In the quest to solve the aquatic problem, plesiosaurs raise the art of paleontological guesswork to new heights. Even with so much fossil material, there is still great debate raging on how they use their four flippers for propulsion, how they hunted, how they mated, and whether they cared for their young. As we move into this next section of the lesson, we will focus on gathering the clues as to how plesiosaurs solved the aquatic problem. Let's start this section by talking about locomotion. There are many ways to move through the water. Most modern marine vertebrates including fish, sharks, sea snakes, and crocodilians use axial locomotion to generate thrust. Even ichthyosaurs which had four flippers, still generated thrust using a vertical caudal fin. Plesiosaurs were almost unique to the animal kingdom in that they probably used their four large powerful flippers to generate thrust. The were not axial swimmers because their short stubby tails are not effective for propulsion. Their gastralia and ribs also caused them to have rigid bodies and that would prohibit any sort of undulatory motion, meaning the plesiosaur body would have remained absolutely stiff while swimming. No axial locomotion here, they would have been purely appendicular swimmers. However, unlike most other appendicular swimmers, they have four well developed flippers to generate thrust, not just two. Their body plan is unique among all living and dead organisms. Even sea turtles, which also have four flippers, are significantly different because their front and back flippers are different in size and it's the front ones that generate most of the trust. Of course, this create a problem: when paleontologists try to figure out how a long extinct animal moved, they usually look at a modern analogue. So what is a <b>modern analogue</b>? It is the assumption that if an extinct animal has a similar skeleton to a modern animal, they probably moved in a similar way. One of the main reasons the plesiosaur locomotion problem is so difficult is because we have no modern analogues. And if we cannot say how they swam, it's very difficult to speculate on how plesiosaurs would breed, hunt, or migrate. This makes a complete picture of plesiosaur life in the Mesozoic oceans even harder to reconstruct. In this section we will be discussing some of the clues that paleontologists have gathered to try to solve the problem of their locomotion, and we will outline some of the leading hypotheses that have resulted from this research. The plesiosaur locomotion debate boils down to one question: how did they move their flippers? There are three main hypotheses. The first is <b>underwater flying</b>. Think of the way a bird moves its wings or the way a penguin or sea turtle moves its flippers underwater. It's kind of a sideways figure eight motion. The second hypothesis is <b>rowing</b>, just like rowing a boat. The flipper pulls back horizontally with its wide edge to generate maximum thrust, and then the flipper moves forwards to its starting position thin edge first to minimize drag. Modern seals can do a similar motion with their flippers and so do we when we do breaststroke in the pool. The third hypothesis is <b>paddling</b> where the feet of the animal are pulled through the water in a vertical plane. This is the way ducks swam, canoes are paddled, and humans do freestyle. Sea lions also paddle using a belly slapping motion to move them forward. How does this swimming dog achieve thrust? A, underwater flying, B, rowing, or C, paddling? This dog is generating thrust by pulling it's feet through the water in a vertical plane. So the correct answer is C, paddling. That is why this motion is called a “doggy paddle”. Before we investigate the skeletal evidence, imagine a plesiosaur swimming. What mode of swimming do you picture? Do you imagine it using its flippers like wings to fly through the water? You imagine it rowing, and using its flippers like a two man rowboat? Or do you imagine the flippers moving down through the water like the paddles of a canoe? Regardless of what swimming style you picture, there are paleontologists who share your point of view. However, one of these hypotheses has gained far more support than the other two. But we aren't going to reveal which one just yet. Instead, clues will be provided as we discuss other aspects of their locomotion. By the end of our discussion, you should be able to combine the evidence and make a more informed guess on how plesiosaurs probably swam. Let's start by looking at the body of the plesiosaur. Plesiosaurs were well adapted to minimize drag. They had smooth skin that was similar to the skin on the porpoise or ichthyosaur, and lowered viscous drag. The body shape of plesiosaurs was also extremely streamlined to minimize inertial drag. They were roughly almond-shaped, five and a half times as long as they were wide. This is the same specific body proportion seen in sea lions, penguins, and some swimming birds including grebes and cormorants. The elongated necks of elasmosauromorphs would have caused additional issues regarding inertial drag, but we'll discuss those a little later. Paddling and flying motions both have a power stroke component from a down and back motion, whereas the rowing motion essentially moves directly backwards without much downward movement. Muscle attachment sites indicate that the muscles use to pull the limbs down and back were huge and powerful. The range of motion of the shoulder and hip sockets also supports the movements suggested by the muscles. They show that plesiosaur flippers could move downwards below their bodies easily, had a limited range of motion front and back, and could not have moved their flippers upwards and behind their backs. This provides evidence against the rowing model which requires a large amount of forward backward motion. The anatomy best supports the flying model as it uses the most up and down motion. However, the flying model does require some upward motion from the flippers, which would have been restricted by the plesiosaur shoulder sockets. Another interesting clue is that a rigid body seems necessary for flyers. Sea turtles and birds both use flying motions, and their bodies are held rigid either by a plastron or an enormous sternum. Both of these structures provide a large bony plate down the front of the