The discovery that dromaeosaurs and dino-birds were covered with downy fibres (Ji et al. 1998,2001) proved that feathers evolved first for insulation and were only secondly used in flight (Sect. 7.8). Birds evolved from cursorial, bipedal, carnivorous, coelurid dinosaurs with feathered forelimbs, hind legs and tails. These took to the air first by using their forelimbs to give added traction when running up sloping branches and, later, by leaping and gliding from one branch to another. By the Middle Jurassic (165 mya), dino-birds had evolved feathers and,by the early Cretaceous (130 mya),were similar in this respect to modern birds (class Aves) descended from them. Archaeopteryx was probably an awkward flier but in Cretaceous times more modern-looking birds such as the piscivorous Ichthyornis and the flightless diver Hesperornis evolved. Toothed birds such as these died out as the Mesozoic Era drew to its close and were replaced by modern types. Hypotheses regarding the origin of birds have been reviewed by Currie and Padian (1997) among others (see also Feduccia 1999; Blount and Crowley 2001).

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Pregnancy And Childbirth

Pregnancy And Childbirth

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