The extraordinary sequence of well-preserved material spanning the 30 to 35 million year history of mosasauroids shows a steady pattern of adaptation from a terrestrial lizard anatomy to a highly specialized suite of marine adaptations that are very similar to the ichthyopterygians and sauropterygians. This is yet another example of convergent evolution between the unrelated groups of marine reptiles. Like early ichthyopterygians from lesson 2 the basal pythonomorphs we examined previously in this lesson, such as <i>Dolichosaurus</i>, were almost certainly anguilliform swimmers. Their long, slender bodies, flexible vertebral columns, and laterally flattened tails are the same features seen in almost all other anguilliform swimmers we've examined in this course, such as thallatosaurs and pachypleurosaurs. Originally, it was thought that the derived mosasaurs were also anguilliform swimmers, with their entire bodies moving in sinusoidal motions like a snake or eel. The absence of fossil evidence suggesting other forms of locomotion such as a caudal fin was used to support this conclusion. Recent analysis of the structure of their vertebrae has revealed that mosasaurs became carangiform swimmers. Meaning that the forward part of their body was stiff, while the rear portion undulated, similar to the way alligators and cod swim. And this change in swimming style from anguilliform to carangiform is reflected in the changing tail morphology. The vertebral column lost its flexibility making the body stiffer. The caudal or tail region became more specialized, and over time, a downward kink developed in the caudal vertebrae. A cartilaginous blade developed on the top surface of the bend, and like the ichthyosaurs before them the mosasaurs evolved a heterocercal tail. This means that the upper and lower lobes of the tail were different lengths. In mosasaurs, the lower lobe got much longer and was supported by vertebrae while a blade of cartilage supported the shorter upper lobe. Although shorter than in the more basal forms, the muscular tail of derived Mosasaurs was still 42 to 50% of their body length. The tail was laterally flattened with a lobed heterocercal fin. This resulted in a higher surface area to push against the water. The base of the mosasaur tail would have been relatively inflexible and would have provided a stable anchor for strong musculature. You've learned a lot about the morphology of marine reptiles, and how it relates to their ecology in the past few lessons. Based on the information you've just been given about mosasaur tail morphology, pick the description below that best fits how you think they would have swam. A, long distance open water cruisers. B, short distance open water cruisers. C, short distance high speed swimmers. D, long distance high speed swimmers. Mosasaur tail morphology, though extremely powerful, is not particularly energy efficient over a long period of time. It's unlikely that mosasaurs were as fast over long distances as the pliosauromorphs or thunniform ichthyosaurs but where they did excel was in bursts of high speed swimming. They could use their enormously powerful tail to accelerate very quickly and maintain a high speed over a short distance. This made them extremely efficient ambush predators. Using powerful acceleration and the element of surprise to overtake and capture prey. So, C is the correct answer. The ambush tactics employed by mosasaurs required them to be able to change direction very quickly. Their large and powerful flippers would have contributed to steering. Differences do exist in the morphology of the limbs between different mosasauroids, which could be important indicators of their ecology. In most early mosasauroids, such as the agialosaurs, the widely spread, thin bone digits would probably have been loosely joined by webbing to form a flexible flipper. In later genera, such a <i>Plotosaurus</i>, the thicker digits were arranged tightly together to form a stiff, wing-like flipper, some what resembling ichthyosaurs and plesiosaur flippers. And still other genera, such as <i>Tylosaurus</i>, the flippers were less ossified and more cartilaginous and highly flexible. Though we're still not precisely sure what these differences meant for mosasaur locomotion, paleontologists are confident that the flippers were not used for propulsion. Most likely, the flippers were held close to the body, reducing drag, while the tail powered the animals through the water, and were only extended to help steer when the mosasaur needed to change direction. This change in morphology in the tail and appendages reflects a progressive shift from lagoonal dwellers to nearshore paddlers to transoceanic animals capable of high speed attack. These patterns of evolution are similar to those observed in marine crocodiles from lesson one, the ichthyopterygians from lesson two, and the sauropterygians from lesson three, and to extant groups like whales. All of these secondarily aquatic tetrapods face the same evolutionary pressures when they return to the ocean. As a result, the evolutionary stages that they progress through, along with the most efficient derived forms show a great deal of convergence. Mosasaurs like sauropterygians and ichthyopterygians developed a streamline body, lost their terrestrial fingers, and developed a fused flipper. Like the sauropterygians, mosasaurs never developed a dorsal fin. But like the ichthyopterygians and the whales, they maintained axial locomotion. However, like the other major marine reptile groups we have discussed, mosasaurs developed some solutions to the aquatic problem that were entirely their own. One solution was to retain scales which ichthyopterygians and sauropterygians appear to have lost. Mosasaurs scales are well known for numerous fossils, some of which are exceptionally well preserved. They had small overlapping scales which would have looked very similar to a snake. The scales are diamond shaped and some had a raised ridge down the center. It may seem that these scales would not be as efficient in aquatic adaptation as smooth skin. However, the small ridges on the scales may actually have reduced the amount of viscous drag