The Predatory Ecology of Deinonychus Figure 3. Correspondence Analysis comparing relative ungual and digit sizes of Deinonychus and extant avians. Deinonychus plots nearest to Accipitridae, emphasizing similarity in pedal morphology. Axis 1 = 50.68% of variation, Axis 2 = 24.15% of variation. Extant avian data (mostly birds of prey) from Fowler et al. [20]. n = 42. doi:10.1371/journal.pone.0028964.g003 positioning. D-IV-1 has slight medial curvature over its length, deflecting D-IV towards D-III. During extension, D-IV articulates in a relatively restricted manner, placing the toe as slightly divergent from D-III. D-II and D-III are roughly parallel as the distal end of MT-III is deflected medially (Figure 5; contra Ostrom [1]). This is important as it alters the interdigital divarification angle, which might be used to interpret didactyl footprints as either troodontid or dromaeo- saurid. In Troodon sp. (MOR 553S.TM068 and MOR 748.TM065) the distal articular ends of the metatarsals are not ginglymoid, and the only pedal phalanges that exhibit ginglymoid articulation facets are all phalanges of D-II, and the distalmost non-ungual phalanges of D-I, D-III and D-IV (Figure 6; as seen in other troodontids. [36–38]. All other phalanges are non-ginglymoid. By comparison, in basal dromaeosaurids the distal articular ends of the metatarsals are either non-ginglymoid or weakly ginglymoid [39–41], developing into strongly ginglymoid in derived forms (MOR 747). In dromaeosaurids interphalangeal articulation facets are usually ginglymoid on all digits ([1]; Figure 7). Discussion The RPR (‘‘ripper’’) model Under the RPR model (Figure 1), the grasping foot of Deinonychosauria is interpreted as an adaptation for holding on to prey as the predator’s bodyweight pins down its victim. Positioning and balance is maintained by anchoring the hypertrophied D-II claw into the prey (also preventing escape), assisted by ‘‘stability flapping’’ (see supplementary videos) and movement of the beam-like tail. Prey escape is restricted as the forelimbs encircle prey (‘‘mantling’’). Deinonychosaurians lack any obvious specializations for prey dispatch, and so were probably similar to accipitrids in eating their prey alive. Detailed discussion of the morphological evidence behind these various components of the RPR model is presented below. Foot proportions and functional interpretation Foot proportions vary considerably among clades of non-avian theropod dinosaurs (Figures 2 and 5). Although combinations of foot morphological characters are not identical to extant raptors (i.e. we do not see one taxon as an ‘‘owl mimic’’ another as a ‘‘falcon’’, etc.) likely predatory behaviours can be elucidated by comparison of individual characters and their functional mor- phology in extant birds of prey. A detailed investigation into all extinct non-avian theropods is beyond the scope of this manuscript, but some initial findings are worthy of reporting, helping us to understand the origin and development of RPR model behaviour (Figure 1). D-II claws and pinning behaviour The large size and high curvature of the D-II claw in Deinonychosauria is suggestive of its use in pinning down prey, as is its function in extant Accipitridae. Positioned on the inside of the foot and on a relatively short toe, the D-II ungual is best served to exert maximum leverage. In extant carnivorous birds (including non-predaceous forms such as crows and turkey vultures), D-II PLoS ONE | www.plosone.org 4 December 2011 | Volume 6 | Issue 12 | e28964