They look like the Loch Ness Monster. We got that out of the way.
In this writer's opinion, plesiosaurs are some of the strangest vertebrate animals to ever live. One the split between reptiles more closely related to lizards and snakes (Lepidosauromorphs) and those more closely related to birds and crocodiles (Archosauromorphs), plesiosaurs fall closer to the lizard side of things. But they don't really look like lizards at all, nor are they terribly closely related to them. Most of the typical plesiosaurs look like someone grafted a snake into the shell of a sea turtle. But there is some degree of variation in body shapes in this group, including ones that looked more like reptilian killer whales than anything else. These creatures were the top reptilian predators of the Mesozoic oceans until giant mosasaurs like Tylosaurus appeared on the scene. Lets dive in (yes that was intended) and learn what we can about the strange, wonderful plesiosaurs.
We know that modern whales came from mammals that went back to living in the ocean (I say "went back to" because all tetrapods come from fish that evolved to live on land), and became highly specialized for living only there. Returns to the ocean are not an unheard of occurrence in vertebrate evolution. In the mammals it happened several times- with the cetaceans (whales/dolphins), the sirenians (manatees/dugongs), and the pinnipeds (seals/sea lions/walrus), as well as to some degree with hippos and Desmostylians. In reptiles, this phenomenon has occurred numerous times. Due to their slower metabolisms, reptiles do very well living in water. The temperature of warm oceans is generally consistent, they can exploit various food sources, and can hold their breaths for long periods of time. This has led numerous groups of reptiles to return to a life in the ocean to exploit open niches present there, an few became so specialized to it as the plesiosaurs. While many aquatic reptiles led a more amphibious lifestyle, returning to land to breed or rest, the more evolved plesiosaurs led their entire lives in the water, much like whales do today. And while reptiles can indeed return to the water relatively easily, plesiosaurs went through a variety of dramatic changes from their ancestral form.
Plesiosaurs, as previously mentioned, have a body much like a sea turtle. They didn't have shells like a turtle exactly, but many other similar features are found in this group. The most obvious similarity is the flippers. It's hard to believe that the turtle-like flippers of a plesiosaur are made up, internally, of the exact same bones found in the limbs of humans. In the front limbs, their upper arm bones are very large, much more so than the lower arm bones, indicating a powerful locomotive ability. Plesiosaurs, in contrast to the ichthyosaurs and mosasaurs, powered their swimming not with their tails (which were rudimentary at best) but with their limbs. Most of the flipper is composed of the "finger" bones, which have become extremely elongated. The digits and wrist make up about half of the length of the limb by themselves. These proportions are similar in the hind limbs. These long, pointed flippers are what was used to paddle through the water. It seems like plesiosaurs were very strong, agile swimmers, and did a lot of moving around. Unlike many other marine reptiles, the trunk region of plesiosaurs was very rigid. The pelvis and chest of these animals were extremely broad and flat, and connected together by a strong network of rib-like bones running across the belly called gastralia. This means that the whole bottom end of plesiosaurs was rigorously sturdy, and gave the animal a barrel-shaped trunk. This may have acted as a bracing device for the body while paddling.
The most obvious and famous aspect of plesiosaurs is their long neck. Actually, not all of these animals had this trait. Scientists split plesiosaurs into two general groups: the long necked plesiousaurids, and the short-necked pliosaurids. We'll talk about the former group first. Anyway, on to that neck. First of all, forget any misconceptions you may have about these animals raising their head out on an s-shaped neck far out of the water, the way a swan does. Plesiosaurs actually seem to have had very little flexibility in their neck, and were just able to break the water's surface with their heads to catch a breath of water. It seems as though wherever the head and neck went, the body would follow. Early paintings of plesiosaurs tend to show its head and neck writhing through the water like a snake to catch fish, but this was also impossible. The plesiosauroids had relatively short, round snouts and smaller, flatter heads than their pliosaur counterparts. Their teeth were many, long, and pointed, suggesting that these animals fed mostly on fish and molluscs. Most scientists view these animals as relatively slow yet powerful swimmers, snatching up unwary fish that swam within range of their necks (which, in animals like Elasmosaurus, could get mind-bogglingly long).
The other main group is the pliosaurids. In contrast to their long necked, short faced cousins, pliosaurs had short, stocky necks and long pointed snouts. They had similar teeth to the plesiosaurids and the smaller species were likely fish eaters, but the larger ones would likely have gone after bigger prey. And pliosaurs could get very big. Animals such as Liopleurodon, Pliosaurus and Kronosaurus could raged from 30-50 feet long. Unlimited by the constraints of land, these creatures grew massive and would feed on large fish (including sharks) as well as other marine reptiles. Pliosaurs were fast, active hunters and dominated the oceans until the mosasaurs showed up to challenge their reign later.
What we aren't too sure about yet is how exactly plesiosaurs moved their great flippers. Two schools of thought exist- one suggest that plesiosaurs "flew" through the water, flapping their flippers up and down in a figure-eight pattern, somewhat like how a sea turtle or penguin swims. The other theory states that plesiosaurs "rowed" their way through the oceans, the same way a sea lion uses its flippers to row itself along. Both theories have their supporters and detractors, and both certainly have their merit, but neither seems terribly more accurate than the other, as the pectoral and pelvic girdle anatomy has some physical traits that make either theory, in some aspects, seem unlikely. It could very well be that some sort of movement, unseen in modern animals, was utilized by plesiosaurs, but no one is sure what exactly that could be.
The reproductive adaptations of plesiosaurs are no less interesting than the anatomical ones. It's fairly safe to assume that, much like ichthyosaurs, plesiosaurs gave birth to live young in order to get around the problem of being an egg-layer living in the ocean. Scientists first envisioned them as reproducing like sea turtles- crawling out onto beaches and laying their eggs there (there are still a small number of scientists who support this theory). While this may have been feasible for smaller, more primitive plesiosaurs, the much larger ones would have been unable to get into land at all, due to their immense bulk and inability to propel themselves on land. While we only have physical evidence of ichthyosaurs giving birth to live young, it's reasonable to assume that plesiosaurs evolved this trait independently as well. Plesiosaur fossils have been found with single, large, well-developed offspring inside their body cavities, which supports the theory that the young hatched within the mother and emerged shortly after.
The history of the study of plesiosaurs is nearly as interesting as the animals themselves. Discovered and named years before the term Dinosauria was given to a certain group of terrestrial Mesozoic archosaurs, the first plesiosaur fossils were discovered in England. In a time where women were unfortunately excluded from most professional activities in English society, the first plesiosaur bones were unearthed by a Dorset woman named Mary Anning, who discovered not only the first plesiosaur, but the first animal to be identified as an ichthyosaur. Anning, who had great knowledge and talent at anatomy and fossil collecting, made countless other advances and discoveries in the fields of biology and palaeontology, in spite of her low social class and gender. While not allowed to partake fully in the dealings of the scientific community at the time, Anning worked tirelessly at collecting and preparing fossils from the seaside cliffs, which allowed others to advance the knowledge of extinct species. I leave you with the request that, whenever you see a plesiosaur or any other beautiful marine reptile, remember Mary Anning, and all the other incredibly bright women who intrepidly contributed to science in spite of the obstacles laid against them.