Current scenarios frequently interpret the Late Jurassic bird Archaeopteryx as having had an avian-type physiology and as having been capable of flapping flight, but only from "the trees downward." It putatively lacked capacity for takeoff and powered flight from the ground upward. Data from extant reptiles indicate that if Archaeopteryx were physiologically reptilian, it would have been capable of ground upward takeoff from a standstill, as well as "trees downward" powered flight. This conclusion is based largely on a previously unrecognized attribute of locomotory (skeletal) muscle in a variety of extant reptiles: During "burst-level" activity, major locomotory muscles of a number of active terrestrial taxa generate at least twice the power (watts kg-1 muscle tissue) as those of birds and mammals. Reptilian physiological status also helps resolve the apparently uneven development of various flight support structures in Archaeopteryx (e.g., well-developed flight features but relatively unspecialized pectoral girdle, supracoracoideus muscles, etc.). Endothermy and capacity for longer-distance powered flight probably evolved only in Early Cretaceous birds, which were the first birds to have a keeled sternum, strap-like coracoid, and hypocleidium-bearing furcula.