Molecular evidence[ edit ] On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. There was one possible patch with animal cellular structures. Feathers are very common in coelurosaurian dinosaurs including the early tyrannosauroid Dilong. A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesis , a type of paedomorphic heterochrony , which resulted in retention of juvenile characteristics of their ancestors. 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.
Feathers are very common in coelurosaurian dinosaurs including the early tyrannosauroid Dilong. Paul , conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich and other ratites. 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. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers. Several proteins have putatively been detected in dinosaur fossils,  including hemoglobin. He used Archaeopteryx as an example of this second stage. Some researchers have demonstrated the presence of color-bearing melanin in the structures—which would be expected in feathers but not collagen fibers. Reproductive biology[ edit ] When laying eggs, female birds grow a special type of bone in their limbs. 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. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Molecular evidence[ edit ] On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. When found in association with fossils , gizzard stones are called gastroliths. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight. Tetrapteryx, Archaeopteryx, Hypothetical Stage, Modern Bird Current thought is that feathers did not evolve from scales, as feathers are made of different proteins. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologists. Skeletal similarities include the neck, pubis , wrist semi-lunate carpal , arm and pectoral girdle , shoulder blade , clavicle , and breast bone. As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mids. Origin of avian flight Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurs , which arose soon after the discovery of Archaeopteryx in The supracoracoideus works using a pulley-like system to lift the wing while the pectorals provide the powerful downstroke. The feathers could also have been used to trap insects or other prey. Gizzard stones[ edit ] Both birds and dinosaurs use gizzard stones. First, animals developed passive flight, in which developing wing structures served as a sort of parachute. 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. The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 thumb and first two fingers in humans.
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