In the previous lectures you have heard the story about how plant life colonized the land about 450 billion years ago during the late Ordovician time, forever changing the appearance of the previously barren continents. About the same time there's evidence that arthropods started to venture onto land in the form of fossil trackways, but the first definitive fossils of terrestrial arthropods appear about 419 million years ago at the very beginning of the Devonian Age. During the Devonian, the land was lush and teeming with plants and arthropods, which was an ample food resource waiting to be exploited by all animal groups. In this lecture I've described one of the biggest steps in the evolution of life on Earth, the colonization of the continents by vertebrate animals. This major evolutionary step started in the middle of the Devonian time, about 385 million years ago, and during the next 20 million years a group of vertebrates gradually achieved a fully terrestrial lifestyle, setting the scene for the later diversification of vertebrates to dominate the terrestrial environment. For an aquatic animal to be able to venture onto land and live on land, several important modifications must be done to its body physiology in order to cope with the new environment. First, it must be able to breath atmospheric air instead of breathing with gills like fish do. The axial and appendicular skeleton all needed to be strengthened in order to support the weight of the animal on land. These abilities seem to have appeared already about 400 million years ago among certain groups of the lobe finned fishes who's swim bladder could act as a primitive lung, enabling the fish to inhabit oxygen starved rivers and ponds by sucking air in from the surface above. Their fins, which were supported by a small lobe of bones, would eventually evolve into the limbs of the first tetrapods. Thanks to a good fossil record from all over the world, we are now able to document this important transition in the history of life in a very detailed way. So where can we find evidence of this important part of the evolution of life? You may find this surprising, but one of the best places in the world to look for fossils from this period of life is in east Greenland, where two of the most important early tetrapods were discovered. Back then, 360 million years ago, Greenland was part of a unified landmass connected to north America and Europe, and was situated close to the equator. The climate was tropical and on land grew dense forest of ferns and horsetails. It was along the river banks of this world that some of the first vertebrates evolved the ability to walk ashore. Today the remains of this prehistoric world can be found on the cliffs of Stensio in east Greenland. The very first finds of the tetrapod ichthyostega were done back in 1971, where their importance was quickly realized and in the subsequent years more material was collected by various expeditions. The first systematic excavation was undertaken in 1987 by a team from the University of Cambridge led by Professor Jennifer Clack. The expedition found a lot of new well-preserved ichthyostega material, and they also found another, now equally famous tetrapod, acanthostega. The skeletal materials were very well preserved. The bones are preserved in light coal-like reddish mudstones so they're easy to recognize, like this skull, where we can easily see the well-preserved bones, or this tail, which are actually very fish-like in appearance. Clack's team returned to Stensio in 1998, and again they returned home with even more fossils that have expanded our knowledge about how the early tetrapods lived and evolved. During the last decades, this material from Greenland has been intensively studied by an international research team led by Professor Jennifer Clack from the University of Cambridge, and thanks to their research we are now able to make extremely detailed skeletal reconstructions of both ichthyostega and acanthostega. One of their projects is a three-dimensional anatomical reconstruction of the ichthyostega skeleton based on CT scanning of the individual bones that have been pieced together to a fully articulated three-dimensional digital model. Such detailed anatomical reconstructions has enabled skilled model makers to build very lifelike reconstructions of how ichthyostega might have looked like when it was alive, like this very heavy looking model from the exhibition at the Natural History Museum of Denmark. Since the first finds of ichthyostega and acanthostega in Greenland, many more tetrapod fossils have been uncovered from all over the world, all showing different stages in the evolution to water life on land. Although it would seem that there's a clear evolutionary path from a fish to a four-legged terrestrial animal, the many new fossils show a very complicated picture with many contemporary evolutionary stages, and even some forms that seem to have secondary return to an aquatic lifestyle barely before they left the water. Many of the transitional forms are only known from relatively few bones, so here's a short run through of the most important and best known fossil forms to illustrate the many evolutionary steps in the process from fish to a land-living animal. Eusthenopteron was a large lobe finned fish that is thought to be closely related to the earliest tetrapods and lived in the late Devonian about 385 million years ago. The pectoral fins of eusthenopteron contained all the bones found in a tetrapod four limb, however, they were too short and frail to support its weight on land. Eusthenopteron is interpreted to be adapted to a life in shallow muddy rivers and lake systems. Panderichtys was a large fish with a body length up to 1.3 metres and it lived in the late Devonian about 380 million years ago. While still looking very much like a fish in all shape, its skeleton contained many tetrapod-like characters, especially the shape of its large skull, and the distal bones in the fins which are developed as individual digits. It is unknown whether it could use its fins to drag itself onto land. Tiktaalik was a large predator which could reach up to three meters in length. It is known from the late Devonian, about 375 million years ago. It had a large, flat skull and the skeleton in its fins had real wrist bones and the humerus were connected to the shoulder girdle so it could support its weight on land. The humerus has attachment scars for strong muscles, so it has been suggested that it was an ambush predator that could spring up from the water and snatch prey on land. Acanthostega is one of the best known late Devonian tetrapods. It lived about 365 million years ago. It was a relative small animal up to 75 centimeters in length, it had long slender legs