Inferring the prevalence and function of finger hyperextension in Archosauria from finger-joint range of motion in the American alligator

Quadrupedal dinosaurs and other archosaurs are hypothesized to alleviate differences in height between shorter forelimbs and longer hindlimbs via digitigrady (standing on fingers). An alternative hypothesis suggests that they trend toward metacarpogrady (vertical, columnar palms with finger reduction and loss) to abandon semi-pronated (mechanically misaligned) wrist and finger joints. Range of motion (ROM) studies provide support for the latter hypothesis by showing that some dinosaurian fingers were held hyperextended away from the palm at metacarpophalangeal (MCP) joints. However, recent studies dispute traditional MCP hyperextension characters. Moreover, in vivoROM is assumed to be less than fossil ROM because of the constraining influence of lost soft tissues, further implying that reports of MCP hyperextension in fossil archosaurs may be overstated. Because crocodilian fingers reportedly hyperextend during the high walk, a ROM study of their MCP joints could clarify these issues. Here a repeated-measures analysis was used to gather ex vivo finger joint ROM data in degrees from fully fleshed to skeletonized conditions in digit III of the American alligator Alligator mississippiensis. Results verify that MCP joints hyperextend, and show that soft tissues significantly affect ex vivoROM. Skeletal ROMs were greater than fleshed ROMs in the three interphalangeal joints; however the MCP joint had the opposite result after losing curved articular cartilages. These results suggest that studies of flattened metacarpo- and metatarsophalangeal joints in fossil archosaurs may underestimate in vivo hyperextension. A comparison of characters in Alligator mississippiensis and the rauisuchid Postosuchus suggests that Postosuchus metacarpal adaptations represent a transitional stage toward vertical quadrupedal support with hyperextended MCP joints. This information provides support for the preexisting hypothesis that quadrupedal archosaurs tend to transform their wrist+palm into a rigid stilt-like extension of the forearm, which implies that locomotor abandonment of semi-pronated wrist/finger joints and alleviation of limb disparity evolve synchronously.