Three‐dimensional skeletal kinematics of the shoulder girdle and forelimb in walking Alligator

Abstract

Crocodylians occupy a key phylogenetic position for investigations of archosaur locomotor evolution. Compared to the well-studied hindlimb, relatively little is known about the skeletal movements and mechanics of the forelimb. In this study, we employed manual markerless XROMM (X-ray Reconstruction Of Moving Morphology) to measure detailed 3-D kinematics of the shoulder girdle and forelimb bones of American alligators (Alligator mississippiensis) walking on a treadmill. Digital models of the interclavicle, scapulocoracoid, humerus, radius and ulna were created using a 3-D laser scanner. Models were articulated and aligned to simultaneously recorded frames of fluoroscopic and standard light video to reconstruct and measure joint motion. Joint coordinate systems were established for the coracosternal, glenohumeral and elbow joints. Our analysis revealed that the limb joints only account for about half of fore/aft limb excursion; the remaining excursion results from shoulder girdle movements and lateral bending of the vertebral column. Considerable motion of each scapulocoracoid relative to the vertebral column is consistent with coracosternal mobility. The hemisellar design of the glenohumeral joint permits some additional translation, or sliding in the fore-aft plane, but this movement does not have much of an effect on the distal excursion of the bone.

Three‐dimensional mobility and muscle attachments in the pectoral limb of the Triassic cynodontMassetognathus pascuali(Romer, 1967)

“…It is worth noting that multiple derived amniote lineages seem to have converged on some degree of pectoral girdle mobility – substantial translational coracosternal mobility has been reported in several extant non‐mammals, including lepidosaurs (Peterson, ; Jenkins & Goslow, ), ornithodirans (Baier et al. ), and pseudosuchians (Baier & Gatesy, ). Little is yet known about the soft‐tissue constraints acting on the joints of the non‐mammaliaform synapsid pectoral girdle.…”

Inferring the prevalence and function of finger hyperextension inArchosauria from finger‐joint range of motion in theAmerican alligator

“…(a) Radiograph in ventral view showing (1) antebrachiocarpal; (2) intracarpal; (3) carpometacarpal; and MCP 1–5 joint locations. (b) Posterolateral view of a vertically oriented carpus + metacarpus with hyperextended fingers, physically positioned to imitate the high walk (after Baier & Gatesy, ). (c) Ventral view indicating location of fleshy weight‐bearing pads underlying MCP joints.…”

Fitting unanchored puzzle pieces in the skeleton: appropriate 3D scapular positions for the quadrupedal support in tetrapods

“…All these factors would affect the estimated appropriate scapular positions ( SP R , SP Y , SP P and SP V ). Future studies of these taxa that combine an electromyographic analysis of the fascicles of all the anti‐gravity muscles involved in body support and 3D kinematic analyses using force plates and cineradiography (Baier & Gatesy, ) would resolve these problems.…”