with eight digits on each foot, but despite its long legs it is not suited to walk on land as its feet were directed outwards and could not be turned into a walking position. Furthermore, the axial skeleton was weakly developed with small connecting surfaces between the vertebrates, and the appendicular skeleton is only weakly attached to the axial skeleton, making it unlikely that it could support its body on land. It is suggested that acanthostega inhabited densely vegetated rivers and swamps where it used its legs to crawl around the roots and seaweeds. Ichthyostega was one of the earliest tetrapods to be discovered and was hailed as the missing link between fish and amphibians. Ichthyostega is often portrayed in the older literature as a fish with four strong limbs and a determined look on its face as it walks ashore from the river. It lived in the late Devonian about 360 million years ago. It was a heavily built tetrapod with strong forelimbs with seven toes on each foot. Its skull was robust and had a wide mouth full of short sharp teeth, as can be seen on this very detailed reconstruction made by Hans Viborg from the Royal Danish Academy of Fine Arts. It also had a very robust rib cage, suggesting it was able to drag itself onto land. The strangest thing however, turned out to be the orientation of its hind limbs. While the pelvis was strongly built as can be seen on this three-dimensional model, the recent anatomical studies have shown that the hind limbs were orientated backwards, making them not very useful for walking on land. The tail of ichthyostega was still very fish-like, so the present interpretation of ichthyostega is that it probably spent most of its life in water, but was able to drag itself on shore occasionally. Tulerpeton lived about 365 million years ago in the end of the Devonian times. It could be up to 1.5 meters long and was the first tetrapod to have a real flexible joint in the neck so it was able to move its head separately from the body. It had powerful legs with six toes on each foot and the legs could be articulated to serve both as powerful paddles in the water and to walk on dry land, and it was perhaps the first tetrapod to really walk ashore. Crassigyrinus was an enigmatic tetrapod that really complicates the picture of tetrapod evolution. It lived during the late Carboniferus, about 318 million years ago. Crassigyrinus was a large tetrapod with a body length of up to two meters. The body was long and slender but the most enigmatic about it was its fore and hind limbs. The limbs are built developed as legs but they are extremely short, in fact so short that it could not be used to walk at all, or even lift the body clear off the ground. Crassigyrinus also had very large eye sockets, suggesting that it could see in dark water. Perhaps crassigyrinus represents the first tetrapod to have secondarily adapted to a life in water, just barely after the evolution enabled them to leave the water. But the debate is still on and future finds might one day cross every interpretation of crassigyrinus. Dendrerpeton from the late Carboniferous times, about 315 million years ago, was one of the first real amphibians fully adapted to life on land. It could be up to one metre long and had powerful skeleton to support its weight. There are no remains of gills left in its skull, so it was apparently fully adapted to breathing atmospheric air. Many of the fossils are found in fossilized hollow tree trunks, suggesting that it inhabited the damp carboniferous forest. At this stage the tetrapods had evolved the ability to progress on land and exploring the many new ecological niches it offered. However, they were still bound to live near water as they were still amphibious animals. The skin of amphibians are not watertight, meaning that water can evaporate through the skin, so the animal dries out if it does not keep its skin moist. The amphibian eggs are also dependent on being laid in wet or moist environments in order to not dry out. So even though the early tetrapods were now fully capable of walking around on land, they were still bound to a life near water and moist environments. The solution to this problem was the evolution of waterproof skin which could prevent the animal from drying out, and the evolution of a watertight membrane in the egg, the amnion. This allows the evolution of the early reptiles to diversify further into the terrestrial ecological niches and through the following geological time periods, diversify further into reptiles, mammals, crocodiles, dinosaurs and birds. It is not only the skeletal record that can give us information about the early tetrapods and their mode of locomotion. In several places around the world fossilized tracks and trackways from early tetrapods have been found. One of the very best places to see this glimpse of evolution captured in the rocks, is on Valentia Island in the southwestern corner of Ireland. On a cliff surface just at the shoreline, the world's longest trackway for a tetrapod is preserved. The trackway is about 370 million years old and can be followed for about five metres across the surface. The trackway shows two parallel sets of alternating manus and pes sets. A second trackway segment show a prominent medial groove from the animal's belly being dragged along the surface between the legs of the animal. When the animals made the tracks back in the late Devonian, the whole area was a flood plain where the animals walked across them on descent, perhaps partly submerged in shallow water. From the current fossil record and interpretations of the finds, it seems that the ability to walk on four legs arose among the early fish-like tetrapods before they started their venture onto land. Perhaps there was an adaptation to life in shallow river systems. Interestingly, today almost 400 million years later, there are still species of fish and sharks that are capable of walking by alternating movements of their paired fins on the bottom in a fashion that could be similar to that of the early tetrapods. Especially the small epaulette shark which inhabits the shallow tropics of Australia and New Guinea, could be a good analog for the early stages of the evolution of tetrapodal walking. This lecture has given you an overview of the important transition from a life in water to a life on land which occurred in the late Devonian and early Carboniferus times. Through the following geological time periods the vertebrates successfully diversified and adapted to a life in virtually all ecological niches on land. Some of them even took evolution one step further and evolved the ability to fly. And some groups went back to a life in water, but their stories are the topic of all the lectures in this course. Thank you for watching.
