Dinosaurs and their Descendents 5 - How did they learn to fly?

` In other entries of Dinosaurs and their Descendents, I have described how birds fit into the maniraptorian clade, why dinosaurs would need feathers, fossils that show that they did indeed have them (and evidently needed them after all), and how feathers could have evolved and how they develop.

` And now, Ascitu-saurus is begging the question:

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` Let us answer the poor, wretched symbol-construed beast...

` How did maniraptors evolve flight, then? It didn't take much. The most birdlike dinosaurs, which are similar to the earliest-known birds, already had hollow bones, the range of motion needed for flight, large brains and eyes, grasping hands and feathers.

` Currently, the most agreed-upon hypothesis for how flight could have evolved is roughly this:

` Tree-climbing dromaeosaur-like animals once existed, perhaps behaving somewhat like tree-shrews or squirrels, probably in the Early-to-Mid Jurassic. As is the case with modern animals, these creatures probably fell an awful lot.
` That's why many living animals need to glide, including the Anomalurid and Petauristine squirrels, Petaurid marsupials, as well as the colugos, Chrysopelea snakes, the iguana-like Draco, Ptychozoon and Cosymbotus geckos, Rhacophorid and other frogs, and Cephalotes ants.
` For most of them, it is essential that they don't ever hit the ground. If they can just land safely and get climbing again, they can prevent themselves from being removed from the gene pool.

` For ants of the genus Cephalotes, it is important to stay in their 'home tree' - they fall frequently enough, but instead of being lost forever, they are able to steer in a 'J' shape toward the trunk by using their oar-like hind legs. And even without limbs, the flying snake can maneuver its body at sharp angles, as this video demonstrates.
` This is most important because the snake doesn't have to climb all the way down a tree, risk being eaten by a ferocious predator on the ground, and then climb up another tree. It's very much an energy-saving technique. This kind of thing is also important to other small, vulnerable tree-climbing animals which would do best if they didn't have to waste valuable energy risking their lives on the forest floor.
` Back in the Jurassic, life was no doubt very similar in most ways as it is now - in the trees, sometimes it boiled down to 'survival of the best steerers'. Tens of millions of years ago, as today, there were different types of reptiles that glided.
` Some used ribs like today's Draco lizards, others used hardened 'ribs' of skin or skin flaps on the limbs somewhat more like gliding geckos. This was because having any slightly webbed skin or flattened body was occasionally a great asset to not falling on one's face and crushing it.

` If a reptile had feathers that stuck out even a little bit, that would have helped it in a similar way. Small non-flying dinosaurs seem to have commonly had winglike feathers on their arms and/or tail for display, or at least contour feathers on the sides of their bodies. If it was these feathers that first caught the air and allowed the animals to parachute, they could have controlled where they landed.
` The animals with feathers best for controlling a fall would be the ones most likely to survive to reproductive age - the same principle operates with modern-day climbing animals. If any animal is able to have a controlled landing further and further away from where it falls or jumps, all the better! Eventually, when the parachuting angle falls below 45 degrees, this is called 'gliding'.

` In order to bolster gliding by adding speed and reducing drag, the flapping of feathered appendages would probably be a useful maneuver, even if this doesn't generate any more lift. Still, gliding 100 feet instead of only 60 is a big advantage!
` If only the best at surviving produced the most offspring over the generations - which is the way it always is with animals - and this ability to produce offpring was greatly helped with this gliding technique, eventually you'd wind up with the strongest flappers.
` And when those were selected for, perhaps the glide angle could be reduced to 20... and then 12, and eventually 0, meaning that lift could be generated if need be.

` Surely, Microraptor had to have been around the end of a process in which is was developing true flight, as it appears to have been a very strongly-gliding or weakly-flying dromaeosaur. It seems to have been able to use the feathers on its feet as a short gliding surface so that it could swoop down and glide back upwards into a tree.
` If this kind of thing could happen with Microraptor, perhaps the same thing happened with the first birds, which were physically very similar to the four-winged wonder. In fact, Archaeopteryx, as well as a recently-discovered enantiornithine bird, both seem to have longish feathers on the hind legs.
` Zhou and Zhang write in Nature:

` "Here we describe a fossil of an enantiornithine bird from the Early Cretaceous period in China that has substantial plumage feathers attached to its upper leg (tibiotarsus). The discovery could be important in view of the relative length and aerodynamic features of these leg feathers compared with those of the small 'four-winged' gliding dinosaur Microraptor and of the earliest known bird, Archaeopteryx. They may be remnants of earlier longer, aerodynamic leg feathers, in keeping with the hypothesis that birds went through a four-winged stage during the evolution of flight."

` There is more to this story, of this stage of evolution, and I may someday add more if I come across something else. Any suggestions?