The Pterosaurs
The study of the evolution of avian flight, thoroughly fascinating in itself, is a window into the heart of the evolutionary process. It is a story which has become much clearer in the last decade, but is still a patchwork, with many missing parts. Long before there was anything that remotely looked like a bird, probably before the most primitive feather, the Pterosaurs (possibly, close relatives of the dinosaurs), had already mastered flight. Throughout the Jurassic and Cretaceous periods, featherless Pterosaurs flew.
The Ancestors
The first dinosaurs were the Theropoda. They were bipedal, and most were carnivorous predators. They were certainly on the scene by 210 mya (million years ago), in the late Triassic. Their legs were large and strong and their arms were short. They had a long neck and a long muscular tail, which extended straight back. Their spine was carried horizontally, with their tail acting as a counterbalance. Coelophysis (“hollow form”), was a Theropod named for its hollow limb bones. The weight of its skull was lightened with openings called “fenestrae”, and strengthened with narrow skeletal struts. Its pectoral girdle was stiffened with a “furcula” (the fused collar bones we find in birds today). It walked, as birds do, on three toes, and its first toe (the “hallux”) was raised. In later groups the hallux swung to the back and opposed the three walking toes.
The Jurassic followed the Triassic, and lasted about 55 ma (mega annum, million years). It was in the late Jurassic, between 160 and 145 mya, that the diverse group of Theropod dinosaurs, the Coelurosauria, began to appear. As the first vertebrate fliers, the Pterosaurs, had already demonstrated, feathers aren’t necessary for flight; insects and bats demonstrate that today; but many, if not most, Coelurosaurians bore feathers on their bodies. Feathers, and many other characteristics that had evolved for other purposes, served as a fortuitous and enduring platform for flight.
Compsognathus lonpipes by Jaime Haedden
(A Jurassic Coelurosaurian)
Over millions of years evolution reshaped the Coelurosaurian skeletal frame. Some bones got smaller or disappeared, others fused together to perform new functions and make stronger, lighter, more compact structures.
The “Hand Seizers”
The Coelurosauria include the Maniraptora (“hand raptor” or “hand seizer”). By the early Cretaceous the Maniraptorans had taken the old Coelurosaurian form in some new directions. Their brains were often rather large for dinosaurs, and more bird-like, and mostly, they were predators. Paleontologists Kevin Padian and Luis M. Chiappe (Scientific American, February 1998), describe the specialized Maniraptoran arm and hand:
“In the wrist, a disklike bone took on the half-moon shape that ultimately promoted flapping flight in birds.... The hand became longer, too, accounting for a progressively greater proportion of the forelimb, and the wrist underwent dramatic revision in shape. The half-moon, or semilunate, shape was very important because it allowed these animals to flex the wrist sideways in addition to up and down. They could thus fold the long hand, almost as living birds do. The longer hand could then be rotated and whipped forward suddenly to snatch prey.”
The Maniraptoran tail was somewhat shorter and stiffened, further bracing it, and improving its ability to counterbalance swift and nimble bipedal motion.
Velociraptor by Jaime Haedden
Groups within the Maniraptora are generally regarded as the most likely antecedents of birds. Foremost among these groups are the Dromaeosaurs, distinguished by a large hyper-extended claw, or talon, on their second toe. They presumably held that toe retracted while walking on toes three and four. They were capable of using hands and feet together to seize and subdue prey, and with the large talon at the end of their powerful legs, to stab and rip. Deinonychus was about 3 meters to the end of its tail and the somewhat smaller Velociraptor almost 2 meters.
Deinonychus, Ostrom of Yale, and the “Urvogel”
Deinonychus is joined in history with a leading light in dinosaur paleontology, the late John H. Ostrom of Yale, and Yale’s Peabody Museum. Ostrom was part of a group which excavated fossils of Deinonychus in Montana in the early 1960s. Years of study of these fossils led him to become an early proponent of the idea that dinosaurs could be fast, active animals, rather than slow moving lizards. He was deeply impressed by the striking similarities between Deinonychus and Archaeopteryx (often called the “Urvogel” or earliest bird, now more for historical than evolutionary reasons, though a recent study suggests it may have been an early Dromaeosaur). It was Ostrom, in 1969, who first proposed, against much resistance and disbelief, that Dromaeosaurs must be closely related to the direct ancestors of birds.
The disbelief has turned to acceptance in the last ten years, with the extraordinary discoveries at the Yixian formation in northwestern China and a few other sites. Fossils in these unusual geological formations were preserved with great surface detail, before the soft features were lost. Dromaeosaurs and other Coelurosaurians have been found at these sites bearing the imprint of feathers.
