Welcome back to Marine Reptiles. In lesson two, we'll examine the diversity of form and function for Ichthyopterygians and their closest relatives. The Ichthyopterygians are a distinctive and highly specialized group of marine reptiles that lived from the early Triassic to the mid-cretaceous, over a span of nearly 150 million years. They all had the same basic body plan but were specialized in a variety of ways ranging in length from 1 to 21 meters. Some were small and snub-nosed, others had long pincer-like jaws and still others had an overhanging upper jaw like a swordfish. The derived members were streamlined like the fastest fishes. They breathe through nostrils located between their eyes and propel themselves through the water using their crescent shape tail flukes. They are an incredible example of convergent evolution, being similar in shape to sharks, tuna, and dolphins. If a casual observer looked at a drawing of a Icthyosaur and identified it as a dolphin, this mistake could easily be forgiven. You can see that the two look very similar. However, a close examination will reveal some key morphological differences between them. If you compare these pictures of an Ichthyosaur and a dolphin, you might notice three major differences. On the Ichthyosaur, the breathing holes, or nares, are in front of the eyes instead of on top of the head. Also, the tail fluke is vertical like we'd see on a fish. It's not horizontal as we see on the dolphin or any other marine mammal. Finally, the Icthyosaur had hind flippers that might of aided in steering. Even though these animals appear to be very similar, these key difference indicate that they evolved from different branches of the amniote tree. Icythyopterygians are best known for the dolphin-like members of this clade, the Ichthyosaurs. Although Ichthyosaur means fish-lizard, they were aquatic amniotes, not fish. Let's take a look at this generalized diagram of an Ichthyosaur, and see if we can figure out which branch of the amniote tree these animals belong to. Ichtyopterygians had only one temporal fenestra and so you might thing that they are synapsid amniotes. But ichtyopterygians and many other marine reptiles we'll discuss in this course have a new kind of skull configuration called <b>euryapsid</b>. Euryapsid skulls were once thought to represent a fourth branch of amniote diversity. But are now understood to be a modified diapsid skull. Therefore, ichthyopterygians evolved from ancestors with two openings in the skull behind each orbit. These openings are the upper and lower temporal fenestra. At some point, the bones behind the eyes got smaller and smaller until eventually the lower temporal fenestra was lost, leaving only the upper temporal fenestra. The second temporal opening may no longer have been necessary after this group started swallowing their prey whole since temporal fenestra are associated with chewing muscles. We can differentiate synapsid and euryapsid skulls based on which bones surround the fenestra. Even the earliest ichthyopterygians show evidence of being well adapted to a fully aquatic lifestyle. On land, the skeleton is essential for counteracting gravity, and a key element of the system is the tight <b>articulation</b>, or close connection, of the leg and hip bones to the spine. Acquatic organisms supported by the water they live in don't need a pelvic girdle capable of supporting their bodies. Even primitive Ichthyopterygians have lost the boney connection between their pelvic girdle and their spines. Like whales today, the pelvic girdle floated in the layers of muscle of the abdominal wall. You can see this quite clearly in this skeleton. See how far apart the girdles are from the spine? The spinal column provides necessary support to an animal. It needs to provide more support on land and less in the water. But the spine also limits the flexibility and type of motion that an animal can perform. Think of the vertebral column is a string of beads. Which of these beads strings would be most flexible? A, B, or C. Think about a string of beads, if the beads are shaped like long narrow cylinders like many vertebrae are, the string can only flex in-between each bead, decreasing the length between each bead increases flexibility because there's less distance between each joint. So the shortest vertebrae make the most flexible spinal column and the long vertical cylindrical vertebrae would result in the least flexible spine. That makes A, the correct answer. Ichthyopterygians have more vertebrae in their torsos and tails than most terrestrial reptiles do. They've got more than 40 vertebrae between their heads and pelvic girdle and up to 80 vertebrae in their tails. The increased number of vertebrae facilitated the axial undulations necessary for anguilliform swimming. Early Ichthyopterygians had vertebrae that were longer than they were wide, kind of like this mosasaur vertebra. Throughout their evolutionary history, the vertebrae developed into the characteristic concave disk shape, like this. Now