Mosasaur evolution appears to have occurred in three distinctive waves, each better adapted to solve the aquatic problem than the one before. The first wave occurred in the Coniacian in the Western Interior Seaway, with the evolution of mosasaurs that had developed aquatic adaptations but still possessed terrestrial features. Mosasaurus continued to evolve growing larger, diversifying rapidly, and adapting solved the aquatic problem even better. About 83 and a half million years ago a second wave of new forms appeared near the beginning of the Campania. Following a possible decrease in diversity near the middle of the Campanian, mosasaurs rebounded and a third wave of highly aquatically derived mosasaurs more diverse and numerous than any before was just getting under way in the Maastrichtian, shortly before their extinction. For the last section, let us look at the incredible rate of evolution and dispersal of mosasaurs and the records of their success that can be found on every continent. Earth at the start of the Late Cretaceous was very different than it is today. The climate was several degrees warmer. As a result, there were no polar ice caps. The extra water in the oceans flooded the continents, forming shallow seas across Europe, North America, Africa, Australia, and India. It was into this environment that the ancestors of the mosasaurs began their transition to the seas. Which of the following factors do you think would have made this transition more likely? Choose all that apply. A, the water temperatures were higher. B, there were fewer predators in the ocean. C, eggs were more likely to survive in the water. D, there were lots of shallow reefs and coastlines. We may never know exactly why these squamates first entered the oceans. However, there are a couple of factors that probably would have made their transition more likely. The ocean temperature of today are too cold for most reptiles. This is why the majority of extant, aquatic, or semi-aquatic reptiles are found near the equator. In the Late Cretaceous the water would have been several degrees warmer, especially in the shallow continental seas. This would have made it easier for reptiles to live in this environment. So A is correct. This time predators were common on land and in the water. Leaving the land for an environment they were not as well adapted for would probably not have made them any safer. So B is incorrect. The amniotic egg is filled with small pores for gas exchange. Under water these pores get blocked and the developing embryo suffocates. So C is also incorrect. The greater amount in reefs in coastlines in the Late cretaceous increase the area and therefore the probability that a squamate could invade this environment. So D is also correct. By the mid-Cretaceous, about 90 million years ago, the continents were arranged roughly where they are now. Except, that the Australia was still attached to Antarctica, and India still attached to Madagascar, had not yet begun its northward journey towards Asia. Worldwide sea levels were 90 to 150 meters higher than they are today, covering a third of all presently exposed land. Much of North America and Europe were underwater, covered by the shallow Western Interior Sea and Tethys Seas, respectively. It was in this warm, wet world that the mosasaurs began their colonization of the world's shallow waters. The mosasaurs did not enter a realm devoid of predators, since the ichthyosaurs and plesiosaurs had colonized the oceans long before the mosasaurs did. However, the ichthyosaurs were in decline by the time the mosasaurs evolved. Many researchers believe that the disappearance of the ichthyosaurs was caused by competition with the larger, more powerful mosasaurs. However, it's unlikely that the first small bodied mosasaurids would ever have seen an ichthyosaur, since the earliest mosasauroids lived in shallow reefs and the last ichthyosaurs were only found in the open ocean. Basal, semiaquatic Mosasauroids are primarily known from Tethyan deposits from the Middle East through Southern Europe, which in the Late Cretaceous were covered with reefs and islands. <i>Aigialosaurus</i> is known from Croatia in a related form, <i>Carentonosaurus</i> is common in Cenomanian deposits across France and Spain. Multiple species have been found in Slovenia including <i>Cominosaurus</i> and <i>Parsosaurus</i>. And these small semi-aquatic lizards were very diverse and would have spread easily across the world from their European African origins during this time when shallow sea ways covered most of the continents. By the Turonian, mosasauroids were relatively widespread. Basal mosasauroids like <i>Dallasaurus</i> are known from the Gulf of Mexico. It was during this time that derived