The Predatory Ecology of Deinonychus Figure 2. Phylogenetic distribution of characters pertinent to the RPR model. 1. D-II ungual larger than D-III; 2. elongate metatarsus; 3. arctometatarsalian metatarsus; 4. short robust metatarsus; 5. dorso-ventrally flattened pedal unguals; 6. D-II ungual smaller than D-III; 7. elongate D-IV; 8. hyperextensible D-II; 9. enlarged D-II ungual; 10. subarctometatarsalian metatarsus; 11. hypertrophied D-II ungual; 12. reduced forelimbs; 13. stiffened tail; 14. ginglymoid distal articulations of metatarsals. Phylogeny from Senter [5]. doi:10.1371/journal.pone.0028964.g002 Although Ostrom [1] considered the range of motion for the D- II ungual and penultimate non-ungual phalanges of Deinonychus, he only briefly mentioned the range of motion for the other digits. In order to ascertain range of motion, we manually manipulated 2 partial, and one complete pes, (MOR 747; following the method of Senter [31,32]). Metatarsal (MT)-I was reconstructed in an unreversed position to match that seen in articulated specimens of Velociraptor mongoliensis ([33,34], contra Ostrom [1]). Phylogenetic nomenclature follows that of Senter [5], except that we use the term ‘‘Ornithomimidae’’ in place of his ‘‘Arctometatarsalia’’. The latter term is potentially confusing, since ornithomimids, tyrannosaurids, troodontids and some other theropods all exhibit the arctometatarsalian condition of the pes (and this is something we discuss frequently in this paper), yet Senter’s use of the term refers to what many workers would understand as Ornithomimidae only, hence our change. Phalan- geal nomenclature follows Moreno et al. [35] where appropriate. Photographs were taken using Canon S400 and SX110 cameras. Figures were processed using Adobe Photoshop. Results Most non-avian theropods measured possess a slightly more curved and enlarged ungual on D-II: in non-paravian theropods it is ,10% larger than the ungual of D-III (e.g. D-II is 14.5% larger than D-III in Tyrannosaurus, 11.3% larger in Allosaurus). The D-II ungual is enlarged in Deinonychosauria relative to non-paravian dinosaurs (one-tailed t-test assuming equal variances, alpha = 0.05, n = 10: D-II/D-III t8 = 3.471, p = 0.004); however, it is only especially hypertrophied in derived dromaeosaurids, where it is associated with further shortening of the non-ungual phalanges of D-II. Correspondence analyses When Deinonychus is plotted in a CA with extant birds of prey, it falls closest to the accipitrids, although even more negative along axis 2. This morphological similarity in pes dimensions is driven by their enlarged D-II unguals and relative proportions of other unguals (Figure 3). The CA of non-avian dinosaur phalangeal proportions clusters specimens by taxonomic group and lifestyle (Figure 4). The first axis, which explains 64.99% of the variation, separates the strongly cursorial Ornithomimidae from other theropods. Intermediate taxa include the less strongly cursorial tyrannosauroids, Archaeop- teryx, Dilophosaurus, Avimimus, Troodon, and Epidendrosaurus. These are spread widely along the second axis, which explains 13.89% of the variation. On the negative side of Axis 2, the tyrannosauroids cluster together, and the unusual Epidendrosaurus appears to possess exaggerated tyrannosauroid phalangeal proportions. On the positive side of Axis 2, Avimimus falls amongst the Archaeopteryx specimens, with Troodon farther along the axis. Troodon and Epidendrosaurus have similar eigenvalues along Axis 1, yet are the most widely separated taxa along Axis 2. While the CA analyses show some grouping, assessment of ratios individually is more informative (see discussion), partly because terrestrial grasping predators must exhibit a compromise of cursorial vs grasping characters. Biomechanical analysis MOR 747 (Deinonychus) The distal articular surface of MT-I has a distinct twist, rotating phalanx D-I-1 to face more laterally, although there appears to be little possible movement at this joint, restricting D-I-1 to a relatively fixed position. The distal articular facet of D-I-1 is ginglymoid, restricting the ungual (D-I-2) to vertical motion. The shaft of MT-II is mostly straight but exhibits a variable amount of medial deflection in the distal third. The shaft of MT- III is straight. The distal articular surfaces of MT-II and MT-III, and those of individual phalanges in each digit, are ginglymoid, limiting these joints to movement in a single dorso-ventral plane. The distal articular facets of MT-II and MT-III show slight medial deflection, so that contra the reconstruction of Ostrom ([1]: Figure 73) it is not physically possible for the phalanges of D-III to articulate in a straight line parallel to the shaft of MT-III without disarticulating MT-III and D-III-1. Instead, D-II and D-III are typical of non-arctometatarsalian theropods in that both are oriented slightly medially with respect to the metatarsus, and very slightly divergent with respect to each other (illustrated in Figure 5). The shaft of MT-IV is straight in the proximal two thirds, but deflects laterally in its distal third. MT-IV has a ball-like distal articular facet, matched by a concave proximal articular facet of phalanx D-IV-1 allowing D-IV some variability in lateral PLoS ONE | www.plosone.org 3 December 2011 | Volume 6 | Issue 12 | e28964