As we have seen in the last two videos, most of the modern vertebrate lineages, including mammals, lizards, turtles, crocodiles and teleost fishes appeared during the Triassic. Many of them encountered great success after the end-Triassic mass-extinctions. There is, however, one major lineage that is lacking to our review, and that is the birds. They represent the most recent modern lineage to appear on Earth. But to understand how they did appear, we need first to look at the history of dinosaurs. Because they are supposed to be extinct, dinosaurs are often considered to be an evolution failure. This is to forget that the first dinosaurs likely originated during the Middle Triassic, 240 million years ago and then dominated most terrestrial ecosystems for more than 160 million years, between the Late Triassic, 228 million years ago, and the end of the Cretaceous, 66 million years ago. The name Dinosauria, which means terrible lizard in ancient Greek, was coined by Richard Owen in 1842 to group together Iguanodon, Megalosaurus and Hylaeosaurus. Be careful, the images we just saw are just historical reconstruction corresponding to the image people had at the time of what a dinosaur was. Since then, many new dinosaurs have been described and today more than 1,000 species are known to science. Our understanding of these animals has thus changed a lot, as we shall see. Dinosaurs can be divided into two main clades based on the structure of their hip: the Saurischia, in which the pubis is projected forward, a condition known as the reptile-like hip arrangement because this is the condition observed in modern lizards, turtles and crocodiles, and the Ornisthischia, in which the pubis is projected partly backward, the bird-like hip arrangement. The Saurischia includes the theropods, the carnivorous bipedal ones, and the sauropodomorphs, the herbivorous quadrupedal ones, whereas the Ornithischia includes a diverse array of bipedal and quadrupedal herbivorous dinosaurs. The oldest dinosaurs were found in the Late Carnian of Argentina and Brazil, which strongly suggests that the origin of dinosaurs occurred somewhere in Gondwana, in the southern hemisphere. However, their exact phylogenetic relationships are still debatable. All these taxa were bipedal, which means they were walking on their hindlimbs only. The first theropods were small animals, not exceeding two metres in length and a weight of 40 kg. The oldest uniquevocal theropods did not appear before the Norian, and they are particularly well known in the Late Triassic of the southwestern United States. In New Mexico, a site named Ghost Ranch has yielded tens of complete skeletons of Coelophysis, a small primitive theropod. Theropods became more diversified and larger after the end-Triassic mass extinction and the disappearance of the last large crurotarsan predators we encountered in our first video. They therefore did not radiate rapidly soon after they originated. The oldest sauropodomorph was found in the Late Carnian of Argentina, which again support the hypothesis that dinosaurs first appeared and radiated in Gondwana. The sauropodomorphs are divided in two categories: prosauropods and sauropods. The prosauropods is made of primitive taxa. One of the best known prosauropod is the European Plateosaurus, which could reach 10 m in length. It is also one of the first dinosaurs in which endothermy has been demonstrated, meaning it was a warm-blooded animal. The prosauropods disappeared at the end of the Early Jurassic and their disappearance is therefore not directly linked to the end-Triassic mass extinction. The sauropods include the well-known quadrupedal herbivorous dinosaurs possessing a long neck, small head and long tail like the iconic Diplodocus. They quickly reached a large size, as demonstrated by the discovery of this one metre long humerus in the Late Triassic of Thailand with the French palaeontologist Eric Buffetaut to give the scale. This early sauropod, called Isanosaurus was between 12 and 15 m long. Only two taxa from the Late Triassic can be confidently attributed to ornisthischian dinosaurs: Pisanosaurus illustrated here in Argentina and Eocursor in South Africa. Again, these taxa were found in the cradle of dinosaurs: Gondwana. Like the first sauropodomorphs, they were small bodied, bipedal, and probably omnivorous animals. Their main innovation was the appearance of a fleshy "cheek". Their success in the Triassic was, however, limited, and it was not before the Early Jurassic that they started to diversify with the appearance of two main groups: the Thyreophora, including armoured forms like ankylosaurs and stegosaurs, and the Neornithischia, including the horned and duck-billed dinosaurs. It was at that time that they started to occupy various niches as both bipedal and quadrupedal herbivorous animals. In the past, it was widely believed that dinosaurs supplanted other animals during the Late Triassic because of their superiority, notably in term of locomotion. However, facts do not support this interpretation. First, as we have already noticed, crurotarsans had evolved an erect gait that was probably as efficient as the dinosaur one. The advantage of the dinosaurs in terms of locomotion was therefore not so important. Second, the fossil record does not show a gradual takeover of the dinosaurs, but two expansion peaks corresponding to extinction events. To summarize, the dinosaur