When Ostrom and others set about revitalizing Huxley's theory that birds were descended from dinosaurs, we had two serious problems, time and size. Having just found a new dinosaur with so many birdlike traits, Ostrom could argue convincingly that no other group of animals had more in common with birds. However, Deinonychus was from the Early Cretaceous Period and Archaeopteryx, the oldest known bird, was from the Late Jurassic. This meant that the theory was stuck with a descendant that predated its ancestor by at least 34 million years. Ostrom and his supporters, which now included Dr Bakker and Dr Currie, were forced to fall back on the old and unsatisfying argument that the fossil record was simply incomplete. Older, transitional dinosaurs, with all the bird like adaptations of Deinonychus must have existed, but their fossils hadn't been found yet. The second problem had to do with the size of Deinonychus, and the classic thinking on how flight must have evolved. Gaining the ability to fly is a game- changing evolutionary innovation, it's a huge jump. It has long been assumed that the best way for a lineage to make that jump is by evolving an intermediate gliding stage. Gliding is not the same thing as flying, it is simply a controlled, slow descent. While gliding, an animal travels further horizontally than vertically. There are many animals today that can glide but cannot fly. For example, there are flying squirrels, sugar gliders, draco lizards, flying snakes and parachute frogs. All these animals are forest dwellers that climb trees and have evolved various membranous proto-wings that let them glide from tree to tree. They're also small creatures and that's critical to their ability to coast through the air. It's easy to imagine any of these forms developing more and more extreme wings for the purpose of making longer and longer glides. Eventually, they could add a flapping motion that would propel them from gliding to true altitude gaining flight. Naturally, that's how everyone assumed birds had evolved flight. They start it with a small, tree-dwelling ancestor that glided among the branches of a Mesozoic forest. Deinonychus did not fit that description. For starters, it was too big. Deinonychus weighed about the same as a grown man. While it could certainly climb a large tree, it was clearly adapted for running quickly on the ground. And the same was true, to a greater or lesser extent, of all the other known, and even less bird-like theropods. Even little Compsognathus had the short arms and long legs of a ground sprinter. Thinking back to all the theropod groups that we have met so far in this course. There is one that is thought to include specialized tree dwellers. Is that group the A, Alvarezsauria, B, Ornithomimosauria, C, Scansoriopterygidae, or D, Therizinosauria? The correct answer is C, Scansoriopterygids are thought to have been specialized tree climbers. Unfortunately they've only recently been discovered, so Ostrom could not have known about them. The theory that flight evolved from tree dwelling animals that first glided down out of branches, and then flew, is known as the 'trees down' theory. Ostrem felt that he needed a theory that could account for the evolution of flight in theropods that were adapted for running on the ground. He wanted a ground up theory, so he envisioned a dinonychous like theropod running along the ground and chasing after prey that could already fly like winged insects. To help catch such prey, Ostrom thought the dinosaurs would need to do a lot of forward grasping with their arms, and a lot of jumping. Over time, perhaps long and complex feathers, could have evolved on the arms, to serve as insect capture nets. Then, these arm feathers might start to help the dinosaur to stay in the air longer, during its leaps. Combined with a forward grasping stroke of the arms, these feathers might even let the dinosaur gain altitude and flutter about. From there, Ostrom theorized true flight might emerge. If that theory strikes you as a little far fetched, you're not alone. Many paleontologists have objected to this theory and various alternative ground up theories have been put forward. Perhaps running theropods developed feathered forearms to help them manoeuvre at high speed. Modern ostriches are thought to make use of their wings in this way. Another modern analogue which many palaeontologists think might be key to solving the problem is 'wing assisted incline running' or WAIR. Many ground birds like turkeys and partridges, and particularly their flightless juveniles, display a curious behavior when fleeing predators. They run up tree trunks while flapping their wings. Although the flapping doesn't create enough lift to get the birds airborne, it does act to keep the birds pressed against the trunk and prevents them from sliding down. This attribute allows them to ascend to safety. Perhaps theropods with early wings used them to escape land-bound predators through WAIR. Larger and larger wings could have permitted faster and faster ascent of steeper and steeper inclines. Eventually increased flapping power could result in a true takeoff. However, it should be pointed out that in order to scale a tree a theropod might have been better off just reaching out with its arms and scrambling up with the assistance of its clawed hands, rather than flapping whatever small proto-wings it might have had. Now, modern birds that display WAIR don't have this option because their hand claws have already been lost. So, the trees-down versus ground-up debate continues. You might think that the trees down versus ground up debate would apply to the evolution of flight in other animals such as bats. However in the case of bats flight is generally accepted to have evolved following the trees down model. Based on your understanding of the two theories, why might this be the case? Is it because A, bats are fast runners on the ground and can use WAIR to ascend trees? B, bats hunt insects and use their wings to help capture their prey? C, the wings of bats are attached to their legs, so bat wings preclude running? Or D, bat claws are blunt and prohibit tree climbing? The correct answer is C, the wing membranes of bats attach to their legs. This is generally regarded as strong evidence that the evolution of bat wings came at the cost of terrestrial running. That makes a ground up origin improbable. Recently, the discovery of two new Eumaniraptorans from China has removed the two biggest challenges that Ostrom faced in explaining how flight could have evolved from dinosaurs. Microraptor is a Dromaeosaurid and Anchiornis is a Troodontid. Both are among the smallest of all known non-avian theropods. Microraptor is from the Early Cretaceous, Anchiornis comes from Jurassic deposits, estimated to be 160 million years old. So, it predates Archaeopteryx by roughly ten million years. Once more both of these little Deinonychosaurs are preserved with extremely long feathers on their forearms, and these feathers are