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Inferring the prevalence and function of finger hyperextension in A rchosauria from finger‐joint range of motion in the A merican alligator
Author(s) -
Hutson J. D.,
Hutson K. N.
Publication year - 2015
Publication title -
journal of zoology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.915
H-Index - 96
eISSN - 1469-7998
pISSN - 0952-8369
DOI - 10.1111/jzo.12232
Subject(s) - alligator , biology , anatomy , range of motion , interphalangeal joint , finger joint , hyperextension , quadrupedalism , joint (building) , palm , numerical digit , metacarpophalangeal joint , tassel , medicine , thumb , surgery , architectural engineering , agronomy , arithmetic , mathematics , zea mays , engineering , paleontology , physics , quantum mechanics
Abstract 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 vivo ROM 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 A merican alligator A lligator mississippiensis . Results verify that MCP joints hyperextend, and show that soft tissues significantly affect ex vivo ROM . 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 A lligator mississippiensis and the rauisuchid P ostosuchus 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.