Deinonychus by Alexandra Lowendahl
The most dramatic find occurred in 2002 with the discovery in Yixian of Microraptor, a small, feathered Dromaeosaur, less than a meter in length, with wings on its arms and its legs, and a long fan-shaped tail. Microraptor did not have the skeletal apparatus necessary to support powered flight, and is thought to have glided from trees. Unlike Yixian, most fossils are found at sites which have preserved only the mineralized bones, or even whole skeletons of animals, but have left no sign of the body surface. While it is now presumed that Dromaeosaurs, like Deinonychus and Velociraptor, and many other Coelurosaurians, bore feathers, the picture is still incomplete.
The Story of Feathers
It is presumed that like fur on mammalian skin, feathers evolved to insulate the body and conserve body heat in animals capable of producing significant heat from their own metabolism (“endothermy”). As evolutionary biologists Richard Prum and Alan Brush argue (Scientific American, March 2003), feathers evolved in much the same way that they develop in living birds today. 
Like hair, they develop in an epidermal follicle. It is the development of the structures of the feather follicle that determines the process and order of feather formation and evolution. The authors describe the stages of feather development, from a simple hollow tube, to a tuft of loose barbs attached to a tubular root called a “calamus”, to a similar tuft of barbs, but now with tiny barbules extending from each barb, to a familiar bilaterally symmetrical (“pennaceous”), planar feather vane, where the barbs extend at equal lengths from either side of the central stem or “rachis”. The barbs have tiny barbules extending from them, but the barbules are loose, so the feather is “open”. In the penultimate stage the barbules form opposing hooks and grooves, which interlock, and create a closed planar surface. These are the smooth contour feathers which cover the bodies of birds today.
Finally, the “flight feather” is planar and closed, but the barbs are short on one side of the rachis and longer on the other. The feather is asymmetrical, and aerodynamic. Prum and Brush conclude:
“The fresh evidence puts to rest the popular and enduring theory that feathers evolved primarily or originally for flight. Only highly evolved feather shapes— namely, the asymmetrical feather with a closed vane,... could have been used for flight.... Rather feathers were ‘exapted’ for their aerodynamic function only after the evolution of substantial developmental and structural complexity. That is, they evolved for some other purpose and were then exploited for a different use.
“Numerous other proposed early functions of feathers remain plausible, including insulation, water repellency, courtship, camouflage and defense.”
Through study of the feathered dinosaurs excavated in Yixian and elsewhere, Prum and Brush identify specific groups of dinosaurs which each exhibit a stage of feather development, from the simple hollow tube, to downy tufts, to bilaterally symmetrical (pennaceous), feather vanes, and finally, to asymmetrical vanes.
Asymmetrical, “closed vane” feathers are the control surfaces essential for the aerodynamics of feathered fight. Called “remiges”, (pl. of “remex”, from Latin “remus”, meaning oar), they are attached directly to the bones of hand and arm, and are positioned on the wing so that the narrow vane is the leading edge cutting through the airstream, the wider part of the vane trailing. This stiffened entry preserves the planar shape, providing smooth airflow over the entire feather surface.
Pouncing Dromaeosaurs and Flapping Chicks
If the downy tufted feather and the pennaceous contour feather, aided “Insulation, water repellency”, etc., what about the long feathers which gradually evolved into a wing? J. P. Garner, et al, in a paper on the origin of avian flight (Garner, JP, GK Taylor & ALR Thomas (1999), Proc. Roy. Soc. London B), proposed a kind of ambush predation, practiced by leaping from a small height, not necessarily a tree, and descending on the surprised prey. Here, drag is what is necessary, not lift. Even before the development of the asymmetrical feather, long pennaceous feathers extending from the hand and arms would provide drag and slow descent. Asymmetrical feathers would later afford reduced turbulence, and greater aerodynamic and directional control.
Alternatively, Ken Dial of the University of Montana has recently conducted studies on juvenile Chukar Partridges, whose wings were not yet fully developed, and found that by flapping rapidly in an up and down, head to foot, motion, they were able to run up inclined surfaces. The downward flapping of the wings, presumably providing a better grip on the inclined surface. Dial also observed that as their wings grew larger, and using the same rapid flapping, the older juveniles were able to run up steeper surfaces including a vertical tree trunk.
Flying Out of the Cretaceous
By the early Cretaceous the tail vertebrae had already been lost or fused in some groups (e.g., Confuciusornis and Rahonavis), and transformed into the “pygostyle”, a bony structure at the base of the sacrum to which the tail feathers attached. In the late Cretaceous, a group called the “Carinatae” developed a keel-like process, or “carina”, extending longitudinally from the sternum, and forming a large and necessary anchor for massive flight muscles. A gull-like Carinate of the late Cretaceous, “Icthyornis”, undoubtedly flew over late Cretaceous coast lines.
Ichthyornis from Paleos by Jaime Haedden
At the end of the Cretaceous, there was a great extinction, and dinosaurs and pterosaurs disappeared. Carinates are the group most likely to have survived the extinction and continued to evolve, over the last 65 ma, into the extraordinary birds we observe today.
