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What Is a Dinosaur? |  Anatomy & Evolution | General Behavior | Where Did They Live? | Why Did They Go Extinct?

Anatomy & Evolution
General Dinosaur Anatomy | Dinosaur Evolution

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TriceratopsPermanent bipedalism is an unusual locomotor mode for vertebrates, having appeared elsewhere only in apes, some rodents, and kangaroos. Thus dinosaurs show an unusual evolutionary trend: many dinosaur species reverted to a quadrupedal posture. This was especially true of larger herbivorous dinosaur lineages. Early sauropodomorphs, ceratopsians, ornithopods, and thyreophorans were all small and bipedal, but the later members of each group were larger and partly or fully quadrupedal. In addition, many different theropods develop the long, slender limbs that characterize "cursorial" animals, but only a few herbivorous dinosaurs seem to have done so (including hypsilophodontids, dryosaurids, and early ornithischians).

Theropods are perhaps the best-studied group of dinosaurs, largely because their ranks include the earliest birds. The transition from terrestrial, "reptilian" non-avian dinosaurs to flying, feathered birds has occupied paleontologists for decades. New fossils of feathered dinosaurs have helped to clarify many of the steps in this complicated evolutionary sequence, but much remains to be learned. It is now clear that feathers must have evolved prior to the origin of flight, and therefore were first adapted for some other purpose. This may have been insulation, display, or some as-yet-unknown function. Many other "avian" features appear to have evolved relatively early in dinosaurs and theropods, and were also probably not directly related to the origin of birds or flight. For example, the specialized wrist of birds, which is used in the complex wing-stroke during flight, was present in many non-avian coelurosaurs. Components of this wrist were present in theropods such as Allosaurus. The furcula (or wishbone) appeared in theropods as primitive as the Triassic Coelophysis, while many features of bird legs are holdovers from the earliest dinosaurs and their ancestors. Thus, although the origin of birds from within theropods seems clear, the details of this transition are still being uncovered.


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