mosasaurs first appear in the fossil record of the western interior sea of North America. Though fully aquatic, with a number of adaptations to the aquatic problem, they still retain terrestrial features, such as their hips. These fossils are found in middle Turonian deposits in Kansas, and are approximately 90 million years old. Later deposits show that by the end of the Turonian, tethysaurines and yaguarasaurines lived in shallow seas of today's Italy, Morocco, Texas, and Columbia. tylosaur remains have been found in Mexico. <i>Prognathodon</i> in France and <i>Platycarpus</i> in Brazil other fragmentary Mosasaur fossils had been found around the world and even though they cannot be identified precisely, they indicate that mosasaurs were already, after barely 10 million years, taking over the world's oceans. By the middle Coniacian, the first major wave of mosasaurs was well established around the world. <i>Yaguarasaurus</i> still inhabited the oceans around Colombia and <i>Platycarpus</i> has been found in Brazil. Mosasaur fossils are not abundant from the Coniacian of Europe but specimens of <i>Tylosaurus</i> have been found in France from this time. The western interior sea was inhabited by various species of specialized mosasaurs. Tylosaurs, plioplatecarpines and <i>Clidastes</i> all lived in the western interior sea at the same time. In order to coexist, each species likely filled different environmental niches to avoid being in direct competition with each other. This was a pattern that continued throughout the rest of the Cretaceous. Let's investigate what this could have looked like. Fossils of <i>Clidastes</i>, <i>Tylosaurus</i>, and <i>Globidens</i> are all found in the Western Interior Sea, in the Santonian. Each of these mosasaurs would have had to focus on different types of prey, in order to avoid competition with each other. Based on the clues I am about to give you, see if you can place these three Mosasaurs in their preferred hunting environments on this map. <i>Globidens</i> and <i>Clidastes</i> have been found in the presence of small turtles, small pterosaurs, and toothed birds and this indicates that these mosasaurs lived in shallower waters, near the shoreline. The bulbous, crushing teeth of <i>Globidens</i> indicate that its preferred hunting environment was on the bottom of shallow water areas where it could find the hard-shelled mollusks that it specialized on. <i>Clidastes</i> was not specialized for bottom feeding or for open ocean hunting. It probably fed primarily on squid, fish, and other small vertebrates in shallow water. The stomach contents of <i>Tylosaurus</i>, show that it had a varied diet of near shore and open water prey including fish, sharks, smaller Mosasaurs, Plesiosaurs, and diving birds. This indicates that Tylosaurs hunted in both shallow and deep waters. This is supported by the fact that it has been found in near shore and off shore sediments. However, it was best suited for deep open water hunting further from the shore. The Santonian represents a time when larger mosasaurs became increasingly common worldwide. The largest number of Santonian mososaur fossils has been collected in Kansas, from the formation known as the Niobrara chalk. About 180 meters thick, this layer extends from Kansas in the United States to Manitoba, Canada and represents a time about 87 to 82 million years ago, when the Western Interior Sea covered most of midwestern, North America. A 1993 estimate states that this layer had produced almost 2,000 Mosasaur specimens from Kansas alone, fossils of <i>Clidastes</i>, <i>Halisaurus</i>, <i>Platecarpus</i>, and <i>Tylosaurus</i> have been found in the Niobrara Chalk and from other deposits of similar age around the Western Interior Sea. Santonian age deposits from Europe tell us that the Tethys was inhabited by <i>Halinosaurus</i>, <i>Plioplatecarpus</i>, and <i>Tylosaurus</i>. But these giants were not the only mosasauroids inhabiting these waters. Smaller mosasauroids, such as the Hungarian mosasaur, <i><b>Pannoniasaurus</b></i> reminds us of the continuing diversity of smaller forms that continue to thrive in shallow water environments. Southern hemisphere mosasaurs of this time include, <i>Halisaurus</i> from Peru, and <i>Platecarpus</i> from Australia. The Campanian is the longest stage in the late Cretaceous, spanning 11 and a half million years. The duration of the Campanian is reflected in the prolific fossil record from all around the world. The abundant fossils from this time show the diversity that constitutes the second major wave of mosasaur evolution, represented by large species who had lost all ties to the terrestrial environment. The Western Interior Sea began to recede to the North during this