the unguals are probably still utilized for prey manipulation in troodontids, and so require some resistance to torsion. A grasping interpretation for the pes of dromaeosaurids is further supported by analysis of range of motion in Deinonychus, as the pes forms an enclosed fist even without engaging maximum flexion (Figure 8). In contrast to the condition seen in most modern birds, where a fully reversed D-I opposes D-III (anisodactyly), in Deinonychus a medially directed D-I opposes D-IV, with D-II and III moving in parallel (contra Ostrom [1]), enclosing the fist antero- posteriorly across the ‘‘palm’’ of the metatarsus (aided by the low angle with which the metatarsus is carried relative to the substrate [58]). This arrangement is somewhat similar to when owls and ospreys rotate D-IV into a partially zygodactyl arrangement, thought to provide a more complete or even grasp. Similarity of the Deinonychus pes to other dromaeosaurids suggests that the grasping foot is typical of Dromaeosauridae as a whole (independently confirmed by Senter [32], and comparable phalangeal measurements among dromaeosaurid taxa; Supporting Information Table S1). Although many theropods possess a medially directed D-I and the ability to flex the foot tightly [32], The Predatory Ecology of Deinonychus only paravians possess the extreme digital elongation facilitating greater opposability and hence grasping ability. In most Cretaceous ecosystems, troodontids lived alongside dromaeosaurids. Divergence of pedal morphology between these sister-taxa potentially affords us insight into variation in their predatory ecology, similar to ecologic separation seen among extant raptor families [19,20]. In Troodon sp. (MOR 553), the distal end of MT-I (Figure 9) is more strongly twisted than that of dromaeosaurids (Figure 10), and it bears a more rounded, ball-like articulation facet (also seen in the more basal troodontid Sinornithoides [38]). This suggests greater mobility of D-I in Troodontidae, perhaps allowing D-I to better oppose the other digits. The Late Cretaceous troodontids MOR 553 and Stenony- chosaurus ([36]; and to a slightly lesser extent Borogovia [59]) exhibit shortening of D-IV (by overall shortening of phalanges), which is in contrast with the stratigraphically older (and more basal; [5]) Sinornithoides (which has a more elongate D-IV; [38]). This trend is consistent with derived Late Cretaceous troodontids evolving further towards cursorial-adapted feet, while the more basal forms exhibited more grasping ability. Starting from similar morphology in basal forms (cursorial metatarsus and a grasping foot), dromaeosaurids and troodontids follow opposite morphological trends indicative of increasing ecological separation. Dromaeosaurids appear increasingly adapt- ed for grappling larger prey with strong, but slow feet, and unusually hypertrophied D-II unguals. By comparison, troodon- tids instead evolved towards a more cursorial habit, being fast and nimble with weaker, but quick feet. The more mobile D-I in troodontids perhaps afforded a more even grip, better adapted for snatching and subduing smaller prey. An important part of our interpretation is that ground-based predation need not necessarily be conducted at high speed. It is commonplace for extant terrestrial predators to employ surprise ambush techniques; goshawks and other forest raptor species commonly hunt on the ground, employing ambush and maneuverability as strategies, rather than outright pursuit [60]. Troodontids exhibit limb proportions consistent with cursorial adaptations, suggesting speed and/or pursuit was important to their predatory strategy. By contrast, derived dromaeosaurids such as Velociraptor and Deinon- ychus do not exhibit limb proportions that suggest significant cursorial ability: rather, they were probably more inclined towards utilizing ambush as a main predatory strategy. In Deinonychus and other paravians, grasping adaptations of the digits are not as extremely developed as those seen in extant raptors [15,20]. Although the flexor tubercles on deinonychosaur- ian pedal unguals are larger than in other non-avian theropods, Figure 8. Ventral view of Deinonychus foot (MOR 747) in flexion. D-I is not reversed, but is rotated slightly so that the claw faces laterally into the ‘fist’, as observed in articulated specimens of Velociraptor [22]. Ginglymoid articulation facets of MT-II and III restrict the motion of D-II and III to a parallel dorso-ventral plane, but the distal ball joint of MT-IV allows D-IV to take a variable position, spreading more laterally, or allowing it to reach over the metatarsus, opposing D-I. Not shown at maximum flexion. Scale = 5 cm. doi:10.1371/journal.pone.0028964.g008 Figure 9. MOR 553S-8.6.92.168, Troodon sp. left MT-I in posterior (A), anterior (B), medial (C), and dorsal (D) views. MT-I has a ball-shaped articulation facet, allowing greater movement and positioning of D-I compared to MT-I of Deinonychus (Figure 10). Scale bar = 2 cm. doi:10.1371/journal.pone.0028964.g009 PLoS ONE | www.plosone.org 7 December 2011 | Volume 6 | Issue 12 | e28964