Paleontologists think that some ichthyosaurs weren't just strong, efficient swimmers but also accomplished divers. Yet, most ichthyosaur fossils that we find are preserved in coastal environments. So, how did paleontologists come to the conclusion that some of these animals spent time diving? Check all the answers you think apply. A, They have big eyes to see in the dark. B, They have large tails to drive them down. C, They got the bends, like scuba divers. And or D, They had extra heavy bones to help them sink. Plenty of animals that live only at the surface of the water have large tails and many animals that dive have smaller tails than relatives that live on the surface. So B is not correct. Diving animals also do not tend to have heavier bones. Since being neutrally buoyant is more efficient. So D is not correct. Ichthyosaurs had some of the biggest eyes of any vertebrates which probably would have helped them to see in deep, dark water. So A is correct. Icthyosaurs also show evidence of a bone disease called the bends, that affects scuba divers today. So C is also correct. Now let's go into some more detail about these adaptations to deep diving. The high pressure experienced in deeper waters poses a unique set of problems to an air breathing tetrapod. The gases absorbed into the bloodstream are carried throughout the tissues of the body. During a dive, if the animal ascends too quickly, the sudden drop in pressure can cause the dissolved gases to come out of the blood and form tiny bubbles. This leads to the condition called the bends. And some fossils provide evidence that ichthyosaurs also got the bends. If a bubble in a bone cuts off blood supply, the surrounding bony tissue dies from a condition known as <b>avascular necrosis</b>. Avascular necrosis causes the dead region of the bone to collapse. So, unusual depressions on the surface of bones or collapsed, dense regions seen in X-rays, allow paleontologists to recognize cases of avascular necrosis. We have mentioned a few times, how large ichthyosaur eyes could be. This exaggerated feature provides paleontologists, with additional support, for the diving habits of ichthyosaurs. The cells that line the back of the eyeball and collect light are called <b>photoreceptors</b>. More photoreceptors allows for greater light collection and better vision in the dark. A larger eyeball can have more photoreceptors and therefore greater low light vision. <i>Temnodontosaurus</i>, with it's 25 centimeter in diameter eye, would have had enough photoreceptors to potentially absorb what little light penetrates to a water depth of 150 meters! The maximum <b>aperture</b>, or diameter of the opening that let's light into an eye, also determines how well an animal can see in the dark. For example, cat eyes have very large apertures. If they could swim and dive, cats would be able to see and hunt fish 500 meters below the surface. <i>Ophthalmosaurus</i> had the largest aperture of any ichthyosaur. The proportions of the aperture to the eye were similar to a cat's. These estimations suggest that ichthyosaurs could have hunted by sight, in deep, dark water. Some studies have compared the occurrence of avascular necrosis and the diameter of ichthyosaur eyes, and it does appear that ichthyosaurs with large eyes were more likely to have experienced decompression sickness, which solidifies the evidence that large-eyed ichthyosaurs like <i>Ophthalmosaurus</i> performed deep dives regularly. Unfortunately the fact that ichthyosaurs were pursuit predators who dove to hunt, raises yet another problem. The ocean, especially the deep ocean, is very cold. As discussed in lesson one, water is an extremely good heat conductor, sapping body heat very quickly. When the body temperature of a reptile drops too low, they become sluggish, inactive and not very good hunters. So, how could ichthyosaurs possibly have lived in the heat sapping ocean all their lives, without being slow and sluggish? Check all that could apply. A, They only lived in warm places. B, They produced their own body heat. C, They basked in the sun to warm up. And or D, They had to stay active to keep themselves warm. The ichthyosaur's active aquatic lifestyle itself may have helped them stay warm. Terrestrial reptiles use about 75% of their locomotion energy simply to support their own weight. Since water supports the bodies of aquatic reptiles, all their locomotion energy is available for propulsion. This greater efficiency allows them to fuel high energy, heat generating activities such as hunting, for much longer. So, D is correct. Ichthyosaurs lived worldwide, including in colder Arctic waters. So A is incorrect. It's also unlikely that they would have basked in the sun very often, as this would have left them vulnerable to predators. So C is incorrect. Finally, B may also be correct. Since it's possible that ichthyosaurs actually generated their own internal body heat. We'll now discuss this in more detail. Like all animals, ichthyosaurs would have been slow and sluggish when they were too cold. We know that ichthyosaurs were able to move quickly because they were highly derived pursuit predators, capable of catching fast moving prey like fish. Since water is such a good heat conductor, ichthyosaurs must have had adaptations to maintain their internal temperature. They may have done this by actually generating their own body heat, a process known as <b>endothermy</b>. Many modern fish, such as tuna and the great white shark, do this by partially heating their muscles using a special kind of circulatory system. Their high metabolism allows them to heat up a small amount of blood that they only use in key areas, such as their eyes, and their sprinting muscles. By heating these areas, it ensures