Fig. 5. A in Quadrupedal dinosaurs did not evolve fully pronated forearms: New evidence from the ulna

Fig. 5. A demonstration of the effects that changes in pronation of the distal radial epiphysis would have on manual orientation in a fully pronated tetrapod (the Virginia opossum Didelphis virginiana Kerr, 1792) that is using parasagittal forelimb kinematics with elbows inturned to the body wall. A. An unpronated forearm and manus, which if possible would point the manual digits posteriorly. B. A semi-pronated forearm and manus, which would point the manual digits laterally, analogous to many semi-pronated archosaurs, such as dinosaurs, that utilized quadrupedalism. C. A normal (for D. virginiana) fully pronated forearm and manus, which points the manual digits anteriorly. This demonstration illustrates why it was traditionally assumed that amniotes that evolve posteriorly directed elbows (i.e., archosaurs, therians and chameleons) would require an additional 90° of forearm pronation past the plesiomorphic 90°, because 180° of pronation is required to keep the wrist and finger joints aligned posteriorly during flexion, and therefore to continue operating in a parasagittal plane. Bonnan's (2003) radial hypothesis states that the radii of quadrupedal dinosaurs pronated to condition C, but the findings of this study show that the ulnae in these dinosaurs experienced an opposing supination that would have kept the planes of the wrist and finger joints wholly in condition in B, albeit with a tubular manus (e.g., Fig. 2E); see text for further discussion. The D. virginiana forelimb elements are from an adult specimen (FMNH 166984).