Feathers are very common in coelurosaurian dinosaurs including the early tyrannosauroid Dilong. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.
All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard common basilisk. Most recent refutations of the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on embryological analyses that conclude that birds' wings are formed from digits 2, 3, and 4, corresponding to the index, middle, and ring fingers in humans.
The first of a bird's three digits forms the alula , which they use to avoid stalling in low-speed flight—for example, when landing. The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 thumb and first two fingers in humans. The wing-assisted incline running WAIR hypothesis was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. Note that in this scenario birds need downforce to give their feet increased grip.
Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion.
The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall to achieve minimum gliding speeds. Anchiornis is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx. Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis , compared to those of living species, suggest that the early species may have lived both on the ground and in trees.
One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds. Archaeopteryx was the first and for a long time the only known feathered Mesozoic animal.
As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mids. There has been debate about whether Archaeopteryx could really fly.
Progressive stretch overload of skeletal muscle results in hypertrophy before hyperplasia.
It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes see diagram above of supracoracoideus pulley ; this may mean that it and other early birds were incapable of flapping flight and could only glide. But the discovery since the early s of many feathered dinosaurs means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight.
These include Rahonavis , a ground-runner with a Velociraptor -like raised sickle claw on the second toe, that some paleontologists assume to have been better adapted for flight than Archaeopteryx ,  Scansoriopteryx , an arboreal dinosaur that may support the "from the trees down" theory,  and Microraptor , an arboreal dinosaur possibly capable of powered flight but, if so, more like a biplane , as it had well-developed feathers on its legs.
Archaeopteryx and Rahonavis birds. Confuciusornis bird. A hypothesis, credited to Gregory Paul and propounded in his books Predatory Dinosaurs of the World and Dinosaurs of the Air , suggests that some groups of non-flying carnivorous dinosaurs - especially deinonychosaurs , but perhaps others such as oviraptorosaurs , therizinosaurs , alvarezsaurids and ornithomimosaurs - actually descend from birds. Paul also proposed that the bird ancestor of these groups was more advanced in its flight adaptations than Archaeopteryx. The hypothesis would mean that Archaeopteryx is less closely related to extant birds than these dinosaurs are.
Paul's hypothesis received additional support when Mayr et al.
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This result supports Paul's hypothesis, suggesting that the Deinonychosauria and the Troodontidae are part of Aves, the bird lineage proper, and secondarily flightless. The paper was criticized by Corfe and Butler who found the authors could not support their conclusions statistically. Mayr et al. Current cladistic analyses do not support Paul's hypothesis about the position of Archaeopteryx.
Origin of birds - Wikipedia
Instead, they indicate that Archaeopteryx is closer to birds, within the clade Avialae , than it is to deinonychosaurs or oviraptorosaurs. However, some fossils support the version of this theory that holds that some non-flying carnivorous dinosaurs may have had flying ancestors. In particular, Microraptor , Pedopenna , and Anchiornis all have winged feet, share many features, and lie close to the base of the clade Paraves. This suggests that the ancestral paravian was a four-winged glider, and that larger Deinonychosaurs secondarily lost the ability to glide, while the bird lineage increased in aerodynamic ability as it progressed.
This was seen as an indication for secondary flightlessness in heavy basal members of that group. From Wikipedia, the free encyclopedia. Main article: The Origin of Birds book.
Further information: Coelurosauria , Maniraptoriformes , and Avemetatarsalia. Main article: Feathered dinosaur. Blank out numbers in image Vane. Hollow shaft calamus. Main article: Origin of avian flight. Dinosaurs portal. Evolution: Education and Outreach. Retrieved Bibcode : Natur. London: John Murray. Philosophical Transactions of the Royal Society of London.
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- Progressive stretch overload of skeletal muscle results in hypertrophy before hyperplasia..
In Weishampel, David B. The Dinosauria Second ed. Berkeley: University of California Press. JHU Press. See the summary and pictures at "A wish for Coelophysis ". Archived from the original on February 3, November Journal of Paleontology. CS1 maint: multiple names: authors list link - full text currently online at "The Furcula in Suchomimus Tenerensis and Tyrannosaurus rex ". Archived from the original on This lists a large number of theropods in which furculae have been found, as well as describing those of Suchomimus Tenerensis and Tyrannosaurus rex.
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The origin of birds. Annual Review of Earth and Planetary Sciences. Biological Journal of the Linnean Society. Willi Phylogenetic Systematics. Urbana: University of Illinois Press. In Padian, Kevin ed.
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