radiation was gradual and proceeded opportunistically in taking advantage of two extinction events at the end of the Carnian and of the Triassic. Their domination of the Jurassic and Cretaceous terrestrial ecosystems appear therefore to be very much due to chance rather than to any kind of superiority over the other Triassic archosaurs. Now that we are better acquainted with the early steps of dinosaur history and the processes that lead to their dominance in terrestrial ecosystems, let's talk about the most recent of the vertebrate lineage: the birds. During the Jurassic, small theropod dinosaurs developed feathers. The reasons why feathers first appeared is not fully understood. It could have been for insulation purposes, but recent studies have revealed that these first feathers were colourful and suggest that their function was also display, in order for the males to attract the females. Such a theory implies that theropod dinosaurs, contrary to most mammals, were able to see a broad range of colours. These feathers became increasingly larger and there are two hypotheses to explain how birds became able to fly. If feathers were only display features, they became larger to be more efficient in their function until the animals were able to glide from branch to branch in trees, and then fly. This is the "top down" hypothesis, and it supposes that these feathered dinosaurs were arboreal, living in trees. The feathers on the other hand may have also helped the animals to catch small prey like insects, being used as some kind of net, until they were large enough to allow their owners to take off while they were running after their preys, leaping in the air and trying to catch them with their arms. This is the "bottom up" hypothesis. The top down hypothesis appears today more likely because of the discovery of small four-winged feathered dinosaurs like Microraptor. Microraptor possessed very elongated feathers on both the fore and hind limbs, forming four wings. The feathers on the hind limbs prevented Microraptor from being a good runner, and favoured instead an arboreal mode of life, well in accordance with its small size: 90 centimetres long including the tail. The oldest flying bird is Archaeopteryx, of which there is a cast of the famous Berlin specimen. As you can see, Archaeopteryx still possesses a number of characters typical for dinosaurs: presence of teeth and of well-developed fingers with claws and a long tail made of numerous vertebrae. This is why it is such an iconic "transitional fossil". It is known from twelve fossils from the Late Jurassic, all found in Germany. It is approximately 150 million years old. The birds are therefore the last major modern vertebrate group to appear on Earth. However, they had to share the world with the pterosaurs, flying reptiles that were well established at that time as we have seen in the last video. During the Cretaceous, birds diversified themselves. Although primitive, the Chinese Confuciusornis was more bird-like than Archaeopteryx. It was devoid of teeth, possessed a beak, and its tail was reduced to a few fused caudal vertebrae, forming a pygostyle like in modern birds. Confuciusornis was a better flyer than Archaeopteryx, and males displayed two extremely elongated tail feathers for display. In addition to primitive birds, like Confuciusornis two main lineages separated from each other, the Enantiornithes and the Ornithurae. The Enantiornithes were a major Cretaceous bird group distributed worldwide. Enantiornithes means "opposite birds", because the articulation of the shoulder bones has a concave-convex socket joint that is the reverse of that of modern birds. Their size varied from that of a sparrow to an estimated wingspan of 1.2 metres in Avisaurus. The best preserved fossils have been found in Spain and China. Most Enantiornithes retained teeth, indicating that their loss occurred several times independently among birds, as they were lost in the more primitive Confuciusornis. Among the other primitive characters retained in the Enantiornithes, there is the presence of claws on their hands. On the contrary, they were the first birds to develop alulas, a small set of forward-oriented feathers on the first digit of their hand, as you can see on this picture, allowing higher manoeuvrability in the air and precise landing. Today, alulas are present in every flying bird. Ornithurae is the lineage leading to the modern birds. In addition to the latter, it includes also two extinct orders: Hesperornithiformes and Ichthyornithiformes. The former were flightless birds that could reach one metre in length. They were eating sea fishes that they caught with their small pointed teeth and they were foot-propelled divers. Very early in their evolution, some birds became thus flightless and conquered aquatic niches, like our modern penguins. Ichthyornithiformes were flying sea birds, the size of a small gull. They were very similar to modern birds, except they still possessed teeth. At the end of the Cretaceous, all Enantiornithes, Hesperornithiformes and Ichthyornithiformes disappeared from the surface of the Earth. Birds were therefore severely affected by this mass extinction, and very few modern birds, the ornithurae, managed to survive into the Paleogene. As their origin is nested among dinosaur, birds are technically feathered dinosaurs. A natural group, also called a clade, should indeed include the ancestor