Fig. 4. A in Quadrupedal dinosaurs did not evolve fully pronated forearms: New evidence from the ulna

Fig. 4. A demonstration of the effects that changes in pronation of the distal radial epiphysis would have on manual orientation in a semi-pronated tetrapod (Alligator mississippiensis [Daudin, 1802]) that is using sprawling forelimb posture and kinematics. A. A forearm and manus forcibly unpronated (0°) to demonstrate the erroneous starting point of dinosaurian pronation according to Bonnan's (2003) radial hypothesis; this dislocation would orient the manual digits laterally in a sprawling forelimb. B. A naturally (for A. mississippiensis) semi-pronated (90°) forearm and manus, which orients the manual digits anteriorly in a sprawling forelimb. C. A forcibly fully pronated (180°) forearm and manus; this dislocation would orient the manual digits medially in a sprawling forelimb. In this and the following figure the upper row of boxes shows the radius and ulna in proximal view, while the lower row of boxes shows their positions in distal view. The orientations represented serve to demonstrate why stem tetrapods with laterally-directed forelimbs are assumed to have evolved semi-pronated forearm morphology from an unpronated morphology, in order to pre-axially rotate the wrist and finger joints 90° so that these joints could participate in locomotion via posteriorly directed flexion (Hutson 2010). Note also that, as a consequence of being a semi-pronated tetrapod, if an A. mississippiensis inturns its elbows to the body wall, then the manual digits will then point laterally (Vialleton 1924). Bonnan's (2003) radial hypothesis assumes that condition A was the starting point of quadrupedal dinosaur evolution instead of condition B; see text for further discussion. This and all subsequently figured A. mississippiensis forelimb elements are from the left forelimb of a juvenile specimen (FMNH 284695).