Welcome to lesson three. In the last lesson, we looked at the characteristics and diversity of ichthyosaurus and their close relatives. In this lesson, we're going to explore another group of aquatic reptiles, the sauropterygians. The sauropterygians are best known for their derived members, the plesiosaurs. Like the ichthyosaurs, the first sauropterygians appeared in the early Triassic. And these early forms were small, semi-aquatic animals with small heads, elongate bodies, and four limbs adapted for paddling around in shallow water. By the end of the Triassic, they had grown into fully aquatic animals, incapable of returning to shore. Throughout the Jurassic and the Cretaceous, these reptiles developed a large number of diverse specialized body plans adapted for life in the water. The last members went extinct with the dinosaurs at the end Cretaceous mass extinction. We'll begin with the basic questions about these animals: what were they and what did they look like? Based on other aquatic tetrapods you have seen so far in this course, where do you think the head was positioned on this animal. Did it have a short neck and a long tail or a long neck and a short tail. If you pick the Plesiosaur with the short neck and long tail you chose the arrangement that is most convergent among aquatic tetrapods. Many aquatic tetrapods you have seen so far, such as ichthyosaurs, thalattosuchians, and whales lengthen their tails for use as a propulsive structure. However, the plesiosaurs evolved a different solution to the aquatic problem. Their tails got quite short and in some cases, their necks grew to extraordinary lengths. So the long neck, short tail configuration is correct here. But don't feel like you missed something if you got it wrong. One of the most famous paleontologists in history, Edward Drinker Cope also put the head on on the wrong end of a plesiosaur specimen before putting it on display. When his bitter rival, Professor Othniel Charles Marsh, gleefully pointed out his error, Cope was so embarrassed he blamed a colleague at the museum for his mistake. The Sauropterygia includes the long neck plesiosaurs and a whole host of other interesting Mesozoic marine reptiles that don't usually receive as much attention in the popular media. Like the ichthyopterygians, sauropterygians have been somewhat hard to place, on the tree of life. The general consensus today is that sauropterygians are a separate lineage of diapsids. But in some phylogenetic analyses, they are grouped in the lepidosauromorpha. Take a moment to familiarize yourself with the placement of the sauropterygians in relation to the other groups we have looked at so far. Use the interactive phylogenetic tree to find the sauropterygia and pay close attention to where they sit in relation to the ichthyopterygians, lepidosaurs, and archosaurs. The word sauropterygian means “winged lizard” which refers to the paddle-like limbs common to the group. Sauropterygian share several other characteristics in their skull and skeleton. Sauropterygian skulls usually have retracted nares close to the orbits, and this is an adaptation for air breathing swimmers. Like ichthyosaurs, they have <b>euryapsid</b> skulls, meaning the lower temporal fenestra has been lost and only the upper one remains. However, the euryapsid condition has been achieved very differently in each group. Think back to the lesson on ichthyosaurs. These animals evolved the euryapsid condition due to a reduction in the postorbital region. In sauropterygians the euryapsid morphology results from the loss of the bony bar that forms the bottom edge of the lower temporal fenestra. And this leaves them with an archlike space and a complete upper temporal fenestra. The loss of the lower fenestra has been used to suggest common ancestry of sauropterygians, ichthyopterygeans, and thallatosaurs, but it is now understood that the lower fenestra was lost independently in these groups. Sauropterygians can also be identified by their distinctive pectorial girdle. The scapula, or shoulder blade, partially overlaps the outside of the clavicles or collar bones. While in other amniotes the clavicles overlap the outside of the scapula. Those that study the sauropterygians are still not sure why this adaptation occurred and what function it provided. Despite these shared features, sauropterygians evolved into a wide variety of body shapes, enabling them to occupy many different ecological niches throughout their evolutionary history. We will explore these in more detail in the next few minutes. As we mentioned, sauropterygians came in various shapes and sizes, which indicates that they had a variety of ecological specializations. See if you can apply what you have already learned in the past two lessons and decide whether each of these basal sauropterygians would have been fully aquatic, semi-aquatic or terrestrial. Select each animal and drag it over to its most likely environment. So let's go through each of these basal sauropterygians to see where they should be placed. First, the placodont, don't let the legs fool you. Placodonts with their compressed bodies and unique toothplates were specialized bottom feeders in shallow marine settings. Even though it may look like placodonts might have been able to climb out and walk, paleontologists think that their entire life cycle could have been performed in the water, making them aquatic. Nothosaurs did not have the specialized flippers of strong swimmers, so we think they were likely semi-aquatic. With their highly derived flippers, it is likely that pistosaurs were fully aquatic and couldn't have gone onto land at all. Pachypleurosaurs, like the nothosaurs did not