In the last video, we have seen how some of the major lineages of vertebrates that make our modern faunas have appeared in the Triassic and evolved during the Mesozoic. We shall now carry on this overview, starting with the crocodiles. The first crocodiles appeared in the Late Triassic during the Norian, but at that time, they were small, 50 cm long on average, fully terrestrial, lightly built and delicate animals as we can see on this reconstruction of Terrestrisuchus from the Late Triassic of Great Britain. As crocodiles belong to the crurotarsans, a clade we have encountered in the first video and well-known for having developed an erect posture during the Late Triassic, the fact that they were highly adapted to terrestrial environment should not surprise us. The question is much more why do they occupy today such a different ecological niche? At the time of the appearance of the first crocodiles, the niche of large predatory aquatic tetrapods is already occupied by two different clades: the temnospondyl amphibians and the phytosaurs. The temnospondyls amphibians appeared in the Carboniferous, and although most of them did disappear at the end of the Permian, during the worst mass extinction the Earth has ever known, some of them did make it into the Triassic and became major predators in rivers and lakes. Their appearance is quite crocodile-like and they possessed a very large skull armed with well-developed teeth as we can see on this ventral view of a skull of Mastodonsaurus. Mastodonsaurus was a formidable predator, which could perhaps attain 6 metres in total length. The phytosaurs on the other hand appeared just before the crocodiles, probably at the beginning of the Carnian and directly invaded the aquatic niches. Being crurotarsans like the crocodiles, they are actually very similar morphologically to the latter. When looking at their skulls, the main difference appears to be the fact that their nose is situated very close to the eyes and not at the end of the snout as in modern crocodiles. Crurotarsans are amniotes and as such, and contrary to amphibians, they lay their eggs on land and not in water. That probably gave them an advantage over temnospondyls during droughts. But anyway, the niche of large aquatic predator was quite overpopulated when the first crocodiles appeared, and they could not invade this niche at the beginning of their history. It was the mass extinction that occurred at the end of the Triassic that led most of the temnospondyls and all the phytosaurs to extinction, freeing the niche for the crocodiles to invade at the beginning of the Jurassic. It is probable that the crocodiles invaded this niche because they were in competition on land with the new lords of the terrestrial realm: the dinosaurs. However, terrestrial crocodiles can also be found during the Mesozoic, living along the dinosaurs, as you can see on this reconstruction of Anatosuchus, which was found in the Lower Cretaceous of Africa. During the Mesozoic, some crocodiles became also adapted to life in the sea. Some, like the Jurassic Metriorhynchus, became so well adapted to marine life that they developed a fish-like tail, and it is unclear whether they were still able to go back to land to lay their eggs as modern sea-turtles do. As you can see the present diversity of crocodiles is a pale reflection of their past success. Together with the appearance of the modern tetrapod lineages, the end of the Triassic saw the appearance of a major innovation for vertebrates: the first winged animals capable of powered flight. They are called the pterosaurs and they are closely allied to dinosaurs. Be careful, they are NOT dinosaurs though. To be able to fly, their skeleton is made of hollow bones and it is therefore quite difficult to fossilize. Moreover, as we can see on this delicate fossil from the Upper Triassic of Greenland, the first pterosaurs were small animals. Hence, complete fossils are quite rare in the fossil record, and the one I am showing you is actually exceptional. The wing of these animals was supported by a single, very elongated finger, the fourth one. In addition, a bone unique to pterosaurs, known as the pteroid, connected to the wrist, helped to support a forward membrane, called the propatagium, between the wrist and shoulder. As we have just seen, the Triassic pterosaurs were rather small animals, with a wingspan of no more than 1 m, but during the Jurassic and the Cretaceous they diversified dramatically and the largest Cretaceous species had a wingspan in excess of 15 m. Of course, pterosaurs cannot be qualified as "modern tetrapods", but as we shall see in the next video, their evolution impacted the evolution of another modern group: the birds. Adaptation of crocodiles to marine life was our first opportunity to discuss the marine life. What about the early evolution of the fishes that make the vast majority of modern fishes: the actinopterygians, or ray-finned fishes, and the sharks? The Triassic was the time of appearance of the first modern-like bony fishes, the teleosts. The most primitive teleost to appear in the Late Triassic was probably Pholidophorus. These fishes, compared with the more primitive ones from the Palaeozoic, developed two characteristics that greatly improved their speed and mobility. First, they reduced their scale covering. As we can see when comparing these two fossils from our collection, Palaeozoic and many Mesozoic fishes had their body protected by thick scales covered by enamel-like tissue. These provided good protection against potential predators, but made them heavy, and therefore pretty slow. On the contrary, when looking at this fossil teleost, we can see that the scales are so thin that they are not even preserved in many fossils. Second, the caudal tail became symmetrical and was made more efficient by the presence of modified neural arches, called uroneural bones, which ensure similar stiffness to the dorsal and ventral lobes of the caudal fin. The appearance of the teleosts corresponds therefore to a change of selection pattern among bony fishes, putting more emphasis on speed and agility rather than on thick protective armour of scales. In a certain way, this is the equivalent of what happened on land with the appearance of an erect posture, which allowed the dinosaurs and the crurotarsans to become more active than their predecessors. The appearance and development of the much faster teleosts impacted on the evolution of sharks. The dominant sharks during the Triassic were the hybodonts, a survivor group