To some extent, the patterns of sauropterygian evolution and distribution closely resemble those of the ichthyopterygians that we explored last lesson. These groups shared the seas during the first 150 million years of sauropterygian evolution, before the icthyopterygians went extinct during the mid-Cretaceous. Sauropterygians originated and diversified during the early Triassic. The near-shore forms went extinct by the end of the Triassic. Fully aquatic forms dominated during the Jurassic. And a small number of derived taxa hunted the Cretaceous seaways until their extinction. Let's go back to our maps to explore the patterns of sauropterygian distribution, and some famous fossil localities. The sauropterygian story starts by the end of the early Triassic, in the shallow coastal waters of the Tethys Sea. Their development appears to have been closely tied to global sea level rise and the resulting continental flooding during the triassic. The abundance of separate marine basins favoured <b>endemism</b>, or the evolution of unique species in different localities. Placodonts, nothosaurus, pachypleurosaurs, and pistosaurs all inhabited the same shallow seaways during the mid-Triassic, at the time when the first dinosaurs evolved on land. This variety of similar looking basal sauropterygians managed to live at the same time without directly competing with each other, and this was probably accomplished by endemism and <b>niche partitioning</b>. The animals would either had to live in slightly different places or would've had to specialize on different resources and niches within the same environment. By the middle Triassic, all of the basal sauropterygian lineages were well adapted to the aquatic environment and looked very distinct from each other. This means all the basal sauropterygian lineages probably originated in the early Triassic. But for some reason, fossil evidence of these groups from this time is rare. Can you think of any reasons why their fossils are not found until stratigraphically younger rock layers? Check all that may apply. A, early Triassic strata are rare or poorly explored. B, early Triassic sauropterygians were likely small, and smaller animals do not fossilize as well. C, early Triassic sauropterygians lived in a near-shore environment, which does not favour the preservation of articulated fossils. D, early Triassic sauropterygians did not have bones and would not have fossilized easily. Nothosaurs, pachypleurosaurs, placodonts, and pistosaurs are morphologically distinctive and diverse by the middle Triassic. So we assume that they evolved early, even if we have not yet found the fossils to support this. These groups are all vertebrates, so they would have had bones that could have fossilized, so D is incorrect. However, the earliest sauropterygians were small, with fragile bones that are less likely to be preserved. This is especially true in the turbulence of shallow water environments, so B and C are correct. The lack of fossils from the early Triassic could also represent a collection bias. The strata that were deposited at that time could have been eroded away by subsequent geological processes. Less of those strata could currently be exposed, or these strata may be unexplored. Any of these cases would lead to an artificially low number of known fossils from that time. Therefore, A is correct as well. Additionally the early Triassic is a much shorter time period than the middle or late Triassic. So, less rock was deposited during this time. By the middle Triassic the first placodonts had appeared around present day Israel. They diversified and spread around the western Tethys Sea to the Germanic Basin and the Alps. Pachypleurosaurs and nothosaurus also became abundant around the Tethys Sea and the western region of the developing Pacific Ocean in what is southeastern China today. The abundance of extinct reptile fossils has long been known in the Guizhou province of China, though for centuries they were considered to be petrified dragons and were symbols of good fortune. The eastern Pacific stretched to modern day Wyoming, and pistosaurs are known from deposits from those basins as well as the western Tethys Sea. During the middle Triassic, all sauropterygian groups were taxonomically diverse and morphologically disparate. However, an extinction event at the end of the late Triassic wiped out the placodonts, nothosaurus, and pachypleurosaurs. Most pistosaurs went extinct as well. The only group to survive was the Plesiosauria, which originated during the late Triassic. These sauropterygians were larger and better adapted to living and feeding in the open ocean. This likely helped them survive the sea level fluctuations at the end of the Triassic. By the beginning of the Jurassic the sauropterygian family tree was pruned to a single lineage, the plesiosaurs. Plesiosauria diversified dramatically during the Jurassic, from the basal <i>Archaeonectrus</i> and <i>Attenborosaurus</i>, which are found in the United Kingdom. <i>Plesiosaurus</i> is also known from the United Kingdom but is from a slightly younger deposit known as the Blue Lias, which is exposed in the sea cliffs of Lyme Regis, where Mary Anning made her discoveries. One of the most diverse and specialized clades of plesiosaurs, from the early Jurassic, was the Rhomaleosauridae, which were the first plesiosaurs to achieve a truly worldwide distribution. By the middle Jurassic, the massive Pliosauridae had replaced Rhomaleosauridae as the top predators in the world's oceans. Plesiosaur fossils are found in Australia, France, India, United Kingdom, and the United States. Pliosaurs were not alone in the middle Jurassic seas because the cryptoclidids also diversified during this time. The Oxford and Kimmeridge Clays of the United Kingdom are Middle and Upper Jurassic strata that were heavily mined for building materials. Miners unearthed a multitude of fossils of dinosaurs, thalattosuchians, and excellently preserved plesiosaurs. These strata preserved abundant cryptoclidids and pliosaurid fossils such as <i>Cryptoclidus</i> and <i>Paleoneustes</i>. During the late Jurassic, global sea level rise resulted in the formation of an interior seaway, the Sundance Sea which