In 1808, Baron George Cuvier, who’s known as the father of comparative anatomy and paleontology, described the fossilized jaws of an enormous reptile which he called, the Grand Animal de Maastricht. The fossilized jaws had been found deep in the mine in the Netherlands. They looked like the jaws of a modern monitor lizard, but were more than one meter long. At the time, fossils were not well understood. Extinction was not a well-accepted fact. And Darwin's Origin of Species would not be published for another 50 years. Naturalists at the time had no idea what to make of the unfamiliar fossils, classifying them as fish, whales, or crocodiles. Cuvier was in the minority by classifying this fossil as a lizard. Although Cuvier was not the first to recognize the lizard features of the skull, he was in a prominent enough position that his words carried great weight. At the time, he was an anatomist at the National Museum in Paris. The Dutch fossil would go on to become the first mosasaur to be described, named for the Meuse River or Mosa in Latin where it was found. Cuvier's fossil was an example of Mosausaurus Hoffmannii, which inhabited the world's oceans about 70 million years ago, well into the second half of the Cretaceous. While Tyrannosaurs ruled the land, Mosasaurs dominated the seas. At this point in time near the end of the Cretaceous, the Mosausaurs had been around for about 25 million years. Their ancestors were small terrestrial lizards that left the land for a life in the water about 95 million years ago. Once in the water, the shore-dwelling Mosausaur ancestors began to evolve rapidly. What is most remarkable about Mosausaurs is their leap to the top of the aquatic food chain. Within moments, geologically speaking, they went from small terrestrial lizards to the apex predators of the Cretaceous seas. For comparison, Ichthyopterygians and Sauropterygians evolved for about 50 million years before they could be considered dominant marine predators. Mosasauroids took only 10 million years. Their adaptations to solve the aquatic problem included the evolution of paddle-like flippers and the lengthening of their tails which also developed the hypocercal tail fin. Recall that the hypocercal tail fin has one lobe that is more pronounced or larger than the other lobe. And this is the type of tail fin we've seen in some fish, sharks, and ichthyosaurs. These adaptations made them proficient at pursuing and ambushing their prey and allowed them to conquer the oceans. During the 25 million years that they prospered, the Mosausaurs spread from pole to pole, leaving their remains in North and South America, Europe, Africa, Australia, Antarctica, and Asia. This incredible group will be the last we investigate which is appropriate, since they were the last major marine reptile group to evolve and represent the last time that reptiles ruled the seas. So, what exactly are Mosausaurs? Mosasaurus are squamates, the group of reptiles that includes all extent ant fossil lizards and snakes. The relationships of Mosasaurus among squamates is a subject of ongoing controversy, though most scientists agree they're closely related to Anguimorph lizards. Anguiomorphs evolved during the Jurassic and have survived into the present time, including modern forms such as the Gila monster, Komodo dragons, and numerous small lizards. All squamates, including Mosasauroids, have a lizard-like skull with two openings behind the orbit. Remember that the diapsid condition, where the skull has two openings behind the orbit, also occurs in crocodiles and dinosaurs. All Mosasaurs also have an intramandibular joint located in the middle of each jaw. These lower jaws are loosely connected at the front of the mouth, forming the intermandibular joint. And the flexibility of these two joints would have allowed Mosasaurus to open their mouths very wide. And gave them the ability to swallow large objects. This unique characteristic is also shared with snakes and is one of the reasons that snakes and Mosasauroids are thought to be sister groups. Mosasaurus also have another special feeding adaptation. They had teeth on the pterygoid bones in the roof of their mouths, and these pterygoid teeth kept slippery prey from escaping the mouth while they were being swallowed. Yet another feature that Mosasaurus share with snakes. If this snake Mosasaur sister group hypothesis is accurate, then it's possible snakes evolved from aquatic lizards like the Mosasaurs did. These two lineages, the snakes and the Mosasaurs are often placed together with their common ancestors in the group of lizards called the Pythonomorpha which means snake-like. As mentioned, many of the characteristics that make a Mosasaur a Mosasaur, including the diapsid skulls, intramandibular joints and pterygoid teeth are also common to snakes. However, unlike snakes, Mosasaurs have well developed, aquatically adapted limbs that exhibit hyperphalangy and a much longer and more laterally compressed tail than is seen in snakes. One particularly important bone when it comes to differentiating Mosasaurs is this bone at the back of the skull called the quadrate. In life, it acted as a hinge that connected the lower jaw to the skull. And it also supported cartilaginous structures involved in hearing. The quadrate was robust and therefore, is commonly preserved. And this is fortunate because variations in this bone help to identify different groups of Mosasaurs. Even a partial quadrate helps identify the genus of a fragmentary Mosasaur specimen. Of the following sets of fossils, select which one comes from a Mosasaur, were you able to recognize the diagnostic characters of Mosasaurs? Remember to look for the intramandibular joint as seen in jaw A, pterygoid teeth as seen in mouth B. And a diapsid condition of two temporal fenestra as seen in skull B. We've found enough Mosasaur fossils that scientists have been able to construct hypotheses regarding early Mosasaur evolution. This is a very different scenario from the icthyopterogians and sauropterygians who appear suddenly in the stratigraphic record and have no current record of transitional forms. The entirety of Mosasaur evolution fits into the late cretaceous, a span of only 30 million years. Unlike the 150 million year history of sauropterygians and icthyopterygians. While 30 million years seems like a long time, in geologic terms it is very short. And so we need to introduce some smaller units of time in order to clarify our discussion on Mosasaur evolution. Think back to what you may know about the geologic time scale. Try to put the following