Figure 1.
RPR “ripper” behavioural model, illustrated by a small dromaeosaurid.
(A) grasping foot holds on to prey. (B) hypertrophied D-II claw used as anchor to maintain grip on large prey. (C) predator's bodyweight pins down victim. (D) beam-like tail aids balance. (E) low-carried metatarsus helps restrain victim. (F) “stability flapping” used to maintain position on top of prey (see Supporting Information Videos S1 and S2). (G) arms encircle prey (“mantling”), restricting escape route. (H) head reaches down between feet, tearing off strips of flesh (may explain unusual deinonychosaurian dental morphology). Victim is eaten alive or dies of organ failure.
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].
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.
Figure 4.
Correspondence Analysis comparing relative proportions of pedal non-ungual phalanges among non-avian theropod dinosaurs.
Separation along Axis 1 (64.99% of variation) discriminates cursorial Ornithomimidae from less-cursorial Dromaeosauridae. Troodontidae plot closer to Ornithomimidae than their sister-taxon Dromaeosauridae, indicating a more cursorial habit. Archaeopteryx plots close to Dromaeosauridae, but in a more intermediate position, as do Tyrannosauroidea, Allosauroidea, and Ceratosauridae. The separation of Archaeopteryx from Tyrannosauroidea and Allosauroidea along Axis 2 (13.89% of variation) suggests an additional discriminatory aspect of phalanx proportions. n = 30.
Figure 5.
Variation in foot proportions consistent with cursoriality or grasping.
Cursorial-proportioned feet of Gallimimus (A) and Allosaurus (B) exhibit D-II and D-IV of subequal lengths, with D-IV significantly shorter than D-III. This is contrasted with Deinonychus (C) where D-IV is significantly elongated, being subequal in length to D-III, with distal-most non-ungual phalanges of D-III and IV subequal in length to the preceding penultimate non-ungual phalanx; features consistent with a grasping habit [20]. Scale = 5 cm. Modified from original sources [1], [42], [100].
Figure 6.
Comparison of ginglymoid vs non-ginglymoid articulation facets in first pedal phalanges of Troodon sp. (all dorsal view).
The distal articulation facet is ginglymoid in D-II-1 (A; MOR 553S-6.29.9.89), but not in D-III-1 (B; MOR 553S-8.11.9.209) or D-IV-1 (C; MOR 553S-8.11.92.213). Specimens are derived from a multi-individual bonebed and may not be from the same individual, hence differences in size are not relevant. Scale bar = 2 cm.
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.
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.
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.
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.
Figure 11.
(A) Archaeopteryx. (B) Variation of wing aspect ratio in extant birds, from left (low) to right (high): goshawk (Accipiter gentilis), golden eagle (Aquila chrysaetos), northern royal albatross (Diomedea sanfordi). The short broad wings of Archaeopteryx are similar to the goshawk, where they afford great maneuverability. Image in (A) altered from Longrich [101].