A plesiosaur’s long neck was likely a steering problem, as any movement would have caused deviation from a straight line as the animal swam through the water. Imagine having two steering wheels, one at the front and another at the back of a boat. As you try to steer, it would require a lot of corrections to keep going straight. To combat this problem, and also to avoid breaking their necks, Elasmosaurs had thick, rigid muscles at the base of their neck, that would've prevented any excessive movement. These issues actually make the rowing and paddling hypothesis less likely. Both of these motions require a recovery stroke. This means that thrust is not generated continuously. As the animal accelerated and decelerated with each stroke, the stiff neck would have been jerked around causing changes in the animal’s direction. Underwater flying, generates thrust continuously during the entire stroke, allowing constant course correction and making this swimming style more likely, especially for the Elasmosauromorphs. Another interesting adaptation of a long Plesiosaur neck, was it had a slight curve. This curve would have generated a small amount of lift and stopped their heavy necks from dipping and continuously steering them downward. Now that we know more about plesiosaur anatomy as it relates to locomotion, let's go back to the problem we introduced earlier in this lesson. How did they actually propel themselves through the water using four giant flippers? Did they paddle like a four footed duck? Did they row like a two man Olympic rowing team? Or did they fly underwater like a penguin? Let's review our clues. The power stroke would have come from a downwards and backwards motion, which is seen in both paddling and flying styles. Conversely, the rowing motion essentially moves directly backwards without much downward movement. The sockets of the shoulder and hip also prohibit a rowing motion. The range of motion best supports the down and back movement of underwater flying. But the upward recovery stroke would have been restricted. The Gastralia give Plesiosaurs a rigid thorax, which is a necessary feature for a flyer. One of the most important clues is the shape of the flippers. The cross-sectional shape of Plesiosaur flippers is an efficient hydrofoil able to create lift for flying underwater. Finally, the degree to which the long neck would have affected steering means that, for Elasmosaurs at least, they would've have benafitted from a continuous motion of underwater flying. Based on the evidence we've put together, let's rank the three modes of locomotion from least to most likely. Rowing, thrust comes from the back stroke requires a recovery stroke and provides discontinuous thrust. The problem? Plesiosaur limbs are not shaped like an efficient oar and are restricted in their horizontal movement, making this the least likely mode. Paddling, thrust comes from the backstroke and requires a recovery stroke where little thrust is generated. The problem? Plesiosaur limbs are not shaped like efficient paddles, but the required range of motion is possible making this the second most probable mode. Flying, all four limbs would have moved primarily in the vertical plane with an axial figure eight rotational motion. And this powerful motion provides constant thrust and keeps the animal moving more smoothly. In Plesiosaur limbs, the plane of moment is vertical, and the hydrofoil shape of the flippers is designed for flying. The only problem is that the limbs can't be raised above and behind the animal's back, which restricts the recovery stroke. This is the most likely mode. Together, these physiological clues provide the most support for the underwater flight model, but it's not perfect. It's more likely that Plesiosaurs used a modified swimming style that combined elements of flying and rowing. The downstroke or power stroke would provide lift and thrust as in flying. But the recovery stroke was probably more similar to the horizontal recovery of the rowing style. It's probable that plesiosaur flippers would have moved in near synchronization, with the front and back flippers moving almost at the same time resulting in maximum thrust and minimum drag. All four flippers would have been used as propulsive elements, with the front limbs generating most of the thrust and the back flippers generating some thrust and the majority of the steering. This would result in more speed, acceleration, and maneuverability. Since it's energetically costly to constantly use all four flippers, it is possible that Plesiosaurs would have glided between strokes or used only the front flippers at low cruising speeds. The modifications for locomotion that we just discussed would have impacted the lifestyles of the long-necked Elasmosauromorphs and the large-headed Pliosauromorphs. Recall that these two body forms evolved multiple times in the Plesiosaur lineage and each form probably had different lifestyles. Which morphotype do you think was specialized for which feeding behaviors? A. Both morphotypes ambushed small prey. B. Both