units of time in order from longest to shortest. Pick the ordering of time units that correctly arranges them from longest to shortest time unit. A, period, era, age, eon. B, eon, era, period, age. C, age, period, eon, era. D, era, age, eon, period. As you may know, the mesozoic is a unit of time known as an era in the phanerozoic eon. The mesozoic is divided into the three periods, the Triassic, Jurassic and Cretaceous. Periods span millions of years and can be subdivided into even smaller units of time for more precise communication. We call these units ages. So the correct answer here is B, eon, era, period, age. The Late Cretaceous, which spans the entirety of Mosasaurus existence, is divided into six ages. The Cenomanina, which began a 100 million years ago, followed by the Turonian, Coniacian, Santonian, Campanian and Masstrichtian, which ended 65 million years ago with the End-Cretaceous extinction event. You should notice that these ages do not represent equal amounts of time. As we did with the icthyopterygians and sauropterygians, we will begin our exploration of the major Mosasaur groups with the most basal and earliest members of the Pythonomorpha. These are the dolichosaurs, pontosaurs, and adriosaurs. Around 90 million years ago, several lineages of ancient, terrestrial, and guiamorph lizards took to the water and very quickly became fully aquatic. One of the groups, the Dolichosauridae were small, long bodied lizards with long necks, short to medium sized skulls, reduced limbs and a very long laterally compressed tail. They lived at the beginning of the late Cretaceous in the Cenomanian and Turonian in the near shore reefs around the northern hemisphere. One particularly interesting specimen was Adriosaurus Microbrachis. Although it had well developed rear limbs, it's front limbs had been reduced so much, that only a tiny humorous was left, indicating that it's front limbs played no significant role in any aspect of it's life. Another remarkable specimen of Pontosaurus, preserved the trachea and scales. The scales were smooth and represented an early adaptation to the aquatic problem of viscus drag. The earliest basal Mosasauroids are known as the Aigialsauridae. They lived during the Sandemanian and Tironian in Europe and North America. They inhabited shallow marine environments such as reefs and lagoons where they would probably have preyed on small fish and invertebrates. They were very slender, up to two meters long, and had extremely long tails that were much longer than the head and body combined. This long, flattened tail would have been their primary source of propulsion. Steering would have been accomplished by their large feet with elongated and slender digits that may have been webbed since these early forms had not yet evolved paddles. In addition, the pakistonic bones of some species would have maintained neutral buoyancy. Their skulls are almost indistinguishable from Mosasaurs, but their bodies more closely resembled terrestrial lizards. There are two main sub groups within the derived Mosasaur lineage. On one side is a lineage that contains groups like the tylosaurinae and the tethysaurinae, and these Mosasaurs are characterized by simpler limb bones and a lesser degree of ossification in their delicate and more elongate flippers. This means that many wrist and ankle elements remained as simple cartilaginous structures and never became bony. The other major mosasaur lineage is the Mosasaurinae, and they have robust flippers where all the wrist and ankle elements are ossified, blocky, and the limb bones are more complex. Each of this claydes is composed of multiple subfamilies or smaller groups of related genre, each with their defining sets of characters. We're now going to explore these groups to learn about their diversity, trace their inner relationships. And discover what makes each one unique. Lets start on this side of the lineage. The Basel Mosasauroids in this lineage include Carsosaurus and Komensaurus who lived during the Cinemania. This Mosasauroids were likely semi-aquatic and retained primitive limbs and pelvic girdle morphologies, that is, they did not have flippers. The only Carsosaurus that's been found is particularly interesting as it is a pregnant female containing at least four embryos. Which you can see outlined in this photo. It's the only evidence of viviparity or live birth in basal Mosasauroids, and it shows that viviparity developed early in Mosasaur evolution. The next most arrive group known from Turonian age deposits also likely retains some primitive terrestrial characters in the limbs and pelvic girdle. One of these groups, the tephesaurinae is known to have the more typically terrestrial structures of both its limbs and pelvis. For example, the pelvic girdle of tephesaurus would have articulated to the spinal column. A connection that is lost in more aquatically adopted species. Additionally most arrive Mosasaurs have only a few pterygoid teeth n the roof of their mouth. But taphesaurus could have as many as 19 pterygoid teeth. Another closely related group, the Yaguarasaurinae. Likely represents the transition from semi-aquatic to a fully marine limb morphology. This means it probably had both paddles and unattached hip bones. Unfortunately, no limbs and girdles have yet been found from this group. So, their positions as a transitional form is just a guess based on other intermediate features of their skulls. These Mosasaurs were relatively small, usually 3-5 meters in length. And had slender jaws with posteriorly curved teeth. The groups we have just discussed show the transition from a terrestrial morphology with well attached pelvic girdles and weight bearing feet to an aquatic morphology, with loosely attached pelvic girdles and flippers. But in what order did these two adaptations happen? Which scenario do you think is correct? A, They evolved aquatic flippers then an aquatic pelvis. B, They evolved an aquatic pelvis then aquatic flippers. C, They evolved both at the same time. Evolving an aquatic pelvis while retaining weight bearing legs would not work. Since even though their legs could bear weight, their pelvis would collapse. So B is incorrect. Flippers connected to weight bearing pelvis are still functional. So A could be possible from a logical perspective. C is also unlikely to be correct. There is more evolutionary pressure on the limbs than the pelvis, because limbs plays a bigger role in locomotion. So limbs tend to adapt faster than the pelvis, which is what is seen in the fossil record. Therefore A is the correct answer.
