The best known and most primitive tetrapods, by which we mean vertebrates with four limbs and feet with digits, are <i>Acanthostega</i> and <i>Ichthyostega</i>. Both taxa are from the latest Devonian of Greenland. <i>Acanthostega</i> is generally considered to be the more primitive of the two, although, it had some features seen in the later tetrapods that <i>Ichthyostega</i> lacks. Do you notice anything weird about their feet? Early tetrapods had more than five digits. <i>Acanthostega</i> had eight digits in it's manus, or hand, and probably just as many in its pes, or foot. It has a very tetrapod-like skull roof with a long snout and large eyes on the top of its head. Its skull also has a pair of notches at the back that may have been spiracular notches. It had an ossified vertebral column but its vertebrae were not differentiated into distinctive neck and trunk regions. It also had fully functioning gills as an adult, a lateral line system and a well- developed tail with lepidotrichia. Compare the skeleton of <i>Acanthostega</i> to a living terrestrial tetrapod, in this case, a dog. Do you think that <i>Acanthostega</i> would have been able to walk very well on land? A. Yes, because it has feet with toes. B. Yes, because it doesn't have wrist and ankle bones. C. No, because it has limbs that point posteriorly. Or D. No, because it doesn't have a neck. The limbs in <i>Acanthostega</i> were tetrapod legs with digits but they probably weren't used for walking. The limbs point posteriorly and the shoulder joints anatomy means it couldn't have swung the limbs forward enough to allow it to walk, therefore, C, is the correct answer. In addition to the limb orientation, there are a few other features in <i>Acanthostega</i> that suggest it didn't regularly support its weight on its limbs. The radius is much longer than the ulna, meaning the elbow probably couldn't bend much. The wrists and ankles are poorly developed. The hind limb is paddle-like, with flattened and overlapping tibia and fibula. Good for swimming, but not for bending at a knee. Its pelvis is also small, but was probably connected via ligaments to a sacral rib. In terrestrial tetrapods, the sacrum connects the pelvic girdle to the vertebral column. And the presence of a sacral rib means there was some connection between the hind limb and the back bone, although the back legs probably weren't weight bearing. <i>Acanthostega</i> was almost certainly a primarily aquatic animal. It also happened to have feet with digits. Now digits have traditionally been thought of as being virtually essential terrestrial adaptations. And their presence in <i>Acanthostega</i> suggests that this is not the case. Although we have no idea what function they performed in this aquatic animal, it probably used its limbs to push itself around the vegetation in the river channels in which it lived. However, the presence of a sacral rib means it may have been able to support it's weight out of the water for brief periods of time, but it couldn't have been very good at it. <i>Ichthyostega</i> also from the Late Devonian of Greenland was the first Devonian tetrapod to be found and described, in 1932. It's probably the best known early tetrapod. Its skeleton is much more solid and robust than that of <i>Acanthostega</i>. The vertebrae in <i>Ichthyostega</i>, are differentiated into distinct regions and the thoracic vertebrae, they bear expanded overlapping ribs. The ribs overlap so extensively, that the animal's torso couldn't have moved much side to side. <i>Ichthyostega</i>, must have flexed it's spine more vertically then laterally. <i>Ichthyostega</i> also has large forelimbs and robust pectoral girdles. The cleithrum is still the largest part of the pectoral girdle, but the radius is shorter and more tetrapod-like than in <i>Acanthostega</i>. The wrist and manus aren't known. The pelvic girdle is robust and articulates directly with a sacral rib, but the hind limb is paddle-like. It has seven toes in its pes, which points posteriorly. It wouldn't have been able to rotate its toes forwards, making it unlikely that the hind limbs were used for walking. So was <i>Ichthyostega</i> a terrestrial tetrapod as it's so often reconstructed? It's possible that it could have supported its weight on at least its front limbs out of the water. However, it's elbows and knees couldn't have flexed very much and the forelimb couldn't fully extend. It also has gill arches and a small opercular element suggesting it retain functional gills. And it had a lateral line system and a tail fin. Taken all together it seems <i>Ichthyostega</i> was primarily aquatic. Maybe, it lurked at the water's edge and could lunge out like a crocodile when it had to using its front legs to lurch along and its back legs to stabilize itself. Many of the features that we think of as tetrapod features therefore evolved primarily in aquatic animals. So no one can deny that <i>Acanthostega</i> and <i>Ichthyostega</i> have limbs or digits, but they were in many other ways still highly aquatic. Later tetrapods would evolve increased skeletal support allowing them to support their bodies outside of the water. Although, early tetrapods remained aquatic as juvenile they would eventually become entirely terrestrial as adults. There are a few ways to tell whether tetrapods were primarily terrestrial and these have to do with the kinds of adaptations that are necessary to be successful out of the water. What changes would you expect to see in a tetrapod living on land that are not necessary for vertebrates living in water? A. Smaller tails. B. More robust bones. C. Bigger eyes. Or D. Smaller teeth. Terrestrial tetrapods have to be able to support their weight on their limbs, without any help from the buoyancy of water. Even limbless vertebrates like snakes need to protect their organs with strong ribs that encircle