morphotypes chased down large prey. C. Elasmosauromophs ambushed small prey and Pliosauromorphs chased down large prey. Or D. Pliosauromorphs ambushed small prey and Elasmosauromorphs chased down large prey. Elasmosauromorphs, with their pointy teeth and long necks, were most likely fish eaters. The long neck allowed them to snap up their prey without having to chase them. The small heads and thin necks would've prohibited them from eating large prey. Pliosauromorphs, with their short necks and large flippers, had high speed agility and the ability to dive. They chased down their prey and would have used their large, powerful jaws to dismember it. So the correct answer is C. The Elasmosauromorphs with their long necks and their high aspect ratio flippers were specialized for long distance cruising. And their fine pointed, recurved teeth were used to catch and hold on to small slippery prey. As we discussed earlier, it's now accepted that the long-neck plesiosaurus almost certainly swam with their necks stretched out in front of them. Fish encephalopods might not have recognized the relatively small heads as much of a danger. Since the large, threatening bodies would've been so much further away. Fossilized remains of plesiosaurus stomach contents show that their most common prey items were fish and soft bodied cephalopods. But other small prey items have been found including small pterosaurs, and even an ichthyosaur embryo. The short necked pliosauromorphs are thought to have been diving hunters. Like modern sperm whales, they would have been the predominant marine predators of their time. Their flippers, which could be as long as two meters, had a low aspect ratio specialized for power, maneuverability, and prodigious speed. The bigger flippers also provided the downward acceleration needed to dive. Their massive, stubby, conical teeth are characteristic of cephalopod feeders, but would have also been efficient at dismembering large prey by shaking and twisting them. This diving lifestyle is supported by the presence of avascular necrosis in their limb bones. This suggests that they suffered from decompression syndrome, also known as the bends. Fossilized stomach contents show that they were likely opportunistic hunters, eating mostly fish and cephalopods. But sharks, ichthyosaurs, and other plesiosaurs are also found in their stomachs. Even dinosaur bones have been found in the stomachs of pliosaurs. This was likely from a floating carcass, since pliosaurs almost certainly would not have hunted land bound dinosaurs. One thing found in the stomachs of all types of plesiosaurs, regardless of what they ate, are gastroliths, sometimes called gizzard stones. Plesiosaur fossils are very often found associated with a mass of these small, smooth stones in their abdominal region. Why would plesiosaurs swallow them? A, Their bodies needed the minerals found in the rocks. B, They ate the rocks when they couldn't find real food. C, They needed them to aid in the digestion of their prey. Or D, They accidentally swallowed them when they ate prey off the ocean bottom. Today gastroliths can be found in the stomachs of lizards, fish, crocodilians, turtles, snakes, mammals, and birds, where they help break apart large pieces of food that the animals swallow. So, the correct answer is C. The minerals an animal needs to survive are usually obtained from its food. And the animal would not eat rocks if they were hungry, as they would not make the animal less hungry. Finally, the gastroliths are too large to have been accidentally swallowed. So, A, B, and D are not correct. Some scientists think the stones may also have acted as ballast and helped achieve neutral buoyancy. Some paleontologists have guessed that the plesiosaurs actively swallowed stones or regurgitated them depending on whether they wanted to float or sink. However, with a five to six meter long neck, it was probably very difficult to regurgitate stones. And in the open ocean, it would be hard to find the appropriate rocks every time they wanted to dive. It seems unlikely that they were using them to actively control buoyancy and so they were probably digestive aids only. Since all plesiosaurs were active predators, they must have had highly developed sensory systems in order to locate their prey items. Like the Icthyopterygians, they had bony rings inside their large eyes. Possible functions include supporting the eyeball under the pressure of deep water, and facilitating quick changes in focus and aperture. Just like using the focus knob on binoculars. This adaptation would have made them extremely effective visual predators in dark or murky water. Plesiosaurs, like many aquatic animals, possessed no ear drum, which is an adaptation to hearing airborne sounds. Aquatic animals hear by direct transmission of the vibrations to their inner ear through their skull, and therefore do not require the specially adapted ear drum. Smelling under water might have been facilitated by scoop shaped openings in the roof of their mouth that directed water into channels where