Based on what you know about how comparative morphology is used to infer phylogenetic relationships, we want you to think about which of these sauropterygians are most closely related to each other. If you were to group them into two pairs of sister groups. How would you do it? Would you group together the individuals with the small heads and long necks, and the animals with large heads and short necks? If so, you would recreate the morphology base groups that paleontologist used for over a 150 years. The long necked <b>Plesiosauria</b> and the big headed <b>Pliosauria</b>. It seems like the most straightforward answer would be to group the fossils that most closely resemble each other but when biology and millions of years of evolution are involved, the situation is not always that simple. Detailed study of more subtle and complex characters, such as the structures of the bones in the skull and the limb girdles, suggest that the plesiosaur lineage looks more like this. As you can see, the pliosaur morphotype evolved multiple times. In addition, the plesiosaurs with the extraordinary long necks associated with the elasmosaur morphotype also appeared in numerous clades. So, as we have discovered, these two morphotypes reflect the true relationships very poorly. When paleontologists want to talk about groups of animals that look the same, but they're not related, we can talk about them by grouping them into morphotypes, or groups that look similar. In plesiosaurs, there are two main morphotypes. The first are the <b>pliosauromorphs</b>. These are the plesiosaurids with a pliosaur morphotype. They have short necks and big heads with jaws full of large teeth. The second morphotype are the <b>elasmosauromorphs</b>, the plesiosaurids with the extraordinarily long necks and small heads. Notice, that groups referring to morphology and not relationships always end in morph. The <b>Plesiosauria</b> is a clade of derived sauropterygians that is nested within <b>Pisotosauria</b>. The Pisotosauria is the clade composed of the Pistosauroidea and Plesiosauria. Fossils of a Plesiosauria have been found from early Jurassic through late Cretaceous strata. All members of this clade had four long narrow flippers connected to the limb girdles, which were modified into characteristically broad sheets of bone to which the powerful swimming muscles were attached. Another <b>diagnostic character</b> of plesiosaurs which differentiates them from other marine reptiles is a pair of holes on the ventral surface of the cervical vertebrae. And these holes are called the <b>subcentral foramina</b> and they allowed the nerves to pass through. A final feature shared by plesiosaurs are <b>gastralia</b>. Along with the ribs, these bones would have enclosed the torso in a shield of internal body armor. To practice identifying plesiosaur fossils, select the fossil in each pair that comes from a plesiosaur. Were you able to recognize the diagnostic character of plesiosaurs? Remember to look for large plate like girdles as seen in girdle A, paired subcentral foramina, as seen in vertebrae B, and a dense basket of gastralia as seen in body A. Now that you know how to identify a plesiosaur, let's continue on to the diversity within Plesiosauria. <i><b>Archaeonectrus rostratus</b></i> and <i><b>Attenborosaurus conybeari</i></b> are two primitive plesiosaurs from the early Jurassic, that fit outside the two major plesiosaur superfamilies, that we'll introduce you to in a moment. They represent an intermediate form between the basal sauropterygians we just looked at, and the derived taxa we will introduce you to next. Even though both of these species were probably fully aquatic they were not very specialized in any particular mode of life in the ocean. For example, while the limbs of <i>Archaeonectrus</i> show some hyperphalangy, they are not particularly large relative to the rest of the body, especially when compared to later, more derived taxa. On one side of the plesiosaur lineage, is a clade called the <b>Pliosauroidae</b>, which has the typical pliosaur morphology. This group is composed of two main families: the primitive, early to middle Jurassic, <b>Rhomaleosauridae</b>, and the more derived middle drastic to late Cretaceous pliosauroidea. All the pliosauroids have a big skull and a short neck, but can be differentiated from other pliosauromorphs by their lower jaws. In pliosaurids, the two-halves of the lower jaw are connected by a short joint between the massive front few teeth. The jaws were wide like a spoon to support these powerful teeth. Species within <i>Rhomaleosauridae</i> looked