Figure 10. MOR 747, Deinonychus left MT-I in posterior (A), anterior (B), medial (C), and dorsal (D) views. MT-I has a ginglymoid articulation facet, limiting movement, but increasing strength, contrasting with the more mobile MT-I of Troodon sp. (Figure 9). Scale bar = 2 cm. doi:10.1371/journal.pone.0028964.g010 the tubercles are still relatively much smaller than those seen in birds of prey. As such, the grip strength of the foot may not have been great enough to constrict small prey as seen in extant raptors. Further, while pedal digits are elongated in deinonychosauria, phalangeal proportions still reflect a degree of cursoriality ([61], i.e. non-ungual phalanges shorten distally, unlike that seen in grasping-adapted birds). This may reflect the fact that the largest deinonychosaurians (and possibly all included taxa) were terrestrial animals, which necessitated a compromise between a locomotor and predatory grasping function. Eating, jaws, and teeth After prey have been captured and immobilized, the task of dismemberment and consumption presents a different challenge to the predator, with important morphological implications. Eating habits vary among birds of prey; owls (small prey specialists) usually swallow prey whole, while falcons and hawks dismember prey before consumption. Prey are typically pinned down between the feet by the claws of both left and right D-II, while D-I, III, and IV contact the ground, steadying the bird for feeding [20]. For larger food items, one or both feet are used in their entirety, which may cause instability, requiring correction by flapping and extending the wings and tail (see Supporting Information Video S2). To feed, the head reaches down between the feet, gripping tissue in the hooked beak, then pulls upwards, plucking away the feathers or tearing off strips of flesh. We envisage deinonychosaur- ians feeding like accipitrids in holding prey under the feet, with the head reaching down between the feet to feed. Manning et al. [2,62] assert the jaws of dromaeosaurids were the main tool in killing their prey, but the mandibles of Deinonychus, Velociraptor, and Saurornitholestes were not particularly robust, lacking a strong bite force [63,64] and were therefore poorly suited for the primary offense in attack and restraint of large prey. The RPR model better explains this morphology. Here the jaws are employed only for dismemberment; the prey item is typically of a much smaller size, and since it is fully restrained under or within the feet, it subjects the jaws to lower stress when in use. A weak bite force might be considered disadvantageous for a predator; however, accipitrids have a relatively weak bite force (especially compared to other birds of prey [28]), and it does not decrease their predatory effectiveness. Rather, a weaker bite force merely forces accipitrids to adopt a different immobilisation strategy than other raptors [20]. In the absence of any other apparent structure The Predatory Ecology of Deinonychus for quick dispatch of prey, it is likely that deinonychosaurians were like accipitrids in simply eating their victims alive. Head orientation and movement during prey dismemberment may help explain the unusual tooth morphology of Deinonycho- sauria. Tooth morphology is a conspicuous indicator of diet and feeding strategy. In general, the teeth of theropod dinosaurs are similar in form to extant varanids, being laterally compressed, posteriorly recurved, and possessing denticles on both the anterior and posterior carinae which pinch and tear through flesh, rather than slicing like a knife [65]. The peculiar teeth of Dromaeosaur- idae (with the possible exception of Dromaeosaurus) differ from typical theropods in that the denticles of the posterior carina are particularly elongate, distally hooked towards the tooth apex, and much larger than those of the anterior carina [66]. This character is particularly pronounced in derived Late Cretaceous taxa (e.g., Velociraptor, Saurornitholestes); indeed anterior denticles may be entirely absent in some Saurornitholestes teeth [66,67]. Denticle reduction on the anterior carina would enhance a piercing function, but the peculiar hooked shape of the posterior denticles would not appear well-suited for tearing through flesh, suggesting behaviour that deviates from more typical theropods. Under the RPR model, hooked posterior denticles may enhance the effectiveness of the jaws’ grip on the prey. When the head reaches down between the feet, the jaws become oriented nearly perpendicular to the prey (see Figure 1). With the predator’s teeth embedded into its victim, subsequent backward jerking motion of the head (as seen in birds of prey) would pull impaled tissue against the posterior denticles. The denticles’ hooked shape potentially enhances grip as tissue is torn away. It is also possible that the peculiar denticles are not an adaptation for hooking flesh, but actually helped in removing feathers or fur from prey items. The teeth of troodontids are similar to those of dromaeosaurids in that denticles are reduced or absent on the anterior carina, with large hooked denticles on the posterior carina [66,67]. Troodontid denticles appear proportionally much larger than those of dromaeosaurids, and compared to the crown height this is true. However, troodontids possessed many more teeth in their jaws than would have a similarly sized dromaeosaurid [67]. For a given fixed jaw length, troodontid teeth are comparatively much reduced in size; the crown height of troodontid teeth would have been only about half as much as those of a dromaeosaurid. Therefore it is probably more accurate to say that troodontids do not have large denticles; rather, they have short crowns, with similarly sized denticles as might be expected for a dromaeosaurid of similar body mass. This makes sense if denticles have a size below which they are no longer able to function effectively. Low-carried metatarsus helps restrain prey Evidence from extant acciptrids suggests the metatarsus itself may be used by Deinonychosauria to help restrain prey. Upon contacting with their prey on the ground, the red-tailed hawk (Buteo jamaicensis) has been observed bringing the tarsometatarsus into a horizontal position parallel to the substrate [68]. It is not known whether this is typical behaviour for accipitrids as a whole, but it is consistent with video evidence [20]. Footprints show that non-avian theropods carried their metatarsus at a lower angle to the substrate than do extant birds [58]. If Deinonychus brought its metatarsus close to the horizontal during prey restraint, this would bring D-I closer to the prey animal, with the metatarsus forming the ‘palm’ of an enclosable fist. Mantling and possible use of the forelimb It remains paradoxical that the manual digits of paravians seem well-suited for flexion and grasping, yet would have borne flight PLoS ONE | www.plosone.org 8 December 2011 | Volume 6 | Issue 12 | e28964