from the Palaeozoic. In general aspect, they were quite similar to modern sharks but they were not very quick swimmers and lacked endurance because their vertebral column was made only of cartilage, and lack the necessary stiffness to anchor powerful muscles. The first modern sharks, called neoselachians, appeared at the end of the Permian, but they remained relatively rare in the seas until the end of the Triassic. Contrary to the hybodonts, they possessed calcified vertebral centra like the one I am showing you that stiffened their vertebral column, and they were therefore better swimmers than the hybodonts, both in terms of speed and endurance. These better swimming abilities gave a clear advantage to the neoselachian sharks when the bony fish assemblages started to change at the end of the Triassic with the appearance of the fast swimming teleosts. They supplanted the hybodonts as main marine predators during the Jurassic and the Cretaceous, and by the beginning of the Cretaceous most of the modern families that we can see today already existed. The hybodont took refuge in fresh waters and finally disappeared at the end of the Cretaceous, at the same time as the non-avian dinosaurs. We have now reviewed most of the modern vertebrate lineages, and we have noticed that most of them appeared during the Triassic. Appearance of new lineages is one thing, but dominance over existing faunas is another. The rise of these new lineages to a dominant position in the ecosystems was mostly the result of a global crisis known as the End Triassic Mass Extinction. It basically shaped the vertebrate faunas as we know them today. This extinction event, however, is a complex phenomenon showing two different phases, one at the end of the Carnian, 228 million years ago, and one at the end of the Triassic, 201 million years ago. At the end of the Carnian, there was a major climatic change on most of our planet. While most of the Triassic was characterised by a climate with heavy rainfall, the Late Triassic had a more arid climate. What triggered this climate change is currently unclear, but it had important consequences on the floras, and the forests of the seed ferns Dicroidium, which is a primitive gymnosperm of the southern hemisphere were replaced by forests made of more advanced gymnosperms. This was the time when most of the modern families of gymnosperms appeared. The gymnosperms are seed-producing plants that include, among others, conifers, cycads and ginkgo. Such changes in the floras caused a collapse of the food chain and the extinction of the dominant and very specialized Triassic herbivorous tetrapods: the dicynodonts, like this Placerias from the Carnian of North America, the herbivorous cynodonts, close to the mammal ancestry like this Exaeretodon from the Carnian of South America and the rhynchosaurs, like this Hyperodapedon from the Carnian of India, which represent a clade of primitive archosaurs, the group to which belong crocodiles and dinosaurs. They are easy to recognize with their very well developed premaxillae mimicking teeth. These are not real teeth, however, but bone. As we have seen in the last video, the small but not yet very successful generalist dinosaurs were on the other hand able to adapt quickly to this changing world and rose to dominance, as the more arid climate that predominated at the beginning of the Norian obviously gave them an advantage and allowed them to outcompete the first mammals. The Norian offered a world very different from the Carnian one, in which new ecological niches were filled by small-sized new-comers that are now parts of our modern ecosystems: lizards, turtles and crocodiles, although the latter were still at the time fully terrestrial animals. Although present, the mammals were not particularly favoured by these new conditions. The change in the marine vertebrate faunas appears to follow a very different pattern than what happened on land. The end-Carnian mass extinction did not affect much the marine ecosystem, apart from the reef communities. 27 million years after the end-Carnian mass extinction, which represents a rather short time at a geological scale, another major event greatly modified the ecosphere. This time the changes were triggered by continental drift and associated volcanism. During the Triassic, all the continents were fused together and formed a single "supercontinent" called Pangaea. At the end of the Triassic, this continent started to fragment, and North America separated itself from Africa and Eurasia, opening the North Atlantic Ocean. Extensive volcanism was associated with this event and triggered another global mass extinction. The last terrestrial crurotarsans disappeared, leaving the dinosaurs without real competitors so that they diversified greatly during the rest of the Mesozoic. The phytosaurs, and the large temnospondyl amphibians disappeared as well, allowing the crocodiles, under pressure from the diversification of the dinosaurs, to take refuge in the aquatic niche they occupy today. On the contrary, the mammals remained a rather minor component of the terrestrial ecosystems. Again, the effect of this event on marine vertebrate faunas does not seem to have been very important. However, during the last stage of the Triassic, called Rhaetian, which started 208 million years ago and ended 201 million years ago, a very shallow sea established itself on most of what is now Western Europe. Shallow seas represent today areas with maximum diversity, and this large Rhaetian sea provided perfect conditions for the appearance and diversification of new lineages like the teleosts and the modern sharks. If the faunal changes on land seem mostly to have been an abrupt opportunistic replacement of extinct lineages by new ones, better adapted to the new conditions, the changes in marine environments appear more gradual and involved more competition between the "new" and the "old" fish. Anyway, the faunal changes at the end of the Triassic were a complex, two-steps phenomenon. It is at that time that almost all of the modern lineages appeared and develop themselves, apart perhaps the mammals, for which the success was quite limited as we have seen in the first video. In the next video, we shall study the appearance of the last modern vertebrate group: the birds, but in order to do that, we shall need to have a closer look at dinosaurs, because, as we shall see, birds ARE dinosaurs! Finally, we shall have a look at the end-Cretaceous mass extinction, which will finally give their chance to the mammals.