by maintaining a thin layer of water around the animal. The ridges and the scales would trap water against the body and that way, as the Mosasaur moved, friction would be generated between water and water, instead of between water and the animal's body. This resulted in much less drag, and it is similar to the way a shark's skin works. Sometimes these scales can even show which sections of the body were darkly colored, which ones were lightly colored. This indicates that some species of Mosasaurs may have had bands of dark and light shading over their bodies. Similar patterns in modern animals such as tigersharks help to break up the light hitting the animal, making them harder to see and therefore better ambush predators. Mosasaurs were sharing the seas with sauropterygians such as the long-necked elasmosaurss and the large-mouthed polycotylids. In order to co-exist, each of these groups specialized on different types of prey. See if you can match the marine reptile to its prey. As we've just discussed, derived mosasaurs were powerful sprinters and ambush predators. Like other marine reptiles, these aquatic predators needed to adapt feeding strategies that allow them to acquire and swallow prey in the aquatic environment. Recall that for long necked plesiosaurs, food was fairly small and was swallowed whole. So, small fish is the correct answer. The large mouthed polycotylids ate correspondingly larger prey but their narrow teeth and jaws restricted their diet to soft-bodied prey. So the correct answer is big fish and squid. Mosasaurs, with their massive teeth and jaws that were 10 to 14% of their body length, were able to snap up small prey, crunch through hard prey, or dismember prey too big to swallow. They were opportunistic feeders who could have eaten anything they came across, including other marine reptiles. So the correct answer is ammonites, fish, and marine reptiles. In order to be able to eat such a wide variety of large prey, mosasaurs needed special adaptations in their skull. One such adaptation unique to Mosasaurs was their highly <b>kinetic</b> or flexible skulls and jaws. Together, flexible elements such as the inter and intramandibular joints would have allowed mosasaurs to open their jaws wide enough to swallow large prey. Your own arms actually demonstrate this quite well. Imagine that each arm is a lower jaw. Your clasp hands represent the intermandibular joint, your elbows represent the intramandibular joint, and your shoulders represent the quadrates. If you pull your hands towards you and spread them slightly apart, you notice that your elbows bend and flare out and your shoulders rotate outwards too. This is essentially what happens in the mosasaur jaw. The intermandibular joint allows the two sides of the jaw to separate slightly and the intramandibular joint and quadrates allow the mouth to expand. The extra space created by all these joints would allow the mososaur to fit large prey into its mouth. Once in the mouth, the curved pteragoid teeth in the throat would prevent any prey from struggling away. Even with their impressive gape, sometimes the prey was indeed too large to swallow. Some of the larger mosasaurs like <i>Tylosaurus</i> and <i>Prognathodon</i> ate other marine reptiles such as turtles, plesiosaurs, and smaller mosasaurs. It's thought that these large mosasaurs could have dismembered their prey by shaking it like a shark. Having highly flexible skulls is not advantageous when feeding in this way. And so in many larger mosasaurs the skull became less and less kinetic. The flexibility of the skull is not the only adaptation mosasaurs developed that enabled them to consume large tough prey. Their teeth, the tools used to actively tear, crush, and dismember their prey also developed specialized growth patterns. You're probably most familiar with the typical mammalian tooth growth pattern of developing one set of teeth as an infant, and a second set of replacement teeth through maturity. Mosasaurs grew teeth continuously throughout their lives with replacement teeth growing beside the functional teeth. Each replacement tooth would develop behind the functional tooth, slowly dissolving the base of the old tooth until it fell out and the new tooth move into the socket to become the new functional tooth. This is a typical pattern of tooth replacement for all lizards and snakes. Tooth replacement was especially important for mosasaurs that ate harder prey items. Because hard shell prey wore down their teeth faster, making the teeth less effective. Tooth wear is found in mullusk crushers like <i>Globidens</i> and <i>Carinodens</i>, and can also be seen on the tips of the large pointed teeth of <i>Mosasaurus</i>. So what were the mosasaurs biting that would have worn down the tips of their teeth? Select which of the following prey you think would have caused the most wear to mosasaur teeth. A, fish. B, squid. C, Marine Reptiles. D, Ammonites. Though mosasaurs commonly ate fish and squid, they were soft bodied and would barely have worn down the teeth. So A and B are not correct. Marine reptiles with their thick bones, would have caused some damage to the teeth, but were probably not the main cause of tooth wear. So C is not correct. One of Mosasaurs most common prey items, and one of the toughest to eat were Ammonites, common invertebrates in the Mesozoic oceans. Several fossils of the round disc like ammonites have been found with an odd pattern of holes in the shells. These holes can be connected to form a large V shape. The size of the V and the spacing between the holes corresponds well with the arrangement of teeth in a large mosasaur jaw. Some of these bitten ammonites even have multiple sets of teeth marks as if the mosasaur bit in and either couldn't chew it or couldn't swallow it and so the mosasaur released the ammonite to try again. Paleontologists debate whether mosasaurs swallowed ammonites whole and let the shells dissolve in their stomachs or whether they crushed the ammonite shells with their teeth to get to the soft animal inside. Either way, the hard shells of the ammonites would have caused considerable wear to the teeth, and so D is the correct answer.