in terrestrial vertebrates the vertebrae have complex <b>neural spines</b>, and these are blade-like processes that come off of the vertebrae like you can see on this bison vertebra. And processes on the neural spines allow for tight articulation to help support the body against gravity. Ichthyopterygian neural spines are less complex and poorly attached and this increases the flexibility of the spinal column. As is typical of many marine reptiles that evolved during the Triassic period, the exact evolutionary relationships of Ichthyopterygians are still unresolved. They appear very suddenly in the fossil record, and the oldest examples are fully adapted aquatic forms. We have not yet discovered any transitional forms. So we really don't know exactly how they evolved or from what. There are three major hypotheses about which Sauropsid group Ichthyopterygians belong to. Ichthyosaurs might be basal Diapsids, Lepidosauromorphs or even Archosauromorphs. In this course we will use the most conservative hypothesis and place ichthyopterygians as a separate branch of Diapsida. Now let's look at some of the members of the group. Since ichthyopterygians appeared suddenly in the fossil record and no transitional forms have been found, it's difficult to know what other reptiles they're most closely related to. Two possible candidates are the <b>thalattosaurs</b> and the <b>hupehsuchians</b>. Thalattosaurs are large Triassic marine reptiles with paddle-like feet and long snouts. Hupehsuchians are also from the Triassic. They have fusiform bodies, similar to ichthyosaurs, but had paddle-like feet instead of flippers. Bony plates along their backs and gastralia on their stomachs would have helped them achieve neutral buoyancy. Now that we've looked at some possible relatives of the ichthyopterygians, let's take a closer look at <b>ichthyoperygia</b> proper. The most famous members of the ichthyopterygia are the <b>ichthyosaurs</b>. And the most derived ichthyosaurs have fusiform bodies with a crescent shaped vertical tail fluke. Their vertebrae have a morphology that we call <b>amphicoelous</b>, which means that both faces of the disk are concave. In the skull, ichthyosaurs typically have a long snout and the teeth are set in a groove rather than in sockets like most marine reptiles. They also have a large orbit and a reduced cheek region. Ichthyosaur teeth are distinctive because in most species, the base of the tooth has a fluted, grooved external appearance. This easily recognizable external texture reflects an internal pattern of folding in the dentine of the tooth known as <b>plicidentine</b>. Of the following pairs of fossils, select the one in each set that comes from an ichthyosaur. Remember that ichthyosaur vertebrae are amphicoelous, meaning that they are concave on both sides like we see in vertebra B. Ichthyosaur jaw bones have a groove for teeth instead of sockets, like we see in jaw C. And finally, ichthyosaurs have the characteristic feature called plicidentine, which can be identified by the folding in the base of the tooth, as in E. Now that you know the key Ichthyopterygian characteristics, let's go back in their evolutionary history, and trace how they developed. Ichthyopterigians first appear in the fossil record during the early parts of the Triassic period. The earliest ichthyopterigian is a small one meter long animal called <i><b>Parvinatator</b></i>, which remains poorly known to this day. However, paleontologists do know that this taxon displayed the beginnings of a trend that more derived ichthyosaurs would take to an extreme: <b>hyperphalangy</b>. If you look at your own hand or foot, you will see that you have two or three segments to each finger or toe, which are known as <b>digits</b>. Now, each of these segments is an individual bone called a <b>phalanx</b>. Many terrestrial tetrapods also have two or three phalanges in their digits. However, it's common for aquatic tetrapods to develop many extra phalanges as their terrestrial limb lengthens into a fin. <i>Parvinatator</i> was already well adapted to its aquatic environment, and had up to five phalanges in each digit. Some of the more derived ichthyosaurs we'll learn about later had as many as 30 phalanges per digit. Other early Triassic ichthyopterigians include the fully aquatic <i><b>Utatsusaurus</b></i> and <i><b>Grippia</b></i>. The teeth of <i>Utatsusaurus</i> were still set in shallow sockets, but the dental groves seen in more derived ichthyosaurs had also started to form. <i>Utatsusaurus</i> also has a long, narrow humerus, which is typical for terrestrial tetrapods. And the more aquatic <i>Grippia</i>, the humerus is wider then it is long. Unlike more derived ichthyosaurs, <i>Utatsusaurus</i> and <i>Grippia</i> lacked dorsal fins, probably didn't have prominent tail flukes, and had longer, more sinuous bodies because they had more vertebrae in their spinal column. And these traits tend to be representative of anguilliform swimmers. Related to <i>Grippia</i> is a genus from the early triassic