Now that we've examined some of the diversity of Ichthyosaurs let's now investigate their paleobiology. Just like the mammals that evolved into whales and dolphins, the reptiles that returned to the sea, eventually acquired characteristics that made life in the water possible. They evolved the fish-like shape, flippers instead of feet, a mouth full of sharp teeth for capturing slippery prey, and the ability to give birth in the water. Like mammals, ichthyosaurs were air breathers, this restricted the amount of time they could spend underwater. Ichthyosaurs occupied both coastal environments and open oceans. Fossilized remains, in their stomachs tell us that they ate mostly cephalopods and fish but also supplemented their diets with turtles, smaller Ichthyosaurs, and the drowned carcasses of other animals. They had large eyes capable of seeing in deep, murky water and likely hunted primarily using their sense of sight. In the next section, we'll explore how the fossils of Icthyosaurs have informed paleontologists about their possible <b>paleocology</b>: feeding, locomotion, and reproduction. One of the most important aspects of the biology of an animal is what and how it eats. Because water is so dense, aquatic animals face unique challenges in simply getting food close to their mouths, let alone inside. What do you think happens to a fish when an Ichthyosaur tries to close its mouth to catch it? A, The fish gets sucked inside the Ichthyosaur's mouth. B, The fish remains stationary relative to the Ichthyosaur's mouth. Or C, The fish gets pushed out of the Ichthyosaur's mouth. An animal swimming through the water will always push some water ahead of it, which means, the water pushes the prey away from it. And the same thing happens when it starts to bite down on its prey. The water displaced by the closing jaws, shoots away from the mouth, taking the prey with it. So the correct answer here is C. So how did Ichthyosaurus catch their prey? One feeding strategy is called <b>ram feeding</b>. Ram feeders, like whale sharks, let the water flow into their mouths and right back out through their gills. The food is strained out of the water by special structures called gill rakers. Now, humpback and blue whales use a special kind of ram feeding called <b>lunge feeding</b>. They accelerate through the water towards their prey, and then open their mouth extremely wide and they have special folds in their throat that allows the skin to rapidly expand outward like a parachute. They then push the water back out of their mouth through <b>baleen</b>, which is a fibrous material that has replaced their teeth. Since ram feeders often filter their food out of the water, we can also call them <b>filter feeders</b>. It's possible that some of the large, toothless Ichthyosaurs, may have used this feeding strategy, but we've never found fossils of any specialized mouth parts that prove it. Most ichthyosaurs used <b>manipulation</b> or biting to capture their prey. This can be used for acquiring prey that lives on the bottom of the sea floor, like shell fish, which won't be pushed away by water as a predator approaches. It's also used for snatching up small prey in the open water and for biting pieces off of larger prey. We can use details of the teeth, like the shape, size, or wear patterns, to understand what kinds of prey ichthyosaurs ate, and how it was caught. Using those features, we can group Ichthyosaurs into <b>feeding guilds</b>. Guilds are groups of species which are not necessarily closely related, but that use similar resources in their environment. Some of the guilds that have been identified for marine reptiles are the <b>Crush, Smash, Pierce, and Cut Guilds</b>. Take a look at the tooth shape seen in each of these guilds. Tooth shapes can be matched to the type of prey they were best adapted for. Crush guild members had robust crushing teeth for eating hard preys so they most likely ate mollusks and ammonites. Marine reptiles in the smash guild, had small teeth with rounded points, crushing and smashing might sound similar, but crushing was used for hard prey items and smashing was used for softer prey, like squid. Long pointed teeth for trapping and piercing soft prey were characteristic of the pierce guild so they ate small fish. Pointed teeth with cutting edges perfect for tearing off chunks of large prey were characteristic of the cut guild, which specialized in eating very large prey. Species in the cut guild were usually the <b>Apex Predators</b> in their ecosystem, meaning they were at the top of the food chain. We find independent support for these guilds by looking at preserved stomach contents. Most ichthyosaurs occupied the smash and pierce gills and fed on soft-bodied fish and cephalopods. We know this because ichthyosaurs' stomach contents have fish bones And hooks from the suckers of squid. Most scientists agree that they probably swam through schools of fish or squid and snapped up the smaller animals, similar to the way dolphins hunt today. It's likely they detected prey using extremely acute vision and a strong sense of smell, as indicated by their enormous eyes, and olfactory channels. They probably couldn't hear very well, having uninsulated middle ears, and thick, non sensitive inner ear bones. Most derived ichthyosaurs actively chased down prey and used manipulation to get their prey into their mouths. Now let's talk about some of the evolutionary adaptations enable the ichthyosaurs to be such