asymmetrical. So, Microraptor and Anchiornis were clearly not just using their arm feathers for display, they were using them as airfoils. By the way, the full name of Anchiornis is Anchiornis huxleyi, named in honour of Thomas Henry Huxley and his original theory of a dinosaur origin of birds. Anchiornis provides a critical missing link for that theory. What's really surprising about Anchiornis and Microraptor is that they each also have a second set of wings. Long, asymmetrical feathers are also present on their hind limbs. Both Anchiornis and Microraptor lived in forested environments. Their long leg feathers could certainly not have been flapped while simultaneously running up inclines and some researchers think that the leg feathers might have gotten in the way during any ground running. It's not clear if either dinosaur had the arm and chest strength for sustained flapping flight, however, it is clear that they could have made use of both wing sets during gliding. For these reasons Anchiornis and Microraptor are often interpreted as strong evidence supporting the theory that dinosaur flight evolved following the trees down scenario. And that's not to say that the trees-down, ground-up debate is over. It continues, and many researchers would be quick to point out that both scenarios might be too simplistic. Microraptor is generally regarded as among the most primitive and basal of the Dromaeosaurids. And Anchiornis is genuinely regarded as among the most primitive and basal of the Troodontids. Assuming that the many similar traits of both animals did not evolve convergently, what does this suggest about the most recent shared ancestor of all Deinonychosaurs? Is the ancestor A, small bodied, B, a Dromaeosaurid, C, from the Jurassic Period and or D, a winged animal that could fly or glide? More than one answer might be correct, so select every answer that you think applies. That both Microraptor and Anchiornis are small bodied suggests that their shared ancestor was as well. So, the evolution of later large Dromaeosurids and Troodontids constitutes another, if less dramatic example of gigantism. This by no means suggests that their ancestor was a Dromaeosaur or a Troodontid, although Troodontids have fewer derived characteristics and may well be closer to the ancestoral Deinonychosaur form. Anchiornis is from the Late Jurassic, so the ancestor could not have evolved any later than that and almost certainly lived at some point during the Jurassic. Finally, the presence of gliding or flight capable wings in both does suggest this was a trait shared by their ancestor. If that is the case, it would be that most later Deinonychosaurs including Deinonychus and Velociraptor were secondarily flightless like today's ostrich. So A, C and D are the correct answers. When dinosaurs finally took to the air, they were not alone. Another group of Archosaurs had already conquered the skies tens of millions of years earlier, the Pterosaurs. This groups includes such famous flying reptiles as Pteranodon, and Pterodactylus. The avian invasion of Pterosaur airspace would pit these two groups of fliers against each other in an ecological competition. >> When dinosaurs took to the air at the end of the Jurassic they weren't alone. There had been a type of animal that had been in the air a long time, it's distantly related to dinosaurs, and that's an animal called a Pterosaur. Now, Pterosaurs had been around since Late Triassic times and basically were active flyers. And they share many characteristics that an active flyer would have, like a bird. So for example when you look at their bones, the bones are elongate, but they're also hollow, and they're very lightly built. Pterosaurs do it a little bit different than birds, the bone is denser, and so they can make them even more hollow than bird bones are. This is Rhamphorhynchus, and Rhamphorhynchus is typical of an early Pterosaur in that it has a very long tail and a relatively short neck and skull. As time went on though, Pterosaur evolved and changed and so you have animals like the Pterodactylus and Pterodactylus lived at the same time as Rhamphorhynchus. You can see that the skull is very long, the neck is long but the tail is short. And the wing proportions are somewhat different than Rhamphorhynchus as well. The fact that this animal looks more unstable indicates that it probably was more unstable. But instability isn't always a bad thing, instability also means you have increased manoeuverability. And the same kinds of evolutionary changes can be seen in birds. Early birds like Archaeopteryx have a long tail like Rhamphorhynchus it's a stabilizer, advanced birds do not. Now Pterosaurs continued to change throughout the Cretaceous. And in the Cretaceous, we could see that you had both actively flying forms in the early Cretaceous, that were incredible diverse. And those actively flying forms were doing things a little bit different again than birds. The wing itself, because it's formed of an elongate finger with a membrane attached to it rather than feathers, became a bit of a problem for Pterosaurs in the sense that it interfered with them walking and moving on the ground. Their legs weren't as well developed as what we see in birds and very often had a membrane connecting the legs and so they were probably very awkward animals on the ground as well. The muscular associated with the wings was also different. Both were active flyers, but in the case of birds, the musculature had all been attached to the sternum. And so you have a large crest or keel on the sternum for attachment of the muscles that are both raising the wings and lowering the wings. In the case of Pterodactyls and Rhamphorhynchoids, you don't have a keel, but that's because the muscles that are pulling the wings down were attached to the sternum, whereas the muscles that were pulling the wings up were attached to the backbone. Some of these differences probably worked against the success of Pterosaurs in the long run. Certainly we know that as the Cretaceous advanced, the diversity of Pterosaurs, the ones that were competing directly with birds, seemed to be decreased. The one area that Pterosaurs continued to dominate however were the large forms. And because of those hollow bones that Pterosaurs have, it may have in fact given them the capability of reaching much larger size. Certainly we know that by the end of the Cretaceous, the largest flying animals that ever existed were Pterosaurs and they were almost double the size of the largest birds that have ever existed. At the end of the Cretaceous though, large size was clearly a detriment and like the dinosaurs, all Pterosaurs disappeared. >> Whatever their competitive advantages were, birds ultimately gained mastery of the sky. This brought about another kind of renaissance. With the power flight, bird evolution erupted with diversity. As you will explore in the next and final lesson, birds succeeded where all other dinosaurs failed and became one of evolution's greatest success stories.