bisected North America. The Sundance was home to two species of cryptoclidids and one pliosaurid. One cryptoclidid, <i>Tatenectes laramiensis</i>, appears to have been particularly well adapted to shallow water. Its body was wide and flat, kind of like a sea turtle, which might have helped it to stay upright in strong currents. The formation of the Sundance Sea is the first time in the history of sauropterygians that the major sea way covered the continent of North America. So, it makes sense that the first evidence of Canadian sauropterygians can be found from the late Jurassic rocks. In fact, in 2008 a new species of rhomaleosaurid called <i>Borealonectes</i> was discovered on one of the Arctic Islands of Canada. However by the end of the Jurassic, erosion and tectonic uplift from the formation of the Rocky Mountains filled in the Sundance Basin, returning it to terrestrial habitats. Cryptoclidid plesiosaurs went extinct by the end of the Jurassic in the marine Tithonian extinction, though some species of Pliosauria survived into the Cretaceous. Early Cretaceous marine deposits are poorly known, but plesiosaurs from this time are represented by species of leptocleidids from Germany and the United Kingdom. The middle Cretaceous saw the development and diversification of the extremely long-necked elasmosaurids. The two earliest elasmosaurid species are known from Columbia and Australia. Leptocleidids such as <i>Nichollssaura borealis</i> from Alberta, were largely replaced by polycotylids during the middle Cretaceous. One of the last pliosaur mega-predators is <i>Brachauchenius</i>, who measured in at more than 11 meters, and lived during the earliest part of the Late Cretaceous. Even though it's one of only two pliosaurids to survive into the late Cretaceous, these animals still went extinct long before the end-Cretaceous extinction event. Sea level fluctuations during the Cretaceous caused the rise and fall of the western interior sea many times. Inhabiting this sea were polycotylids like the three-meter long <i>Trinacromerium</i>, who possessed the longest flippers ever known, equal in length to its torso. These long flippers allowed it to reach unprecedented speeds. Polycotylids shared the sea with other record-holders, like the long-necked elasmosaurids, such as <i>Elasmosaurus</i> from Kansas and <i>Albertonectes</i> from Alberta. Canadian Cretaceous sauropterygians represent the community of the northern half of the western interior sea, which covered most of what is now Western Canada. Multiple species of elasmosaurid, pliosaurid, and polycotylid remains have been found in late Cretaceous strata from all of the western Canadian provinces. These discoveries include the giant-flippered <i>Trinacromerium kirki</i> from Manitoba and <i>Albertonectes vanderveldei</i> from Alberta, which has the most cervical vertebrae of any known elasmosaur. The abundance of elasmosaur fossils from the late Cretaceous Canada reflects their diversity all around the globe. During this time, elasmosaurs achieved worldwide distribution, and their fossils have been found on every continent including Antarctica. Interesting finds, like <i>Cimoliasaurus</i> from the northwestern territories of Canada tell us that plesiosaurs almost certainly went extinct, due to the end-Cretaceous mass extinction event. <i>Cimoliasaurus</i> is found in rock layers that date from the very end of the Cretaceous so we know that at least some lineages of Plesiosaurs lived up until this time. The late Cretaceous mass extinction caused the extinction of all Plesiosaurs, 66 million years ago, ending sauropterygian dominance. Based on what you know about the law of superposition and biostratigraphy, and what you have learned about the evolutionary history of sauropterygia, which of the following three stratigraphic columns correctly depicts how assemblages of sauropterygian fossils might be found? Think back to what you know about sedimentary rocks. The law of superposition states that in undisturbed sedimentary rocks, the younger layers are deposited later than the older layers and must be found above them. You have also learned when different groups of sauropterygians lived. So you should have identified the stratigraphic column in which the most primitive sauropterygians are found in the lowest marine layer. Placodonts and nothosaurus are basal sauropterygians and are found in the oldest layers at the bottom. Cryptoclidids and pliosaurids lived during the Jurassic, and elasmosaurs and polycotylids inhabited Cretaceous seaways at the top of the stratigraphic column. Therefore, stratigraphic column A is correct. Sauropterygia was the longest lived lineage of the extinct marine reptiles. They showed a huge variety in form and function, from the durophagous turtle-like placadonts of the Triassic to the long-necked fish-eating elasmosaurs of the late Cretaceous. We now know that the two main body plans, seen in the derived plesiosaurs, the pliosauromorphs, and the elasmosauromorphs evolved multiple times and do not necessarily indicate close relationships between species. Plesiosaurs were unusual among secondarily marine tetrapods in that they did not rely on axial locomotion, but rather retained four functional limbs that they likely moved in a modified flight pattern. The pliosauromorphs in particular likely used these hydrofoils to generate impressive speeds to pursue their prey. Strong evidence has been found that plesiosaurs gave birth to live young and likely invested a great deal of effort into raising a single offspring at a time. Plesiosaurs eventually colonized the world's oceans, which they shared with the ichthyopterygians for the first 150 million years and the mosasauroids, which are the subject of the next lesson, for the last 30 million years of their existence. Their fossils are found on every continent and likely inspired many myths and legends. The lives of these animals were no less wonderful than the stories they inspired. I hope you enjoyed this lesson on sauropterygians and their evolutionary journey. Let's move on to the final lesson, where we will be discussing the evolution, ecology and diversity of the Mosasauroids: the last major marine reptile group to rule the seas.