have the specialized flippers of strong swimmers. Additionally, pachypleurosaurs had thickened bones, characteristics of animals that live in near shore settings. That means that the pachypleurosaurs were most likely semi-aquatic. As all this animals have some degree of aquatic adaptation none of this would have been fully terrestrial. Now let's go into more detail on these basal sauropterygian groups. Let's begin our exploration of the major sauropterygian groups with the most primitive aquatic form, the placodonts. These animals first appeared in the Early to Middle Triassic during a period of global sea level rise. Placodonts tended to have one of two body plans, or <b>morphotypes</b>: some were barrel-shaped and others were wide and flattened. Both had fairly rigid bodies and must have used their webbed feet and laterally compressed tails to propel them through the water. Placadonts were likely durophagous, feeding on hard shelled mollusks with their distinctive dental morphology. They had large, wide and flattened teeth found not just along the edges of their jaws but also across the roof of their mouths! <i><b>Placodus</b></i> is a good example of some of the specialization of the placodonts. It was 2 meters long with a massive skull, gastralia and spines down it's back. It looked a bit like an over grown buck-toothed iguana. It had large shovel-like teeth jutting forward out of its jaws and probably use them to pry shellfish off of underwater rocks, before crushing them like a nutcracker between the heavy plate-like teeth on its palate and lower jaw. On the other end of the spectrum, <i><b>Henodus</b></i> was a flattened armored placadont that was as long as it was wide. It had protective armour on its ventral and dorsal sides, composed of bony plates, covered in horn, and looked superficially similar to a turtle. This is yet another good example of convergent evolution. The most intriguing thing about <i>Hedonus</i>, it that it was almost entirely toothless, but grooves in the jaw supported a baleen-like material, similar to filter-feeding whales. Like the baleen whales of today, it probably would have eaten zooplankton or small fish. What is this kind of feeding called? If you're having problems determining which one you should select, think back to the lesson on ichthyopterygian feeding. A, manipulation feeding, B, lunge feeding, or C, ram feeding. Remember that manipulation feeding involves the animal using its teeth to bite, tear, or scrape its prey. So A is incorrect. Ram feeding and lunge feeding are both types of filter feeding, where small prey is filtered out of the water using specialized structures. In fish, the prey is filtered out of the water using specialized structures on their gills. This is called ram feeding, so C is incorrect. Amniotes do not have gills and so they have to take in a mouthful of water and push it back out of specialized structures that have replaced their teeth. This is called lunge feeding, so B is correct. The next most derived group of sauropterygians to evolve during the Triassic was the <b>Nothosauroidea</b>. They had slender bodies, long necks and tails, and webbed feet. They flourished in Europe during the middle Triassic and were probably able to come out of the water like modern seals. Their nostrils were not at the tip of the nose like most reptiles, but were set further back suggesting at least a semi aquatic lifestyle. The <b>pachypleurosaurs</b> may have been a subgroup of nothosaurs, a sister group to nothosaurs, or more likely, a series of species that lead towards more derived sauropterygians, including the nothosaurs and plesiosaurs. Pachypleurosaur means “thick ribbed lizard” and the name reflects the thickened ribs of these sauropterygians. They're found in European and Chinese deposits from the middle Triassic. They have a wide range of sizes from 20 centimeters to 4 meters long. Pachyplaurasaur skeletons have poorly ossified limb girdles suggesting that they could not walk very well. However the feet had not become specialized into flippers suggesting they probably moved by undulating their long tails. It's possible that the pachyostotic ribs provided neutral buoyancy and stability in the water column. They had small heads, moderately long necks, and small, cone like teeth, that suggest they ate fish. <b>Pistosauroidea</b> is thought to be the sister taxon of the derived Plesiosauria. Now only a few pistosaurs have been found in Triassic strata, including localities in Germany and China, but these rare fossils suggest that pistosaurs represent an intermediate form between the nothosaurs and the plesiosaurs. They had a nothosaur-like body and a plesiosaur-like head. Pistosaur limbs have wide, more expanded shoulder and hip girdles, and their fingers show some hyperphalangy. This indicates that pistosaurs were better adapted to an aquatic life than the nothosaurs and pachypleurosaurs that came before them. Placodonts, pachypleurosaurs, nothosaurs, and pistosaurs show a huge variety of body plans and specializations and likely filled a wide array of niches in the Triassic oceans. Similar to what you learned about ichthyosaurs in the last lesson, the end Triassic extinction event led to an extreme drop in sauropterygian species' diversity. Only one clade survived into the Jurassic, but this lone group of survivors went on to become very diverse and successful: the plesiosaurs. The name plesiosaur means “near reptile”, because it was recognized that they were more like saurians, or lizards, than the fish-like ichthyosaurs. These creatures evolved into dominant roles in the Mesozoic seas and were powerful predators that evolved around two basic body plans.