and all its descendants. This means that not all the dinosaurs disappeared at the end of the Cretaceous, even if very few survived. Their diversification after the mass extinction was however very successful, and with nearly 10,000 living species, birds, and thus dinosaurs, are today more diverse than mammals. The Cretaceous/Palaeogene event, which occurred 66 million years ago, is the last of the major mass extinctions to affect the Earth and to shape the vertebrate faunas as we know them today. Among terrestrial tetrapods, the non-avian dinosaurs and pterosaurs disappeared, as well as several families of birds and marsupial mammals. In the sea, marine reptiles like the plesiosaurs and the mosasaurs and some families of sharks, including the last hybodonts and teleost fishes disappeared. However, many groups were apparently little affected: some fishes, amphibians, turtles, lizards and many crocodiles. All in all, 30% of all vertebrate families disappeared at the boundary, and it seems that no tetrapod over 50 kg survived the event. The existence of a huge, nearly 200 km in diameter, asteroid impact crater at Chicxulub in the Yucatan Peninsula of Mexico is now well established and dated as corresponding to the mass extinction event. However, although the role of this asteroid impact in the mass extinction process cannot be denied, one can wonder whether it was the sole culprit for the extinction. The impact caused massive extinctions by throwing up a vast dust cloud that blocked out the sun, preventing photosynthesis, and caused freezing. Hence, plants died off, followed by herbivores and then carnivores. But is that enough to explain everything? It seems that the asteroid hit the Earth at a time when the global ecosystem was already destabilized. The destabilization came from two other contemporaneous events: climatic changes linked to changes in sea-level and a vast outpouring of lava in the Deccan Traps of India. Long-term climatic changes would have replaced most of the tropical lush habitats, which were preferred by dinosaurs, with strongly seasonal temperate conifer-dominated habitats, which were preferred by mammals, whereas the volcanic activity of the Deccan Traps would have had a similar effect as the one caused by the asteroid impact, in addition to causing acid rains. The asteroid impact might therefore be only a coup de grace to a very fragile ecosystem and if this impact would have occurred 10 million years before or after the boundary, perhaps its effect would not have been so devastating. The disappearance of all non-avian dinosaurs and pterosaurs freed numerous ecological niches that were filled by mammals and birds. The 'replacement' of non-avian dinosaurs and pterosaurs by mammals and birds was therefore not a matter of competition with one clade being in some way superior to the other. In the same way as the dinosaur would have been unable to replace the crurotarsans in the Triassic ecosystems without the help of the end Triassic mass extinction event, the mammals and the birds would have been unable to replace the dinosaurs and the pterosaurs without the help of an asteroid impact. However, recovery phases after a mass extinction are always times of experimentation and the replacement process is never as simple as we may think. For example, the mammals did not evolve any large carnivores during the Paleocene and the most successful tetrapods of the Paleocene recovery time were certainly the birds, which occupied most of the flying niches as the first bats did not appear before the end of the Paleocene, 10 million years after the asteroid impact. They also developed giant, carnivorous flightless forms like Diatryma and Paraphysornis. During the Paleocene and the Eocene, these birds dominated many terrestrial ecosystems as top predators. The birds, which are indeed dinosaurs, therefore kept the ecological niche of the large theropod dinosaurs for another 20 million years. Another very successful clade after the Cretaceous/Palaeogene mass extinction was the crocodiles. As aquatic predators, they were quite resilient to extinction, but in the absence of large competitors during the Paleocene, the crocodiles also evolved into large fully terrestrial predators, the teeth of which were very similar to those of the theropod dinosaurs as you can see here on this skull of Sebecus from the Eocene of Argentina. Such crocodiles survived until the Pliocene, 2 million years ago. Probably because of the competition with large carnivorous birds and terrestrial crocodiles, the appearance of modern large carnivores was delayed until the end of the Eocene, 35 million years ago. The 'replacement' of dinosaurs by mammals was therefore a rather long and complex phenomenon, contrary to what we might intuitively think. The expansion of mammals did not mean that the evolution of other tetrapods stopped or were at risk of being supplanted by the mammals. Looking at the numbers of vertebrate species counted in 2004, one might even wonder why we are talking about an age of mammals. There were 28,900 species of fishes, 9,917 species of birds, 8,300 species of reptiles, including lizards, snakes, turtles and crocodiles, 5,743 species of amphibians and only 5,416 species of mammals. After all, in terms of number of species, they are the least diversified vertebrate on Earth! However, we are mammals, and we shall discuss our own evolution later in this course.
