associated with increase in flapping adaptations of the forelimbs, culminating in small body-size at the base of Paraves [94,98]. Thus, Deinonychus, Velociraptor, and other relatively large-bodied deinonychosaurians were probably derived from small-bodied ancestors. Even if stability flapping does not represent a condition ancestral to true flapping flight, it may help explain the prevalence of apparent flapping and aerial abilities in otherwise terrestrial taxa. The presence of secondary flight feathers on the forelimbs of Velociraptor [88] might be unexpected since the large body size of this taxon appears to preclude a flighted or gliding function (although Velociraptor was of similar mass to the largest extant flying birds, e.g. bustards at ,19 kg [99]). However, stability flapping (especially in its less vigorous forms) may still be a viable use for a wing, even in a taxon as large as an adult Velociraptor. Conclusions The Raptor Prey Restraint (RPR) model presents multiple new concepts that give functional explanations for the morphological peculiarities of Deinonychus and other paravians. These findings open many novel lines of research into the predatory ability of extinct theropods, and emphasize the importance of exaptation in the evolution of novel structures and behaviours. The hypertro- phied D-II talon of Accipitridae represents the closest analogue yet presented for use of the similarly hypertrophied D-II talon of Deinonychosauria. Gradual divergence of foot proportions within Deinonychosauria is potentially indicative of ecological separation between Dromaeosauridae and Troodontidae as large and small prey specialists (respectively). Future research on the diet of extinct theropods should include analysis of foot functional morphology, which has the potential to test hypotheses recently presented by Zanno and Makovicky [44]. The grasping foot of paravians demonstrates a shift in emphasis for prey restraint from the manus to the pes, as the forelimbs became increasingly feathered and adapted for flapping functions through Coelurosauria. In our description of stability flapping and its importance to predatory success, we hope to have opened a new direction of study in the evolution of flight in birds. The RPR model demonstrates that there need not be a scenario where flight is gained (and lost) numerous times [85]. Rather, we present the more parsimonious ‘‘flapping first’’ hypothesis: that basal para- vians exhibited a range of flapping behaviours unrelated to flight [96,97], but that it was only in Avialae where true flapping flight evolved as a method of aerial locomotion. A more precise definition of stability flapping is in preparation such that future studies can better focus on potential osteological or biomechanical correlates. Further investigation is also required into other flapping behaviours that do not involve flight, including stability flapping executed outside of a predatory role. Hence, much work remains in characterizing stability flapping, but as with other recently proposed models [96,97], recognition of this novel behaviour enriches our understanding [94] of the physical capabilities of the ancestors of modern volant birds. The Predatory Ecology of Deinonychus Supporting Information Text S1 Text S1 includes additional discussion of previous work on paravian claw morphology and function, comparison of Deinonychosauria with extant seriema birds, reconsideration of the origin of the avian pes Tendon Locking Mechanism, and the possibility of stability flapping in Haast’s eagle: an extinct giant accipitrid. (DOC) Video S1 Stability flapping in a wild Eurasian Sparrow- hawk (Accipiter nisus). The vigorous flapping of this sparrowhawk is not an attempt to fly away with its prey. Rather, this is ‘‘stability flapping’’: employed only to get on top of the prey and maintain this position so that the raptor can use its bodyweight to pin down its victim. With the feet employed in preventing escape, the forelimbs must now be used to maintain an advantageous position: the opposite to what is seen in basal theropods where the forelimbs presumably had a greater role in subduing prey, with the feet used for positioning. Filmed 12th March 1998, Nacton, Suffolk, UK. (AVI) Video S2 Prey positioning in a wild Eurasian Sparrow- hawk (Accipiter nisus). Here the sparrowhawk has the prey pinned between its D-II talons, with the other toes used for stable footing. Even though its victim is still alive, the raptor can continue to feed as the prey is well restrained by the predator’s bodyweight and claws. Light stability flapping is intermittently employed to maintain position. Filmed 12th March 1998, Nacton, Suffolk, UK. (AVI) Table S1 This table comprises raw measurement data and calculated ratios for dinosaurs measured for this study, and comparative data from Fowler et al. [20]. (XLS) Acknowledgments For access to specimens and metadata, we thank C. Marshall-Faux, E. Lamm, and J. R. Horner (Museum of the Rockies); J. Rotella (Montana State University); Peg Hart, Carl Mehling, and Paul Sweet (American Museum of Natural History); Cleveland Museum of Natural History, Ohio; and D. Brinkman and W. Joyce (Yale Peabody Museum). Many thanks to the Montana Raptor Conservation Center for MOR specimens used in this study. S. Hallager (United States National Museum), and Torsten van der Lubbe (Niedersa¨chsisches Landesmuseum, Hannover) provided reference materials. Thanks to Jack Horner and the Museum of the Rockies for support. This work was enhanced by comments from D. Csermely, T. Dececchi, A. Farke, C. Glen, S. Keenan, D. Naish, K. Padian, J. Stiegler, and two anonymous reviewers. 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