Abstract

Non-avian theropods present a trend in among-species variation of forelimb proportions, with larger species presenting proportionally smaller forelimbs (negative allometry). This trend disappears in the Avialae, but is especially clear in their immediate ancestors, the non-avian Paraves. Allometric evolutionary trends reflect ontogenetic trends within a clade. A straightforward prediction is that in many theropods, ontogenetically younger individuals should present proportionally larger arms than the adults. This prediction appears to be fulfilled in Deinonychus, where recently described materials of a ju- venile specimen suggest larger arms. Bambiraptor, a Dromaeosaurid with especially long arms, is known from a juvenile specimen. It has been suggested that important locomotor differences (such as flapping) may have existed between young and adult dromaeosaurids. Disarticulated remains of Allosaurus at different ontogenetic stages allow for histological sections and age estimates, showing a slower growth rate for the humerus than the femur. Size reduction and paedomorphosis are well-discussed evolutionary trends near the origin of birds. If proportionally large arms are a juvenile trait of non-avian theropods, then paedomorphosis may have played an important role in the early evolution of flight, fixing these traits in adult birds.