that they are warmed up and ready to go when they come upon prey. It's interesting to think, that with such a similar body plan and lifestyle to these fish, the ichthyosaurs may have also been convergent in their ability to warm their blood. Just as there is ongoing discussion about whether dinosaurs were <b>endothermic</b>, producing their own heat, or <b>ectothermic</b>, relying on external temperatures like most reptiles, there is a comparable question about marine reptiles. Do we have any proof that ichthyosaurs may have been at least partially endothermic? Unfortunately not, but we do have some clues. Large animals, by virtue of their size alone, are better at retaining internal heat created by basic body functions. Small animals lose body heat faster than large ones because they have more surface area to lose heat through, relative to the volume of their bodies. Another clue comes from the microstructure of the limb bones in some ichthyosaur species, which reveal growth patterns like the rings on a tree that indicate a high rate of bone growth. This growth pattern is characteristic of warm blooded animals. Finally, the ability to give live birth is another hint that ichthyosaurs were endothermic. In order to have the embryos develop to term, they must be kept at a constant temperature. Unlike endothermic animals who are continuously warming their blood, it's difficult for ectothermic animals to maintain a constant internal temperature because they are so reliant on the variable temperature conditions around them. We know that ichthyosaurs gave birth to live young, meaning they must have had some way to maintain relatively consistent body temperature. Ichthyosaurs seem splendidly adapted for swimming, but they would probably have been as helpless on land as a stranded whale. In terms of reproduction, ichthyosaurs got around this problem by giving live birth to up to 12 babies, instead of laying eggs on land. Evidence of live birth in ichthyosaurs was actually discovered very early. In 1846, a specimen was found with a tiny ichthyosaur skeleton protruding from the body of a much larger one, prompting scientists to look at other ichthyosaurs that had been found with smaller skeletons inside. The tiny skeletons were found in the pelvic region and not in the stomach. It became clear that these were fetuses and that ichthyosaurs were <b>viviparous</b> which means they gave birth to live young. A difficult birth could have resulted in the death of the mother and her associated fetuses. Perhaps, accounting for the fossils that appear to show the mother in the process of giving birth. Another explanation for these partially expelled fetuses, is that if the mother died before giving birth, the build up of gases inside her decomposing body could provide enough pressure to partially force out the infants. This is a phenomenon seen in beached whales today. Further clues about ichthyosaur social behaviour can be found in an ichthyosaur bone bed that preserves evidence of a mass death. There is a unique site in Nevada that preserves a bone bed full of 15 meter long <i>Shonisaurs</i>. What is interesting about it is that all 40 of these animals are pointing in the same direction and are partially articulated but incomplete. Which scenario do you think could explain this? Check all the answers you think apply. A, A giant predator took bites out of all of them. B, They were trapped under sea ice. C, they were stranded on the beach. And or D, They were poisoned by toxic fish. Finding so many articulated ichthyosaur skeletons together indicates that they probably died at the same time. It is extremely unlikely that a predator killed them all at once, so A is incorrect. In addition, the Mesozoic climate was very warm and no sea ice existed. So B is incorrect. It is possible that they might have been stranded on the beach, as sometimes happens with whales today. So C is possibly correct. However, it was later discovered that these rocks were from the open ocean so stranding is unlikely. Another suggestion is that they may have been poisoned by infected fish. Today, toxic microorganisms called dinoflagellates can sometimes have a season where they are unusually abundant. They infect large numbers of fish and if a predator eats too many, they can be poisoned and die. This phenomenon is called a red tide, and is a cause of some mass strandings seen in whales today. So D is also potentially correct. Either way, finding so many ichythosaurs together suggests that they were gregarious, living and sometimes dying in groups. C and D are the most correct answers. As you have seen, ichthyosaurs were splendidly evolved to overcome the aquatic problem. Even the earliest ichthyopterygians were very distinctive reptiles, completely adapted to marine life. Their hunting armament included large, sensitive eyes, and long snouts filled with small, pointy teeth, perfect for grasping slippery prey. Strong tails propelled their elongate fusiform bodies through the water at high speeds, and two pairs of flippers kept them going in the right direction. They probably hunted deep in the water, diving in search of more abundant prey. It's likely they maintained a very high metabolism, possibly assisted by some endothermic capabilities. Being unable to leave the water, ichthyosaurs were naturally viviparous, giving birth to live young, which may have been raised in family groups. This first major group of marine reptiles is a classic example of convergence, where the same function results in a similar form across evolutionary history. Let us now dive into the evolutionary history of ichthyosaurs in order to develop an understanding of how their aquatic adaptations enable them to rule the oceans for nearly 150 million years.