of China, <i><b>Chaohusaurus</b></i>. This small ichthyopterigian never grew longer than two meters. <i>Chaohusaurus</i> also had multiple rows of crushing teeth in its upper jaw. Its limbs had developed into primitive flippers but the pattern of bones in the fingers more closely resembled terrestrial tetrapods. Despite its primitive number of phalanges, <i>Chaohusaurus</i> is believed to have been fully aquatic, as the limbs are well-developed flippers. New fossils of <i>Chaohusaurus</i> include pregnant females, including this specimen, which shows three embryos in the mother's abdomen. This demonstrates that these primitive ichthyopterigian gave live birth in the water. Another possible basal ichthyopterigian is <i><b>Omphalosaurus</b></i>. And this animal is definitively fully aquatic and shares many features in common with basal ichthyopterygians like <i>Utatsusaurus</i> and <i>Grippia</i>, but its teeth look very different from other ichthyopterygians. Instead of rows of conical fish-eating teeth, <i>Omphalosaurus</i> has a pavement of rounded, bulbous teeth across most of the front of its lower jaw. We would normally expect to see teeth like that in the <b>durophagous</b> animal, one that uses powerful crushing bites to break down hard prey. But the skull of <i>Omphalosaurus</i> was not suited to biting down hard. Having crushing teeth far away form the jaw joint means that the jaw isn't optimized for powerful bites. It's unclear exactly what it would have eaten. Even though the external shape of the teeth is different from most ichthyosaurs, the presence of plicidentine inside the teeth suggest that it is probably an ichthyopterygian. Imagine a basal ichthyosaur that was 10 meters long with an incredible 60 vertebrae in front of the pelvis. Now imagine it also had a tail without a fluke, and no dorsal fin. Finally, imagine that it had very small eyes and a long snout full of robust, conical teeth. Which of the three descriptions below do you think would best describe this ichthyosaur? Remember, we covered anguilliform, carangiform, and thunniform swimming in lesson one. A, An anguilliform swimmer that grazed in shallow water, B, A carangiform swimmer that hunted in deep water, or C, A thunniform swimmer that hunted in shallow waters? The ichthyosaur we just described is called <i>Cymbospondylus</i>. Its large, conical teeth meant that it was a hunter that probably preyed on fish and cephalopods. The lack of a tail fluke indicates that it was probably not a thunniform swimmer. So the correct answer here is B, it probably hunted in deep water where there was abundant fish and cephalopods, and it was most likely a carangiform swimmer. Now, let's go on to discuss this genus and others like it. The most derived ichthyopterygians formed the clade <b>Ichthyosauria</b> and are united by the presence of nares that face laterally rather than being on top of the skull. Some of the earliest true ichthyosaurs were genera like <i><b>Cymbospondylus</b></i> and <i><b>Mixosaurus</b></i>. <i>Cymbospondylus</i> was the first large ichthyosaur, growing to ten meters long, about the size of an adult male killer whale. This ichthyosaur was very elongate. It had 60 vertebrae in front of its pelvis and its very long tail would have assisted in propulsion. <i>Cymbospondylus</i> also had a much longer snout then earlier ichthyopterygians. It also has the smallest orbit and therefore eye of any known ichthyosaur. <i>Mixosaurus</i> represents further steps towards solving the aquatic problem. Instead of a lizard-like body with a paddle-like tail, it had a more fusiform, streamlined body, a small dorsal fin for stability, and a lower tail fluke for added propulsion. The amphicoelous shape of the vertebrae gave greater flexibility to the spine and probably resulted in an anguilliform swimming style similar to more primitive ichthyopterygians. The biggest ichthyosaur of them all has been found in British Columbia and is named <i><b>Shonisaurus</b></i>. It was at least 21 meters long, which is about the size of a finned whale. The skull alone would have been over three meters long. <i>Shonisaurus</i> and other large ichthyosaurs known as shastasaurids may or may not have had dorsal fins. But paleontologists think they likely did have a low tail fluke to generate additional thrust. the vertebrae in this group had developed into the disc-like morphology characteristic of the derived ichthyosaurs. And they also lacked teeth as adults, and so what they ate is a mystery. Some paleontologists think they may have swallowed soft-bodied cephalopods whole. Shastasaurids and their early relatives, though not as derived as later ichthyosaurs, had already evolved to solve many of the issues associated with the aquatic problem. Based on this reconstruction of a shastasaurid, select which aspect of the aquatic problem these ichthyopterygians had probably solved. A, Drag, B, Stability, C, Propulsion, or D, All of the above? Early ichthyosaurs were already extremely well adapted to their aquatic environment. Their