment. The process of overcoming these challenges, what we refer to as the <b>aquatic problem</b>, is the main theme of this course. Over the four lessons of this marine reptiles course, I'll introduce you to some of the amniotes that made the transition back to aquatic habitats, and the ways in which they solved the aquatic problem. To understand the information in this course, you need to know how to read a phylogenetic tree, how the fossil record is interpreted, and the basics of evolutionary biology and comparative anatomy. And if you're feeling a bit rusty on any of these topics, you might want to review your notes from an introductory biology or geology course, or visit some of the videos from our Dino 101 MOOC for a refresher. Before we get started, let's explore your ideas on one of the most fundamental concepts we'll be discussing in this course, the general morphology, or body shape and structure, of an amniote that has returned to the sea. Sort the following animals into the baskets you see below based on whether you think they have terrestrial, aquatic or semi-aquatic lifestyles. All the animals you just sorted are amniotes that have adapted to different habitats and lifestyles. The deer, snake, lizard and tortoise are all terrestrial. These animals might be able to swim if they were forced to go in the water, but would be perfectly happy to spend their entire lives on land. The orca is the only one of these amniotes that is fully aquatic, meaning that everything it needs for all aspects of its life can be found in the water. The crocodile, sea turtle, and penguin are all semi-aquatic, and need both the land and the water to survive. In the previous exercise, we looked at semi-aquatic lifestyles, but it's important to note that there's a full spectrum of semi-aquaticness. On one end are animals like penguins that only go into the water to hunt for food. On the other end of the spectrum are animals like the leatherback sea turtle that spend nearly its entire life at sea. In fact, female leatherback sea turtles only come ashore one day a year to lay eggs. Males never return to shore after they hatch. Even though sea turtles spend so much of their lives in an aquatic setting, they're still considered semi-aquatic because without that one tie to land, they would go extinct. Fish, on the other hand, are fully aquatic. Since all living fishes descended from ancestors that were also aquatic, we consider fish to be <b>primarily aquatic</b>. Any semi-aquatic or fully aquatic amniotes are considered to be <b>secondarily aquatic</b> because they descended from ancestors that were terrestrial. The purpose of this course is to understand the aquatic adaptations of Mesozoic reptiles, but like all aspects of paleontology, our knowledge is based on modern life. Understanding the aquatic reptiles of today will enable you to draw analogies between more familiar modern organisms and the less familiar ones we will discuss later. So, let's start with an overview of some aquatic amniotes that live in the sea today. When we think about secondarily aquatic animals, we normally think of mammals, and several groups of mammals live in the ocean today. Whales and dolphins, the most obvious, but there are also seals, sea lions, walruses, sea otters, and manatees. Most of these animals have long, smooth bodies, flippers, a tail fluke, and no hind legs. This makes it very difficult, if not impossible, for them to move around on land. Other types of modern animals, like penguins and turtles who spend their lives swimming and diving, are also secondarily aquatic amniotes. Reptiles are not generally the first thing that you would think of if asked to name a secondarily aquatic animal. However, many reptiles today are very successful in the marine environment. Which of the following groups of reptiles have aquatic members living today? Check all that apply. A, Turtles, B, Snakes, C, Alligators and Crocodiles, and/or D, Lizards? I wasn't kidding when I said that reptiles were successful in the marine environment. All of these groups have members that live in and around the water today, so all four answers are correct. Let's talk about some of these reptiles. In this section, we're going to introduce you to a variety of aquatic reptiles and some of the adaptations they have to living in water. Likely the most familiar aquatic reptiles alive today are turtles. Numerous turtle species are secondarily adapted to an aquatic life, living in freshwater environments such as lakes and rivers. Sea turtles are adapted to saltwater environments, and spend their entire lives in the ocean, venturing onto land only to lay eggs. Sea turtles tend to display a flatter, smoother body form than freshwater forms, and they have flippers instead of feet. Now, all species of turtles are characterized by a shell, made up of a dorsal <b>carapace</b> and a ventral <b>plastron</b> that encloses the body. Alligators and crocodiles belong to a group known as the <b>Crocodylomorpha</b>. They've got long pointed heads, an elongate body covered with bony armour, webbed toes, and a powerful, laterally compressed tail. These reptiles are generally large, reaching up to 12 meters long, but this one is only a baby. Crocodylomorphs are semi-aquatic, since they carry out important aspects of their lives on land and in the water. They live in and around shallow waters such as rivers, lakes, swamps, and streams. Today, only the saltwater crocodile has colonized the ocean and can move far away from shore. Although they usually live in rivers, estuaries, and along the coast, they are known to use ocean currents to move between Indonesian islands. Among snakes, many <b>extant</b>, or currently living species, have adaptations to various fresh water and marine environments. These include swamps, rivers, coast lines, lagoons, and other shallow marine environments. Of the sixty-two living species of sea snakes, most live in shallow coastal waters, and can dive up to 100 meters to find food near the bottom. All but one of these species bear live young, and all have a characteristically broad and paddle-like tail to help them swim. Many sea snakes have developed permeable skin to help them breathe, and adaptations in the lung and heart to facilitate long, deep dives. The marine iguana is one of the unique animals found in the Galapagos Islands. Although mostly terrestrial, it swims and dives in order to forage for its favourite food, algae. It mostly inhabits rocky shores, but is also found in marshes and mangrove beaches. These large lizards have developed adaptations for swimming, such as a laterally compressed tail. The marine animals we just mentioned look very different from each other, even though their ancestors faced the same challenges and limitations as they transitioned from land to water. What do you think are some differences between a terrestrial and an aquatic habitat that a secondarily aquatic amniote would have to adapt to? Check all that apply. A, Water creates higher drag than air, B, Gravity does not exist in the water, C, Animals walk on land, but have to swim in the water, and/or D, Animals living in water need gills to breathe instead of lungs. As aquatic amniotes evolved, they all had to adapt to life in a different medium. This created interesting challenges that required major changes to their bodies in order to cope. Water is denser and more viscous than air which causes more drag, so A is correct. Animals also have to evolve new ways to generate thrust, since most animals don't walk in the water, they swim. So C is also correct. Gravity still exists in water even though it is counteracted by the force of buoyancy, so B is incorrect. Finally, to breathe water an animal needs gills. But it is <i><b>not</b></i> necessary to breathe water to live in water. All aquatic amniotes still have lungs and breathe air, so D is also incorrect. The challenges we just discussed, and a number of other differences between air and water, combine to form the <b>aquatic problem</b>. The ancestors of secondarily aquatic animals were adapted to a terrestrial life, they breathed air and had legs. How did they survive a transition to the aquatic environment and then go on to thrive in the water? What problems did they face and how did they overcome them? In the next section, we will address several aspects of the aquatic problem, and give examples of how modern animals have adapted to solve them.