animal which prevents any bending. Plesiosaurs also have very rigid bodies caused by their ribs and gastralia. This doesn't mean that the plesiosaurs couldn't have paddled or rowed, but they do possess one of the necessary features for a flyer. Now that we know that the body was stiff and attached to a massive swimming muscles, let's look at the flippers those muscles were attached to. Dividing a flipper's length by it's width yields a measurement known as the <b>aspect ratio</b>. So a flipper 4 feet long and 1 foot wide has an aspect ratio of 4. And the aspect ratio can tell you a lot about how maneuverable an animal was, how much energy it expended to move, and whether it was built for fast acceleration or covering long distances. Let's use airplanes as an example. Now transport planes and most commercial planes have long thin wings. Long length, small width equals a high aspect ratio. These planes are built for long distance cruising since they generate more lift. However, the longer surface also generates more drag, making it harder for those planes to turn. Fighter jets have short broad wings and therefore are small aspect ratio. They're built for maneuverability, they can turn quickly since there's very little drag on their small wings. But they require more energy to stay aloft for a long time since their short wings generate less lift. Elasmosauromorphs and pliosauromorphs generally differed in their flipper shapes. The elasmosaurs had long, thin flippers, and the pliosaurs had bigger, shorter flippers relative to their bodies. Based on what you just learned about aspect ratios, which of the two body forms has high aspect ratio flippers and which has low aspect ratio flippers? From this you should be able to tell which form was more suited for long distance cruising and which was better for maneuvering and fast attacks. Let's figure out the aspect ratio first. The elasmosauromorph flippers were long, divided by a small width, this equals a large aspect ratio. The pliosauromorph flippers were short, divided by a large width, which equals a small aspect ratio. Large aspect ratios like those on cargo planes are efficient at traveling long distances, but they're not very agile. Small aspect ratios, like those on fighter jets, are very fast and maneuverable, but require a lot of energy. So the correct answer here is that the elasmosauromorph has high aspect ratio flippers better suited for long distance cruising, while the pliosauromorph has low aspect ratio flippers, better at fast maneuvering. In plesiosaurs, flippers with different aspect ratios evolved separately and the animals had to develop attack strategies to match their physical abilities. A highly maneuverable pliosaur would be much more suited to chasing prey as it darted through the water. Elasmosaurs were less maneuverable and would likely have had to rely on swimming long distances in search of situations where they could ambush their prey who were spread out across the large expanse of the ocean. The flippers of plesiosaurs contain large numbers of bones. Humans have only 2 or 3 phalanges per digit, but plesiosaurs had as many as 18. In the animal kingdom, only ichthyosaurs had a greater degree of hyperphalangy than the plesiosaurs. These bones would've been fixed in strong connective tissue and cartilage, which is why we often find articulated plesiosaur flippers. Because they really could not bend their elbows or wrists, plesiosaur flippers were quite stiff. And a stiff flipper causes changes in direction by lifting or dropping the leading edge, like in a bird's wing or a shark's fin. While traveling at high speeds, the back limbs of plesiosaurs would probably have preformed most of the steering, leaving the front limbs free to generate maximum thrust. At slow speeds, it's likely that all four limbs would have generated thrust with minimum energy expenditure. The stiff flipper also helped to a maintain a <b>hydrofoil</b> shape. A hydrofoil is a very specific cross sectional shape. If you chopped off the pointed tip of a plesiosaur flipper and looked at the cross section, you would see that the bottom of the flipper is flat and the top is rounded with a tapering back edge. This is the same cross sectional shape that you would see in a bird's wing, or an airplane wing. In wings however, we would call them airfoils, not hydrofoil, because they're used in air. The shape is optimal for generating lift, the force that pulls an object up. Now how does that work? As the water moves over the rounded top surface, it covers a greater distance than the water moving over the straight bottom surface. And this means that the water molecules on top get more spread out. By spreading out, the molecules on top create empty space, which then sucks the animal up in an attempt to fill that empty space. This is the same way a vacuum works. This upward suction is what generates lift and keeps the animal moving in the direction they want to go. If you change the direction your hydrofoil faces, you can control the direction the lift force will pull you in. The hydrofoil shape of the flippers is a major clue to locomotion. It provides some of the strongest evidence for one of the three locomotion types. Based on what you just learned about how hydrofoils work, which mode of locomotion does it provide the most support for? A, underwater flying, B, rowing, or C, paddling. Hydrofoils almost certainly mean underwater flight, since lift is not generated with paddling or rowing. So A is the correct answer. Lets now look at the most recognizable feature of some plesiosaurs, the long necks of the elasmosauromorphs. Based on what you learned about the mechanics of swimming so far in this lesson and the lessons before, which position is the most likely for the long necks of a elasmosauromorphs? A, upright, B, coiled and snake-like, or C, horizontal? Early paleontologists drew elasmosaurs with the necks upright in an s-shaped curve or coiled like a snake. These are common misconceptions propagated in popular media and are not the correct answer. Today we recognize that the vertebral shape, neck muscles and neck weight would've limited lifting, bending, and coiling of the neck. Elasmosaurs swam horizontally with their necks stretched out in front. So the correct answer is C, horizontal. This position also would've been the most streamlined.