age, separating the Gulf of Mexico from the rest of the Western Interior. And the separation of these two bodies of water meant that evolution started to take different paths in each of these seas. And this resulted in different communities of mosasaurs in each environment. Both the Gulf of Mexico and the Western Interior were home to a huge number of mosasaurs, such as <i>Clidastes</i>, <i>Globidens</i>, <i>Halisaurus</i>, <i>Mosasaurus</i>, <i>Prognathodon</i>, plioplatecarpines and tylosaurines. However, <i>Mosasaurus</i> was more abundant in the northern Western Interior, while <i>Globidens</i> and <i>Halisaurus</i> were more commonly found in the Gulf of Mexico. The Western Canadian provinces have produced a vast array of mosasaur fossils that represents this incredibly diverse community. One particularly impressive site is an ammolite gemstone mining operation in Southeastern Alberta. Now, this area is mined for the shells of ammonites that are uniquely and beautifully preserved with brilliant opalescent rainbow hues. Of course, mosasaurs are known to have eaten ammonites and the environmental conditions that preserve the vivid colors in the ammonites' shells also preserved the mosasaurs in great detail. In fact, even some cartilaginous structures like the trachea can be found preserved. Another major community of mosasaurs has been found in the broadening Atlantic Ocean. From the east coast of North America to the remnants of the Tethys. The fact that the same species of mososaurus lived on both sides of the Atlantic, indicates that the deep ocean did not inhibit the migration of these derived forms. One exception to this is <i>Dollosaurus</i>, a prognathodontine which was unique to Europe at this time. The mosasaur fauna around the Pacific was slightly different, though the genera there are likely closely related to the more familiar groups from North American and Europe. Some unique Pacific genera include <i>Kourisodon</i>, which is known from western Canada and Japan; and the New Zealand taxa <i><b>Taniwhasaurus</b></i>, a tylosaurine, and <i><b>Moanasaurus</b></i>, which is related to <i>Mosasaurus</i>. By the Maastrichtian, the sauropterygians while abundant were less diverse than during their Jurassic heyday. The mosasaurs, on the other hand, were more diverse and derived than every before, and lived all around the globe. In fact, mosasaur fossils have been found in Maastrichtian strata from all seven continents. Platecarpines have been found in these locations. giant tylosaurines here, and diverse though not as aquatically adapted halisaurines can also be found the world over. Prognathodontines were nearly everywhere and <i>Globidens</i> had a new relative called <i>Carinodens</i>. In addition to being more diverse, mososaurs were also increasing in size. <i>Mosasaurus</i> had spread out of the Western Interior and some specimens from the East Coast of North America and the Netherlands were truly massive. Even though complete fossils of these behemoths have yet to be found, it's estimated that they could easily have exceeded 18 meters. There are a few localities where some unique genera are found. <i>Plesiotylosaurus</i> and the ichthyosaur-like <i>Plotosaurus</i>, which are both mosasaurians, are only found in California. Two unique African specimens come from yet another transcontinental seaway called the trans-Saharan seaway, which bisected the African continent. Along with the other more common mosasaurs found in Niger and Nigeria, are unique species called <i>Pluridens walakeri</i> and <i>Goronyosaurus nigeriensis</i>. <i>Pluridens</i> had twice as many teeth as any other mosasaur. Their thin, cone-like morphology could have formed a basket to catch small fish and soft invertebrates. <i>Goronyosaurus</i>, whose relationship to other groups is still unclear, has an unusual skull with small orbits, a blunt snout and large anterior teeth. In some ways, it rather resembles a crocodile. <i>Goronyosaurus</i> is not found in marine strata but is actually from sediments deposited by a freshwater river. Perhaps the broad snout supported additional pressure sensors to help it find prey in murky water at the bottom of rivers. In Canada, even though the Maastrichtian age deposits are not as extensive, as Campanian rock layers, numerous specimens of <i>Plioplatecarpus</i> have been found across the west. These Canadian plioplatecarpines show a pattern that is reflected all around the world. The end of the Cretaceous, shortly before their extinction, mosasaurs were not in a state of decline. In fact, the opposite seems to be true. Mosasaurs as a general trend were increasing in size, abundance, diversity, disparity. Then suddenly, they were extinct. The abrupt end of the Mesozoic, brought