their body cavity. This means terrestrial vertebrates need robust bones, so B, is the correct answer. As you travel further into the tetrapod tree, the paired limbs become larger, more robust, and more flexible with well developed shoulder, hip, elbow, knees, wrists, and ankle joints. The digits eventually stabilized at five or fewer in each manus and pes. The sacrum became more robust as did the vertebral centra and the neck became better developed. The first two vertebrae become modified to allow for movement of the head, creating the neck. Terrestrial vertebrates also lose some of the features they inherited from their aquatic ancestors, like gills, a lateral line, and fin rays. These are useful in the water, but serve no function, and may have been a hindrance, on land. Terrestrial vertebrates also had to sense things in their environment in entirely new ways. If you've ever tried to see or hear under water, you know that your senses don't work in quite the same way as the do in the air. Likewise, fishes are very well adapted to finding food in the water but those adaptations don't function as well in the air. Air and water refract light differently, so the lens in the eye has to change to adapt to seeing in one or the other environment. <i>Panderichthys</i> could probably see out of the water, so the ability to see through air could have evolved long before the first terrestrial tetrapods. Although changes in vision don't fossilize well, we can see changes in hearing reflected in the tetrapod's skeleton. The density of fish bodies, is close to the density of water. So, sound waves can pass from the water, right through into the inner ear and otic capsules of the brain of the fish, with no loss of intensity. This means, fishes don't need any kind of special adaptations to transfer sound to the sensory cells of their inner ear. Air, on the other hand, is much less dense than both water and vertebrate bodies. And sound waves traveling through air, when they hit the body tissues, would mostly reflect off, rather than being transmitted to the sensory organs. In order to hear anything, a terrestrial vertebrate needs some mechanism to collect and amplify sound waves and transmit them into a form that can be picked up by the sensory receptors in the inner ear. This mechanism is called an impedance matching middle ear. Impedance is complicated but it basically means the amount of resistance something presents to sound waves passing through it. If the impedance of two substances is very different, sound waves will bounce off of the boundary between the two substances rather than passing through it. In terrestrial vertebrates the middle ear is composed of two main parts: the tympanum, or ear drum, and the stapes. The tympanum is a membrane of skin stretched over the opening that used to be the spiracular notch. The stapes is a modified hyomandibular which supported the jaw arch in most fishes. The middle ear cavity is therefore homologous with the spiracle in aquatic vertebrates. The tympanum collects sound waves and concentrates them. The stapes transmits those sound vibrations to a small opening in the otic capsule called the fenestra ovalis, which means, the oval window. By concentrating the sound into such a small area, it is amplified enough that sound can be detected by the sensory cells in the inner ear. It's hard to tell whether a notch on the back of the skull is a spiracle or had a tympanum, so we mostly rely on the form of the stapes to tell us whether a tetrapod had an impedance-matching middle ear. If the stapes is big and robust, it would not have worked well in transmitting sound waves. So it probably acted as a strut to brace the side of the skull against the brain case. For the stapes to effectively transmit and concentrate sound, the stapes had to be slender and delicate. So, animals that had skull notches, slender stapes, and no obvious aquatic adaptations, like a lateral line, were probably, primarily terrestrial as adults with impedance-matching middle ears. <i>Seymouria</i>, a Permian tetrapod from the United States, is an example of a tetrapod with aquatic larvae, but fully terrestrial adults, with impedance-matching middle ears, robust limbs, stocky bodies, and short tails. Once tetrapods were able to move about and sense airborne stimuli effectively on land, they rapidly diversified and spread all over the world probably taking advantage of the arthropod prey that lived in the first forests. All the features, they inherited from their early vertebrate ancestors would be passed down to their descendants, including ourselves. The transition from aquatic fish to terrestrial tetrapod is a major milestone in the evolution of vertebrate animals, and this is where we end our whirlwind tour of our ancient relatives and ancestors. This journey, from spineless chordate to jawed fishes, to land roving tetrapods, took approximately 200 million years and that's nearly half the time of the entire existence of vertebrates. We squeezed it into a mere few hours. There's much more to the story than we can cover in this MOOC. Paleontologist, Neil Shubin, who is one of the discovers of <i>Tiktaalik</i>, wrote a best selling book titled Your Inner Fish. One could not have chosen a better phrase to capture the core reason why we should study early vertebrate evolution. Think of the long evolutionary road that has lead, well, to you. Each of us is walking proof of a deep ancestry that traces all the way back to the Devonian, Silurian, and even Cambrian. It's our hope that after this course, you've become more acquainted with your inner fish. Where do we take it from here? You can learn more by visiting a natural history museum in your area, registering at the University of Alberta's other new courses in paleontology, or investigating our program here at the University of Alberta. I hope to see you again.