similar to the more primitive <i>Archaeonectrus rostratus</i> and <i>Attenborosaurus conybeari</i>. They had relatively long necks with 25 to 30 cervical vertebrae. They had short snouts and a fairly long tail of about 37 vertebrae. More derived characters include very large flippers and correspondingly large limb girdles. These would have provided muscle attachment sites to power those massive limbs. Species belonging to the Pliosauridae had massive heads, with long snouts and reduced numbers of cervical vertebrae. <i><b>Liopleurodon</b></i> is probably the most famous genus of this clade. Its size has often been exaggerated in public media, but at 7 meters it still was one of the largest predators in the Jurassic seas. A true giant of the clade is the Australian <i><b>Kronosaurus queenslandicus</i></b>, named after the mythical titan who ate his children. <i>Kronosaurus</i> was ten meters in overall length. The crowns of its teeth could exceed ten centimeters, but lacked the cutting edges of <i>Liopleurodon</i> and <i>Pliosaurus</i>, so it likely had a different diet. Let's explore that now. Different large pliosaurids had teeth that were specialized for eating specific prey items. Drag the prey items to the teeth that were best adapted to process that food. These teeth might not look terribly different from each other, but they do have features that would have made each of them better specialized to handle certain prey items. Tooth 1 looks half way between a cut and a pierce tooth. It is long, which is good for trapping fish and squid, but it also has cutting edges so it would have been able to slice up prey. This tooth is from <i>Liopleurodon</i>, which ate large fish, squid, and maybe other marine reptiles. Tooth 2 is definitely a cutting tooth. It has strong cutting edges so it would have been even better at piercing and slicing prey. It is from <i>Pliosaurus</i> which ate large fish and other marine reptiles. Tooth 3 is the largest, but it doesn't have cutting edges. It is long like a pierce tooth, but blunt and conical like a smash tooth. These teeth would have been good at immobilizing small prey, such as small marine reptiles, turtles, fish, and squid that the pliosaurid could have swallowed whole. This tooth belongs to <i>Kronosaurus</i>. These diets are supported by the discovery of preserved stomach contents found with the fossils of these pliosaurs. Take a moment to explore the interactive phylogeny to familiarize yourself with the diversity of these clades and see the differences between them. All other species of plesiosaurs fall into the <b>Plesiosauroidea</b>. This group shows a huge diversity in form, but all members can be identified by the lack of nasal bones, which are replaced by other bones in the skull. Within this group, the family <b>Plesiosauroidae</b> is typified by the original plesiosaur, <i><b>Plesiosaurus dolichodeirus</b></i>. Most early discoveries of plesiosaur fossils were assigned to <i>Plesiosaurus</i> which became a <b>wastebasket taxon</b>. A wastebasket taxon contains many unrelated species that being grouped together because they look vaguely similar or because scientists don't know where else to assign them. This means that the wastebasket taxon is not a true group, and so each species within the wastebasket taxon needs to be critically examined to see if it actually belongs. This is the case for <i>Plesiosaurus</i>. Not all the early taxonomy has been checked, so this clade is hard to define. On the opposite end of the morphology spectrum from the pliosaurids is the <b>Elasmosauridae</b>. Species in this Cretaceous clade had 40 or more cervical vertebrae. Besides the long neck, elasmosaurids can be identified by a wide indentation in the posterior edge of the pectoral girdle. Possibly the most striking of the plesiosaurs, and certainly one of the most famous is <i>Elasmosaurus</i>, one of the longest known plesiosaurs. Is at least 14 meters long with over 7.5 meters of neck, composed of 70 vertebrae, ending in a tiny head. However, <i>Elasmosaurus</i> lost the title of most cervical vertebrae in 2012 with the discovery of <i><b>Albertonectes vanderveldei</b></i> in Alberta, Canada. The animal slightly smaller than <i>Elasmosaurus</i> has a neck of similar length with a whopping 76 cervical vertebrae. The <b>Cryptoclidia</b>, meaning hidden clavicles, are named for their most distinctive feature. Their small, practically invisible clavicles that lie hidden in shallow depressions of the inner surface of the front limb girdle. This clade is quite diverse. It contains numerous species, showing