The ammonites and other large prey items on the mosasaurs' menu were not always easily found in surface water. So mosasaurs would have to dive to hunt. Think back to the types of fossil evidence that had been used to infer which ichthyosaurs were deep divers. This includes their eye size and regions of collapsed bony tissue attributed to avascular necrosis. While no mosasaur had eyes as large as some ichthyosaurs, evidence of avascular necrosis has been found in some species. For example, the genera <i>Clidastes</i>, <i>Platecarpus</i>, and <i>Tylosaurus</i> are known to have cohabited the western interior sea, which covered the middle of North America. This seaway was relatively shallow, likely less than 200 meters. But 200 meters is deep enough to result in the pressure changes that release dissolved gasses from the bloodstream if ascent is too rapid. Avascular necrosis is never seen in <i>Clidastes</i> but has been found in <i>Platecarpus</i> and <i>Tylosaurus</i>. <i>Clidastes</i>, with its less specialized morphology is therefore thought to have inhabited shallower waters, whereas <i>Platecarpus</i> and <i>Tylosaurus</i> pursued prey in the dark ocean depths. Even though their eyes are not as adapted as ichthyosaurs, mosasaur senses were still highly adapted to solve the aquatic problem. Though not as proportionately large as ichthyosaurs, they still had fairly large eyes that were probably useful for seeing in dimly lit waters. Mosasaurs also had sclerotic rings which are used to support the eyeball and to change its shape in order to focus the eye. The large eyes faced laterally from the head, giving the mosasaur a wide range of vision, but would've prevented binocular vision and limited depth perception. Some paleontologists have hypothesized that, like snakes and lizards, mosasaurs may have had a transparent eyelid covering their eye to protect them and prevent water loss from the eye by osmosis. The ears in mosasaurs are also well-adapted to their environment. Recall that, in water, eardrums are not particularly good at detecting vibrations, so many aquatic amniotes hear by picking up vibrations in the water through their skull bones. This is a type of hearing called <b>bone conduction</b>. Mosasaur ears have gone one step further and have evolved a hard, cone-shaped, bony formation that sat where the eardrum was in their terrestrial relatives. In this cone-shaped plate, would have picked up sound in the water, focused it, and directed it towards the inner ear far more effectively than the skull bones. This would've given mosasaurs a fairly acute sense of hearing under water. A similar modification is seen in whales. What purpose would an acute sense of hearing serve in mosasaurs? Check all that might apply. A, detecting prey. B, locating threats. C, finding mates. D, echolocation. Since smell transmits so poorly in water, mosasaurs almost certainly used their sense of hearing and sight to detect their prey and potential threats. So A and B are correct. Additionally, while no evidence has been found, it has been hypothesized that mosasaurs which were likely solitary would have used acute sense of hearing to communicate between other members of their species. This would have been especially important in helping them locate a mate across long distances in the open ocean. So, C is correct. We have no evidence of any reptile ever evolving specializations for echolocation. So D is incorrect. This type of logical deduction often is part of the process within paleontology. Mosasaurs, like the ichthyopterigians and sauropterygians we have already talked about, were viviparous and bore live young. A fossil of the basal mosasauroid, <i>Carsosaurus</i>, which was found with at least four advanced embryos in the abdomen, indicates that this adaptation to the aquatic lifestyle developed early in their evolution. The orientation of the embryos suggests that they were born tail-first like the ichthyosaurs and modern whales to reduce the possibility of the newborns drowning before they were separated from the mother. Vivaparity in these early, medium-sized, amphibious mosasauroids freed them from returning to the land to deposit eggs, permitting the evolution of gigantic, fully-marine Mosasaurs. One of the best places to study juvenile mosasaurs is the Smoky Hill Chalk and the Niobrara Chalk in Kansas. A large number of small mosasaurs had been found in this location which would have been in the middle of the ocean in the time of the mosasaurs. Like the sauropterygians, the fact that mosasaurs were apparently giving birth mid-ocean means that the mothers were probably heavily invested in their young. The mothers gave birth to a small number of well-developed offspring and they probably would have protected and helped feed them until they matured. Even with parental care, the survival rate of young mosasaurs was probably low in an environment shared with large sharks, giant fish, and other mosasaurs. An additional source of data used to infer aspects of the lives of extinct animals is <b>paleopathology</b>, or the study of diseases or injuries as preserved in fossils. We've already discussed one form of paleopathology, the bony tissue deterioration called avascular necrosis, that results from ascending too quickly during a deep dive. Paleopathologies can record diseases such as arthritis and bone cancer. Physical trauma such as broken bones or bite marks and even evidence of healing. A bony <b>callus</b> is an extra growth of ossified tissue which grows around the site of an injury. Such a callus around a break or bite mark indicates that the animal live long enough after the incident for its injury to heal. On the other hand, if there are no signs of new bone growth around the injury then its possible that the incident contributed to the individual's death. Broken bones are usually not fatal to humans, but are fatal for many animals since they impair their ability to hunt, and evade predators. Mosasaur fossils have been found that preserve a wide range of paleopathologies, including diseases like avascular necrosis, infections of bony tissues, and various injuries. Interestingly, mosasaur paleopathologies tend to be concentrated around the skull and jaws. And this could be the result of collection bias resulting from more skulls then bodies in paleontological collections. However, some mosasaur jaws have been found with bone that had been healed around very clear bite marks. This suggests that injuries to the head were common for mosasaurs and may indicate some interesting behaviors that paleontologists are just starting to investigate. Take a close look at this mosasaur skull and jaw as it clearly shows bite marks. What type of behaviour do you think these bite marks on the head and jaw suggest? Remember that paleontologists often have to take the most logical path based on the evidence provided. Select all that apply. A, Mosasaurs preying on each other. B, Mosasaurs fighting each other for resources. C, Mosasaurs fighting with pliosaurs. D, Mosasaurs being preyed on by sharks. It is unlikely that paleontologists will ever know for certain how or why these mosasaurs got bitten. But it does appear that they resulted from being bitten by another Mosasaur. While sharks certainly bit mosasaurs, shark teeth do not match the shapes of the preserved bite marks, so D is not correct. Pliosaur teeth are a better match shape wise but Late Cretaceous pliosaurs were much smaller than the injured mosasaurs so C is not correct. The bite marks do match mosasaur teeth and likely came from an individual about the same size as the one that was bitten. This is probably not an example of attempted mosasaur cannibalism, answer A since even mosasaurs would avoid prey as big as they were. It's just too dangerous. The most likely answer is therefore B. Mosasaurs fighting each other for food, territory, or mates. As you just discovered the wounds on mosasaur skulls are considered to be a record of <b>intraspecific</b> interactions or occasions when individuals of the same species came into contact. Intraspecific aggression is supported by scratches and puncture marks and also by a few rare fossils that preserve the aggressors broken off tooth still embedded in the bone. One of the genera that exhibits several cases of bites around the skull and jaws is <i>Mosasaurus</i>. It's most likely that only another <i>Mosasaurus</i> would have been able to attack the large individuals that had been found injured. The bites could have been inflicted during competition for food, territory, mates, or even during the mating process. Evidence of <b>interspecific</b> interactions occurs when individuals of different species came into contact. For the most part, these are easier to interpret because they come in the form of stomach contents. Large mosasaurs had been found with smaller mosasaurs in their stomach cavities. Unlike the specimens that provided evidence of live birth, in plesiosaurs and ichthyosaurs, stomach contents show fragmentation and erosion from stomach acid, meaning that they were processed and partially digested. The strongest evidence of mosasaurs preying on each other is when smaller mosasaurs in the stomach are complete enough for identification and are different species than the larger mosasaur that ate them. Mosasaurs evolved relatively quickly from small shore dwelling lizards into the dominant marine predator's of earth's oceans. Live birth freed the Mosasaurs from the constraints of terrestrial living, allowing them to evolve into the ultimate predator of the Cretaceous seas. Their adaptations to life in the ocean included major modifications to the axial skeleton and limbs, including a long body and tail with a heterocercal fin and strong flippers in a variety of shapes. Mosasaurs also had small scales covering their body that decreased drag and may have had shading that made them harder to see. The highly kinetic skulls and jaws of some species enable them to swallow larger prey than any previous marine reptile, and wickedly pointed pterygoid teeth prevented the escape of any prey unlucky enough to be bitten. Even animals that were too big to be swallowed were not safe. Derived species of Mosasaurs with less kinetic skulls became adapted to tearing and dismembering extremely large prey. With acute senses of hearing and sight, not much would've escaped these supremely well-adapted predators, the undisputed rulers of the late Cretaceous seas. Fossil evidence in the form of broken bones and bite marks shows that only another mosasaur was big or powerful enough to challenge these reptiles. The next section will give us an overview of the extent of mosasaur dominance. We will explore the patterns of their diversification and distribution as they became better and better adapted to solve the aquatic problem. We will also introduce you to some famous localities where mosasaur fossils have been found.
