Last lecture we saw the origin and evolution of flying dinosaurs ("birds" in the broad sense), and before that the origin of feathers. But we haven't yet addressed the origin of flight, nor how all these things are related to each other.
Evolution doesn't work by massive instantaneous transformations: a fully flying modern style bird did not hatch out of a totally non-flying dinosaurs egg! Instead, each phase along the way functioned in its own way, well enough for that dinosaur to make a living at it. And (as we'll see) it wasn't as if dinosaurs were trying to become birds; instead, each adaptive phase produced several divergent branches, some evolving in one direction and others in others.
Our knowledge of the phases of the origin of bird flight (anatomical, phylogenetic, and behavioral) have all greatly increased in the last decade. But before we see this newer understanding, let's review some terms and take an historical look at the problem.
Some key terms:
In the history of life, only four groups of animals have evolved powered flight:
While there are only three groups of flying vertebrates, there are many modern gliding/parachuting ones: flying fish; flying frogs; flying lizards; flying snakes; sugar gliders (gliding marsupials); flying squirrels; colugos, or "flying lemurs". In addition, there were various extinct gliding reptiles and mammals in the Permian and Mesozoic.
Traditionally, paleontologists have considered two main scenarios for the origin of bird flight:
(NOTE: during the 1970s-1990s, this debate was tangled up with a scientifically separate debate; that is, where birds fit phylogenetically among the archosaurs. The media in particular made the equation "'arboreal model = non-dinosaurian origin of birds'; 'cursorial model = dinosaurian origin of birds'". But these were actually separate debates. Even among those who recognized the dinosaurian origin of birds, some argued for the trees-down model, and others for the ground-up.)
This debate was primarily waged prior to the new discoveries of Early Cretaceous feathered coelurosaurs, which greatly increased our knowledge of the anatomy (integumentary and skeletal) of the basal members of the coelurosaur clades. Additionally, important observations of modern birds revealed a very significant locomotory behavior, previously overlooked.
EVOLUTION OF FEATHERS:
Modern feathers are very complex structures, with a hollow shaft that branches, with those branches branching and the branches of the branches branching. Modern feathers are NOT just the big flight feathers of the wings and tail: there are body feathers (that insulate and streamline the body); down (that keeps pre-flying baby birds warm); display feathers; and others.
Until recently, the basalmost theropod known to have feathers was Archaeopteryx, although some researchers speculated that other theropods had them as well. And the feathers of Archaeopteryx were identical to the feathers of modern birds, so they didn't reveal much about the early phases of these structures. But fossils from lake sediments of the Early Cretaceous (and now possibly the Middle Jurassic) of China have given us a better understanding of the distribution of feathers and protofeather structures.
Megalosaurs and basal coelurosaurs (compsognathids, tyrannosauroids) show a body covering of plumulose protofeathers: simple hair-like or down-like filaments. Based on the developmental biology of modern feathers, this is the expected earliest phases of feather evolution. Protofeathers obviously don't have a flight function, since they don't form an aerodynamic surface. However, they might serve other functions:
However, at least some of the protofeathers of compsognathids are pennaceous, and some of those of the tyrannosauroid Yutyrannus are long strap-like pennaceous feathers of the sort also found in maniraptorans.
Note that the fuzz of the heterodontosaurid Tianyulong is very similar. Whether these turn out to be formed by convergence with theropods or retention in both groups of some ancestral dino-fuzz is not yet certain. (If it is a homologous structure, this means that the concestor of all Dinosauria was fuzzy, at least in part!)
Plumolose protofeathers of this sort seem to be present as part of the body covering of therizinosaurs, oviraptorosaurs, deinonychosaurs, and basal avialians. (In ornithothoracines, they are replaced by body feathers.)
Tyrannosauroids and therizinosaurs in addition have strap-like protofeathers intermediate in size and form between the downy protofeathers and true feathers. At present it is not certain what the exact structure of the integument of ornithimimosaurs and alvarezsaurids was like. However, these pennaceous strap-like feathers (as well as fully modern feathers) are present in oviraptorosaurs.
More advanced groups of maniraptorans for which the integument is preserved (oviraptorosaurs, dromaeosaurids, troodontids, avialians) show true broad pennaceous feathers on the arms and the tail. In eumaniraptorans (deinonychosaurs and basal avialians) long broad true feathers are present on the hindlimbs as well. Additionally, these same dinosaurs show some major transformations of the forelimb: elongate arms; semilunate carpal allowing for folding the hands; laterally oriented shoulder joints allowing the arms to stick out sideways; enlarged sternum for more powerful arm muscles. In modern birds these adaptations are useful in the flight stroke.
Were all these dinosaurs fliers? It appears unlikely, given the anatomy of the non-eumaniraptoran forms especially! What other function could broad feathers have served?
Maniraptorans are known to have brooded their eggs (nests with parents on them are known for oviraptorosaurs, troodontids, and avialians). Broad feathers may have helped insulate and protect these eggs: increased arm length, better ability to stick arms out to the side; increased surface area of feathers allows greater coverage of eggs. So perhaps brooding was a selective feature in these adaptations?
Additionally, the carnivorous members of this clade used their forelimbs to capture prey: elongation of the forelimbs gave them a better range in which to strike, while modifications of the shoulder and wrist would let them fold the arms in tight rather than drag them along.
However, it is observations of modern birds that revealed another possible selective force in the evolution of the maniraptoran forelimb and feathers.
WING ASSISTED INCLINE RUNNING (WAIR) and CONTROLLED FLAPPING DESCENT (CFD):
In the early 2000s research by Ken Dial of the University of Montana's Flight Laboratory revealed a locomotory behavior in modern birds not previously realized. Birds (in this case chukar patridges) were discovered to run vertically up surfaces, aided by flapping their wings back and forth in order to generate traction against the surface. They called this behavior Wing Assisted Incline Running, or WAIR for short. They have since found many bird species with this behavior, even perfectly good fliers like pigeons:
Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, and also experimented by trimming the feathers of birds to different lengths.
Additional work has shown that this behavior is widespread among modern birds.
The apparatus required to use WAIR is:
All these attributes are present in many maniraptorans (oviraptorosaurs, deinonychosaurs, basal avialians). Additionally, modern birds use WAIR to escape predators: certainly a selective factor present in the Mesozoic, too! Furthermore, there are net selective advantages to slight increases in the length and breadth of the feathered arm surface: the sort of material that can easily be increased by natural selection.
WAIR might represent a "stepping stone" or "behavioral missing link" in the origin of flight. Small (or juvenile) maniraptorans might have used this method to escape predators. Now that they had the ability to get up into trees and other high spots, some lineages of maniraptorans might become specialized for life up on these high spots. Additional natural selection could favor further development of wing size and shape as an aid for getting back down off of high places (controlled flapping descent), or (eventually) from branch-to-branch.
Thus, WAIR serves as a functional link between cursorial and arboreal models (and organisms). It is a cursorial model in that wings begin in part as an aid to running locomotion (just vertical running); it is an arboreal model in that once maniraptorans have an ability to get into the trees, evolution can further develop the forelimbs to get them back down to the ground. And all of these behaviors are still found in modern animals: no speculation of behaviors not currently seen needed.
A NEW SCENARIO FOR BIRD FLIGHT ORIGINS:
The various recent discoveries of the skeletal and integumentary anatomy of various coelurosaurs (including basal avialians) and the behavioral and biomechanical evidence of modern birds suggests a more complete possible scenario for bird flight evolution than the historical "ground up" or "trees down" versions. Note that as with all evolutionary scenarios this would be a simplification, but the following is consistent with our current evidence:
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