a huge variety of morphologies. In fact, all the plesiosaur body plans from big heads to long necks can be found in one species of cryptoclidid or another, just not taken to the extreme of the pliosaurids or elasmosaurids. The wide range of morphologies in this group often led them to be misclassified over the years. However, as I've already explained, paleontologists have recently come to understand that the pliosaur and the elasmosaur morphologies arose multiple times. This realization has allowed paleontologists to more accurately reconstruct cryptoclidid relationships by focusing on subtle characters such as the arrangement of bones in the skull, the unique shape of the pectoral girdle, and the unusual nature of the clavicle. Three families are now recognized within Cryptoclidia: the basal Jurassic <b>Crypotclididae</b>, and the Cretaceous <b>Leptocleididae</b>, and <b>Polycotylidae</b>. Cryptoclididae is named after <i><b>Crytoclidus</b></i>, its <b>type genus</b>. The type genus is the genus that the description of the family is based on and named for. It was about 3 meters long and is known from a large number of individuals from the middle Jurassic of England. These plesiosaurs are quite common, and enough individuals have been found that we know what they would have looked like throughout their development, from very young individuals to very old ones. This makes <i>Cryptoclidus</i> one of the best- studied and understood plesiosaurs. If you've seen a plesiosaurus skeleton in a museum, there was a good chance it was <i>Cryptoclidus</i>. The Cretaceous members of the Cryptoclidia come from two families that can be found all over the world: Leptocleididae and Polycotylidae. Both families looked like pliosauroids, because they had large heads and short necks. However, when you look closely, their skulls show they aren't true pliosaurids. First off, they have distinctive crests, like this. Additionally, their lower jaws were very long and slender. Supporting narrow teeth, and the joint between the two-halves of the jaw is long. Pliosaurids had a short joint between the two-halves of their lower jaws and more robust teeth. Both families also had a triangular arrangement of bones in the roof of their mouth. Now they're common to all cryptoclids. Leptocleidus and polycotylids are also smaller than pliosaurids. Fossils of fully grown specimens are usually under three meters long. An Australian leptocleidid, <i><b>Umoonasaurus demoscyllus</b></i> received a great deal of attention upon its discovery in 2006 due to it's unusual mode of preservation. During fossilization, the bone was replaced with opal. I presented you with a summary of the diversity of the various clades of plesiosaurus, but of course, the diversity is much greater than we have time to present here. Feel free to take a moment to explore these groups in the interactive phylogeny. You've just learned about many different groups of sauropterygians. You may have noticed, that throughout their evolution, these animals became more and more morphologically disparate. Their basal members were generally long, slender, lizard like animals. The derived members, on the other hand, came in a variety of shapes and sizes. What does this pattern of morphological differentiation say about the roles they played in their environments over time? A, regardless of their shape, derived sauropterygians lived in shallow waters, and ate small fish, like the basal members. B, even though they were morphologically diverse, all derived sauropterygians were big, and therefore ate other marine reptiles. C, as sauropterygians became more morphologically diverse their behaviours and diets also became diverse. D, like the strange and diverse animals found in the deep oceans today, the extreme morphologies of the derived sauropterygians were only suited to deep oceans. A general trend in biology is that behaviour follows morphology. So animals that looked different will generally have different behaviours. The derived sauropterygians looked very different from each other and also from their basal ancestors. It's safe to assume that these animals have behaviours and lifestyles as diverse and different as their looks. So C is correct. A and B are both incorrect because they indicate that the derived sauropterygians all shared the same lifestyle. In addition, there may be some strange creatures alive at the bottom of the oceans today, but equally strange things live near the surface. These deep sea creatures are all primary marine animals. They don't need to breathe air like tetrapods. So D is also incorrect.