on by the Cretaceous-Paleogene extinction event which occurred 66 million years ago, cut short a lineage that showed no signs of slowing down. The Cretaceous–Paleogene extinction event, also known as the Cretaceous-Tertiary extinction was a mass extinction of some three quarters of plant and animal species on Earth, including all non-avian dinosaurs. It occurred over a geologically short period of time 66 million years ago. From the moment that the grand animal de Maastricht was unearthed from a mine in 1770 mosasaurs have been among the most intriguing of prehistoric animals. Descended from terrestrial lizards, mosasaurs quickly developed more and more specialized aquatic morphologies, allowing them to dominate marine environments and colonize the oceans and waterways of the world. Some reached enormous sizes, and with their flexible jaws and powerful teeth, became apex predators. Others lurked in the shallows ready to ambush anything that swam by. Still others developed heavy rounded teeth that enabled them to crush the thick shells of invertebrates. All of them were big, fast and dangerous to other Mesozoic marine life. They were among the largest and most powerful carnivorous animals that ever lived. They dominated the world's oceans and inland seas for 30 million years, and then with the Cretaceous-Paleogene extinction event, they abruptly vanished. By then, the ichthyosaurs were long gone, and the plesiosaurs were in decline. So the passing of the mosasaurs marks the end of an era when reptiles ruled the seas. Throughout the history of marine life, many unrelated groups of amniotes have converged on similar adaptations in order to live in aquatic environments. We'll never know why specifically the first reptiles returned to the oceans. But we do know that they started a trend that has continued to the present day. Mesosaurs were the first group of reptiles to re-enter the water and were followed by many others, including ichthyopterygians, sauropterygians and the mosasauroids. All groups acquired or reacquired profound adaptive modifications that enabled them to engage in an aquatic lifestyle. Giving birth to live young allowed each of them to completely abandon the land, adapting instead to the pressures and challenges of the aquatic environment. Again and again, we see similar solutions to the eight aspects of the aquatic problem that we discussed at the start of this course. Adaptations for propulsion in the water fall under two main categories, broad flippers for appendicular swimming, and a compressed tail or fluke, in axial swimmers. Flippers and fins confer stability in a 3D environment and allow changes in direction. The development of a fusiform, hydrodynamically advantageous shape solves the problem of inertial drag. And a slick exterior surface that would pass smoothly through the water counteracted viscous drag. Since all secondarily aquatic amniotes retained air breathing lungs, their nares migrated to the top of their heads to enable easier breathing. The additional buoyancy created by breathing air was counteracted in a number of ways by increasing body density. Pachyostosis and osteosclerosis altered the skeleton to increase weight, and in some cases, gastroliths performed the same task. Maintaining a constant water and salt balance could've been helped by the development of a salt gland. Metabolism was a trickier problem, probably solved by a combination of factors such as size, enzymes, lifestyle, and habitat. Vision and hearing were modified for life and a medium that transmits sound well and light poorly. Other adaptations allowed these creatures to take advantage of a whole new ecosystem and set of of prey. The jaws of some grew long and narrow, and were filled with pointy teeth to gobble up slippery fish and squid. Others grew powerful skulls to kill prey larger than themselves. Still others grew exceptionally long necks that enabled them to snatch fish ten meters away. The support that the water provided allowed members in each of these groups to grow extraordinarily large, some becoming among the largest aquatic predators ever to inhabit the seas. Like so many large and diverse groups of ancient animals, the ichthyopterygeans, sauropterygeans and mosasauroids are gone, extinct by the end of the Cretaceous, leaving no descendants. Just as the terrestrial dinosaurs dominated the land and the flying reptiles dominated the skies, the marine reptiles were the lords of the aquatic realm. These animals may all be long gone, but the allure of their fossils still lives in our imaginations to this day. You've now reached the end of the University of Alberta's MOOC on marine reptiles. We hope you enjoyed your experience and be sure to investigate our other MOOCs.
