Figure 7. Comparison of ginglymoid vs non-ginglymoid articulation facets in first pedal phalanges of Deinonychus (MOR 747; all dorsal view). The distal articulation facet is ginglymoid in D-II-1 (A), D-III-1 (B) and more weakly so in D-IV-1 (C). Specimens found as part of an articulated pes. Scale bar = 2 cm. doi:10.1371/journal.pone.0028964.g007 bears the largest claw (of D-II to IV) and is used for pinning down of food items during consumption [20]. Actively predaceous carnivores (such as Accipitridae and Falconidae) typically exhibit greater curvature of the D-II ungual than taxa that are only scavengers ([20]; although see Supporting Information Text S1). In most extinct carnivorous non-avian theropods, D-II also bears the largest ungual, and we suggest it was used for a similar pinning function. By contrast, in non-carnivorous extant birds (which do not exhibit pinning behaviours) the D-III claw is the largest, and the claws are generally less curved than carnivorous taxa [6,20]. Similarly, in the secondarily herbivorous theropod dinosaurs Ornithomimidae (Figure 5; [42]) and Avimimus [43], the ungual of D-III is the largest, and all unguals have relatively very low curvature (consistent with a cursorial habit for these taxa). Further, ornithomimids and Avimimus exhibit reduction in size of pes flexor tubercles, which suggests considerably reduced strength in the foot. These are all features expected of non-carnivores that do not use the D-II ungual (or indeed any unguals) in pinning or any other predatory behaviour. Thus, curvature and / or relative size of the D-II ungual potentially provides independent indication of carnivory, or active predation [9], which may have implications for recently proposed hypotheses of herbivory in coelurosaurian dinosaurs ([44]; see discussion in Supporting Information Text S1). Pinning of prey by more basal non-avian theropods may have served as a behavioural origin for the RPR model in Paraves (Avialae + Troodontidae + Dromaeosauridae; sensu Senter [5]) where prey manipulation by the feet is hypothesized to have become increasingly important. Cursorial and grasping feet Relative proportions of the feet vary depending on a predominantly cursorial or grasping function (Figure 5). Extant cursorial birds (e.g. ratites; Emu, Dromaius novaehollandiae, MOR OST-1299) and similar cursorially adapted dinosaur taxa (e.g. Ornithomimidae [42]; Avimimus [43]) typically exhibit a robust D- III, foreshortened distal phalanges with shorter, subequally sized lateral digits (D-II and IV) [11,13–15,17,45–48]. An opposite trend is seen in the pedal digits of Deinonychosauria and basal Avialae (e.g. Archaeopteryx): D-IV becomes especially elongated, distal non-ungual phalanges are more elongated than in specialist The Predatory Ecology of Deinonychus cursors, and D-II becomes hyperextensible. These characters are consistent with grasping rather than a cursorial function [15]. Elongation of the metatarsus increases length of the flexor tendons, thus reducing mechanical advantage (and hence, grip strength), but also increases stride length (with other cursorial benefits [46,48]) and ‘quickness’ of the feet. Extant accipitrids have an elongate metatarsus compared to other birds of prey, granting them better ability to snatch prey, an important aspect of their predatory strategy [19,20]. Conversely, acting as the out-lever of the hindlimb, the short robust metatarsus of owls effectively reduces the length of the flexor tendons, increasing mechanical advantage; hence owls have greater force production (grip strength) than that produced by the more elongate metatarsus of accipitrids, but at a cost of less rapid movement [19]. A relatively elongate metatarsus (sometimes subarctometatarsalian) is present in basal paravians (e.g. basal troodontid Sinovenator [49]; basal dromaeosaurid Sinornithosaurus [50]; [51,52]), indicating a cursorial habit is basal for the clade. This is further developed into a fully arctometatarsalian metatarsus in derived troodontids [5,37,38,46,53,54]. By contrast, derived dromaeosaurids like Deinonychus [1], Saurornitholestes (MOR 660), and Velociraptor [33] lost the elongate metatarsus, instead evolving extremely short and robust metatarsi (among the shortest relative to tibia length of all non-avian theropods [55]). This suggests that the plesiomorphic cursorial metatarsus became further adapted towards cursoriality in Troodontidae whereas Dromaeosauridae reversed selection direction, specializing towards grasping strength at the expense of speed (Figure 2). The morphology of interphalangeal articulation surfaces is indicative of strategy for countering stress incurred during foot use. Ginglymoid phalangeal articulations limit movement of the joint to a single plane, affording resistance to torsion but decreasing flexibility [35]. Conversely, non- or weakly ginglymoid articula- tions (‘‘roller’’ joints) are indicative of low torsional stresses, and are especially prevalent in cursorial taxa on digits that are mediolaterally oriented relative to the ground resistant force [35]. In the cursorial ratites (e.g. emu, MOR-OST 1299) and similarly cursorial ornithomimids (e.g. Ornithomimus sp. UCMP 154569), the main weight of the animal is borne on the central digit (D-III; [35,56,57]) the phalanges of which are nearly symmetrical in shape and exhibit a very weak sagittal furrow (i.e. non- ginglymoid). The lateral digits (D-IV and D-II, where present) perform a stabilization role [35,56] and exhibit ginglymoid interphalangeal articulations (except for ungual articulations, which are non- to weakly ginglymoid on all digits). In extant birds of prey, all interphalangeal articulations are strongly ginglymoid, which might be expected as struggling prey exact torsional loads on the predator’s feet. In basal dromaeosaurids, the distal articular ends of the metatarsals are either non-ginglymoid or weakly ginglymoid [41,50,52]. In derived dromaeosaurids, all metatarsal and interphalangeal joints are ginglymoid ([1], except for MT-IV; see above), indicating adaptation for torsional resistance. By contrast, in the derived troodontid Troodon sp. (MOR 553S.TM068; also seen in other troodontids; [36–38]), the only ginglymoid joints are interphalangeal articulations of D-II and all ungual articulations. D-III bears roller joints, and D-IV is only weakly ginglymoid (Figure 6). This suggests a more strongly cursorial habit than seen in Dromaeosauridae (Figure 7), but somewhat different from the cursorial style of extant ratites, perhaps due to the didactyl pes of troodontids. Also, it is interesting that the unguals of MOR 553S.TM068 maintain ginglymoid articulations, even when other non-ungual interpha- langeal articulations in the same digit do not. This is unlike that observed in ratites and ornithomimids, and may be explained as PLoS ONE | www.plosone.org 6 December 2011 | Volume 6 | Issue 12 | e28964