Using comparative statistics

The evolution of powered flight has traditionally been associated with the origin of birds, the most successful clade of modern tetrapode, as exemplified by the nearly 10,000 species alive today. Flight requires a suite of morphological changes to skeletal anatomy to create a light yet resistant framework for an airfoil and advanced nervous motor control. Given the level of morphological integration necessary to create a suitable aerofoil, the origin of flight may be intuitively assumed to be coupled with high evolutionary rates of wing-related morphologies. Here we show that the origin of birds is associated with little or no evolutionary change to the skeletal anatomy of the forelimb, and thus Archaeopteryx is unlikely to be the "" for the origin of flight it was once believed to be. Using comparative statistics and time-series analyses on a data set constructed from all known forelimb skeletal anatomy of non-avian theropod dinosaurs and a diverse assemblage of early birds, we demonstrate three focused peaks of rapid forelimb evolution at Tetanurae, Eumaniraptora, and Ornithothoraces. The peaks are not associated with missing data and remain stable under multiple perturbations to the phylogenetic arrangements. Different regions of the forelimbs are demonstrated to have undergone asynchronous periods of evolutionary peaks and stasis. Our results evince a more complicated stepwise mode of forelimb evolution before and after the origin of Aves than previously supposed. Introduction The origin of avian powered flight has been heralded as a key evolutionary novelty at the origin of birds (Ostrom 1995). Rosetta Stone English The evolution of avian flight required that theropod dinosaurs dramatically modify their anatomy to accommodate the demands aerial locomotion imposes: small body size (Padian et al. 2001), increased cerebellar and cerebral brain volumes to coordinate flight (Alonzo et al. 2004), and an airfoil (Gauthier and Padian 1985; Pr?m and Brush 2002). These evolutionary novelties in early birds were so successful that soon after their first appearance in the Tithonian (ca. 150 Ma) birds radiated into multiple diverse clades (Chiappe and Dyke 2006) and were the dominant aerial vertebrate by the end of the Cretaceous. During this transition, bird forelimbs underwent a profound remodeling as each element in the modern avian forelimb was radically modified from its homologue in the earliest archosaurs. However, previous analyses have shown that many avian skeletal (Sereno 1999), integumental (Ji et al. 1998), and endocranial (Larsson 2001) characters are present in non-avian theropods, suggesting these features were co-opted for flight by Aves from non-volant theropods. We examined forelimb evolution within the non-avian to avian theropod transition to assess the patterns of evolution of the skeleton most involved with avian powered flight. Skeletal changes were ordered in a phylogenetic context to calculate patristic evolutionary rates and allow for quantitative comparisons among selected segments over bird evolutionary history.

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