bodies were streamlined to counteract drag. They had dorsal fins for stability in a 3D environment. They had laterally compressed tails to generate propulsion, and their nares had migrated to the top of their heads to facilitate breathing at the surface. So the correct answer is D, All of the above. Ichthyosaur is more derived than the shastasaurids form the clade <i><b>Euichthyosauria</b></i>, and are united by the loss of the bony attachment between teeth and jaws. The majority of Euichthyosaurians are in the clade <i>Parvipelvia</i>. Ichthyosaurs of this clade are united by straight as opposed to plate-like scapula, a radius wider than it is long, and a reduced, narrow pubis. A unique family within this clade is <b>Leptopterygiidae</b>. They have converged on a similar morphology with swordfish, having a very narrow elongate upper jaw, sometimes greatly overhanging the shorter lower jaw. One species was named <i><b>Excalibosaurus</b></i>, both for its sword-like jaw and for the fact that it was found in England near where King Arthur's legendary sword was supposed to have emerged. In another species, <i><b>Eurhinosaurus</b></i>, the upper jaw bears teeth even beyond the contact with the lower jaw. We're not certain what these strange animals ate, but they probably had similar feeding habits to swordfish that use their extended <b>rostrum</b> or snout to slash at and injure prey making it easier to catch and swallow. The genus <i><b>Temnodontosaurus</b></i> contains many species, including some of the first ichthyosaur fossils ever discovered. Unlike most ichthyosaurs within Parvipelvia, species of <i>Temnodontosaurus</i> had fewer than five digits in their flippers. The phalanges in the first digit were notched, possibly for the attachment of the soft tissues that covered their limbs. The final ichthyosaur group we will discuss is the Thunnosauria. Paleontologists consider this clade to be the best adapted to the marine environment. Take a look at this reconstruction. Why do paleontologists think this? A, They eat only fast moving prey such as tuna, B, Like tuna, they evolved gills so they could stay underwater indefinitely, C, When they were first discovered, early paleontologists mistook them for tuna, or D, Their body plan resembles a tuna, which are some of the strongest swimmers alive today. Since ichthyosaurs are reptiles, they have different skeletons than fish, and they never evolve gills, so B and C are incorrect. Thunnosaurs certainly ate fast moving prey, but like any predator they would have eaten anything they could including slow prey. So A is incorrect. The correct answer is D. Tuna are some of the strongest swimmers in today's oceans. Thunnosaurs looked very similar to tuna and therefore were probably the fastest, best-adapted of all the Ichthyosaurs. As noted in the previous question, the most highly derived clade are the <b>Thunnosauria</b>, named for their resemblance to the strongest open ocean cruisers of today: tuna. The species in this clade are united by fore flippers twice as long as their hind flippers. These ichthyosaurs also have much more exaggerated down turn bends to their tails. The tail bend is formed by a few wedge shaped vertebrae, and the vertebrae behind this kink would have supported the bottom half of a crescent shaped tail fluke. The most basal thunnosaur is <i><b>Ichthyosaurus</b></i> found near Lyme Regis on the south coast of England in the early 1800's. These ichthyosaurs had flippers, well-adapted to an aquatic life. They had advanced hyperphalangy, and also a condition called <b>hyperdactyly</b>. Not only did they have at least 25 bones in an individual digit, but they could have nine or more digits in each flipper. These adaptations would have made the flippers larger, stiffer and solid for stronger, faster, and more precise changes of direction. The genus <i><b>Stenopterygius</b></i> and all the more derived Ichthyosaurs are characterized by fusion of the Pubis and Ischium. into a single pelvic element. <i>Stenopterygius</i> is known from thousands of beautifully preserved fossils, including numerous pregnant females. <i><b>Ophthalmosaurus</b></i> is another thunnosaurid, well-known from around the world. It had the largest eyes relative to it's skull of any vertebrates that has every lived. It's eyes were the same size as those of a blue whale, an animal almost seven times larger. The most derived genus of Ichthyosaur, in one of only a few that survived into the Cretaceous is <i><b>Platyperygius</b></i> and there's several species within this genus but they all exhibit extreme hypertadactyly and hyperflangy. With at least seven digits containing up to 30 short blocky phalanges in their fore flippers. In this first section I presented you with a sample of Ichthyopterygian diversity. Before we go one to discuss some of the ways Ichthyosaurs solved the aquatic problem, take a moment to review the species, included in the interactive file of genetic tree. This is the same interactive resource you used in lesson one.