successful pursuit predators. The most basal ichthyopterygians with long, serpentine bodies like <i>Utatsusaurus</i> and <i>Chaohusaurus</i>, would not have been efficient, high-speed, long distance pursuit predators. However, their long bodies and tails were well suited for short bursts of rapid acceleration and quick turning so they probably ambushed their prey instead. More derived Ichthyosaur like <i>Stenopterygius</i> evolved streamline tuna like body shapes with a shorter torso and a tall, efficient crescent shaped tail fluke. Similar body shapes are found today in fast-swimming sharks, dolphins, and fish like the mako shark, the spinner dolphin, and the tuna. This body shape allows for high-speed cruising with minimal energy expenditure, which is an important quality for pursuit predators. Ichthyosaur tails went through multiple stages before evolving the convergent, highly efficient, crescent shape fluke. Below are several ichthyopterygian tails. Which one is the most derived? A, The crescent-shaped tail of <i>Platyterygius</i>, B, the laterally flattened tail of <i>Grippia<i>, Or C, the low tailfin of <i>Mixosaurus</i>. In basal ichthyosaurs, the tail stretched straight out behind them and was laterally flattened like tail B. Throughout their evolution Ichthyosaurs began to develop a downward tail bend to support the longer bottom lobe of a tail fluke. And a cartilaginous blade to support the shorter top lobe like tail C. Over time the bend got sharper and the lobes of the tail fluke got longer and more equal in size like tail A. The shape of tail C, where one lobe is longer than the other is known as heterocercal. The heterocercal shape helps the animal change directions and accelerate when swimming. As time when on, the crescent-shaped tails of more derived Jurassic and Cretaceous ichthyosaurs became increasingly symmetrical, a shape we call homocercal. A is the most derived tail form of the three and thus the correct answer. In addition to evolving this crescent shaped, <b>homocercal</b> tail, ichthyosaurs also required other changes to their morphology before they converged on the highly specialized tuna like body plan. Support for the tail driven swimming also required the vertebrae to change. Like we discussed earlier, the vertebrae of basal ichthyosaurs were long and narrow, conducive to flexible oscillatory swimming. As time went by, they became shorter, flatter, and less flexible, allowing the Ichthyosaurs to evolve towards thunniform swimming. In order to achieve maximum speed, more is required than a strong propulsive tail to generate thrust. A moving object encounters a lot of inertial viscus drag as it moves through the water. Therefore, maximum speed and efficiency in the water requires a smooth, streamlined shape. As discussed, ichthyosaurs converged on a fusiform body shape that allowed them to cut through the water easily, similar to torpedos and submarines. It's possible that some of the most derived Ichthyosaurs even lost their scales and were covered in smooth skin that reduced viscous drag. A tall dorsal fin made from a blade of cartilage provided stability when moving through the water at high speeds. Minimizing drag and generating thrust were not the end of the locomotive problems facing Ichthyosaurs, they also had to steer. Which would've been primarily accomplished by the flippers. Ichthyosaurs are amniote tetrapods. And so, we share many of the same bones in our own skeleton. But Ichthyosaur hands and feet are different. Compare to a human skeleton, what differences do you notice? Choose all that apply. The Icthyyosaur hand has, A more finger bones, B more fingers, C, no humorous and or D large claws. Ichthyosaurs have four flippers that contain the same arm and leg bones as you and I have. And so, they still have a humerus, which makes c incorrect. Additionally, like you and I, Ichthyosaurs did not use their limbs as weapons, and so it was more important for their limbs to become streamlined than to retain unnecessary claws, so D is incorrect. But, as you can see, Ichthyosaurs have many, many more bones in each finger. Instead of having the two finger bones, called phalanges, in our thumb, or three phalanges in our other digits. A single ichthyosaur digit might have 20 phalanges or more. Some ichthyosaurs even branch some of their fingers into additional digits. We call the addition of more finger bones, hyperphalangy, and the addition of new digits, hyperdactyly. So the correct answers are A and B. In the evolution of ichthyosaurs flippers, we can see a different solution to the problem of steering in water. As you just saw, ichthyosaur flippers are very different from a terrestrial limb. The flippers of whales and seals and manatees all have five internal fingers, demonstrating that these marine mammals are descended from terrestrial tetrapod ancestors. It is a bit of a surprise then to see the derived ichthyosaur solve the problem of flipper design in a completely different way. As with marine mammals, we see a reduction in the length of the arm bones, including the humerus and ulna and radius. But instead of seeing separate fingers as in marine mammals, derived ichthyosaurs have a mass of tile shaped bones packed together so that they form a solid flipper shaped mosaic. In the most derived ichthyosaurs, the number of digits varied from three to nine, and the hyperphalangy became extreme, with as many as 30 phalanges in a single digit, as in <i>Platypterygius</i>.