scent receptors were located. And then out through the external nostrils. This arrangement might have enabled a sense of smell allowing them to hunt by smell, like a shark. Reproduction and sociality are harder to understand than locomotion and feeding, because they don't leave direct skeletal evidence. For a long time, paleontologists have debated whether Sauropterygians were oviparous, meaning they laid eggs or viviparous, and gave live birth. Many paleontologists thought that like modern sea turtles, plesiosaurs might have come ashore to lay their eggs. But, with enormous flippers, a massive body, and in some cases an extremely long neck or oversized head, they would have been extremely awkward out of the water and very vulnerable to predators. In fact, both pliosaurmorphs and elasmosaurmorphs had such massive heads and necks, that they couldn't even have lifted them out of the water. It's extremely unlikely that plesiosaurs ever left the water. But, without solid proof, the egg laying hypothesis persisted. Other paleontologists thought that like ichthyosaurs, plesiosaurs gave birth to live young underwater. If they did, it would mean that these animals would be freed from ever having to go onto land. But for a long time, no evidence of viviparity was found. Now, thanks to the recent discovery of three remarkable specimens, we know that both basal and derived Sauropterygians were viviparous. Two pregnant specimens of a pachypleurosaur, <i>Keichosaurus</i>, were found with embryos in the abdominal cavities, indicating that basal Sauropterygians gave live birth. A subsequent discovery of a pregnant plesiosaur fossil from Kansas in 2010, show that derived plesiosaurs also gave birth to live young. The impressive specimen is a five meter long <i>Polycotylus</i> mother with a single one and a half meter long fetus inside her abdomen. This is unusual, as all other Mesozoic marine reptiles had several babies. The plesiosaurs appear to have gestated only one big baby at a time. Here's some examples of modern animals that generally give birth to a single live offspring. What features do they have in common? Choose all that apply. A, They live in social groups based on extended families. B, They are predators. C, Offspring are provided with a lot of parental care. And/or D, They mate for life. Not all of these examples are predators. Baleen whales for instance are filter feeders. So B is not correct. Only some whales, penguins, and humans mate for life, so D is not correct. However, all these animals that give birth to a single large baby tend to provide them with a lot of parental care. They also commonly live in social groups based on extended families. This evidence suggests that plesiosaurs may have cared for their young and that they could have lived in pods like modern dolphins. There has been no direct evidence of parental care or social living, but it is an interesting idea. So the correct answers here are A and C. A juvenile plesiosaur recovered from South Dakota provides additional indirect evidence for parental care in plesiosaurs. Cretaceous South Dakota was very far from shore. It would have been dangerous for a small predator to be in the open ocean on its own. And so, it is possible, that a parent would have accompanied the juvenile. Further inferred evidence for reproductive behaviour can be found in South Australia, which is a great place to go if you are looking for baby plesiosaurs. Interestingly, there is a high proportion of juveniles found here, more than anywhere else in the world. This great number of babies suggests that south Australia could have been a breeding ground where pregnant mothers would've gathered to give birth. They may have been attracted to these waters because of high concentrations of plankton, fish, and squid. As you have seen, plesiosaurs were uniquely evolved to overcome the aquatic problem. They did not evolve the more commonly used axial locomotion, but retained the locomotive capabilities of all four limbs. There is considerable debate over how these limbs functioned, but most paleontologists support the idea of modified underwater flight. The maneuverability of the Elasmosauramorphs would have been affected by the aspect ration of their flippers, and also by their long neck, likely resulting in an ambush strategy targeting small fish. Pliosauramorphs, freed from the constraints associated with the long neck, were able to actively pursue and consume larger prey. Like the ichthyopterigians, the sauropterygians developed viviparity early in their evolutionary history, but likely invested a higher degree of parental care into fewer offspring. The second major group of marine reptiles solve the aquatic problem in a completely unique way, which has not been replicated by any modern animals. Still, some elements of their anatomy and behaviour converge upon solutions we have seen in other groups. Clearly, though unique, their adaptations ended up being highly successful, resulting in a 180 million year reign, which was the longest of any of the predatory marine reptiles.
