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Published online by Cambridge University Press: 17 July 2017
No area of paleontology has changed more in recent years than the history of birds, both during the Mesozoic Era and the Tertiary Period. The most controversial issue in the study of birds for several decades has been their origin, and the origin of avian flight and feathers, and clearly too much emphasis has been placed on the earliest known bird, the late Jurassic Archaeopteyx, an arboreal form that was already well on its way to becoming a modern bird (Feduccia, 1993a). Over the past two decades this urvögel has mistakenly been characterized as an earth-bound, feathered theropod, and a number of rather bizarre scenarios have been envisioned to account for the origin of flight and feathers in birds. However, the debate has centered on two main themes. Were birds derived directly from small, bipedal theropod dinosaurs (Ostrom, 1991), and therefore evolved via the scenario of the cursorial (ground-up) theory (Ostrom, 1986); or were they derived earlier in time from small, arboreal “thecodonts” or basal archosaurs, and therefore evolved via the scenario of the arboreal (trees-down) theory (Bock, 1986; Martin, 1991; Feduccia and Wild, 1993).