FORELIMB STANCE AND STEP CYCLE IN
A half-scale model of the complete forelimb of Chasmosaurus irvinensis was fabricated to test current hypotheses of neoceratopsid forelimb stance and step cycle. Using this model in conjunction with trackway data, it was determined that the forelimb stance approximated neither the upright stance of extant graviportal mammals nor the sprawling posture of primitive tetrapods. Rather, at the beginning of the propulsive phase, the distal end of the humerus was directed posteroventrally at approximately 45 degrees to the frontal plane. At this point, the elbow was strongly flexed, with the anteroventrally directed epipodials forming an angle of approximately 90 degrees with the long axis of the humerus. As the propulsive phase proceeded, the humerus gradually moves closer to the horizontal, at the end of the propulsive phase forming an angle of approximately 25 degrees to the frontal plane. At this point, the elbow was extended to its maximum, forming an angle of approximately 115 degrees with the long axis of the humerus, causing the epipodials to assume a vertical orientation. At no point in the propulsive phase did the humerus move in the parasagittal plane. Rather, the elbows were moderately everted, and the long axis of the humerus formed an angle averaging slightly less than 30 degrees to the parasagittal plane. This orientation of the humerus, which resulted in compression of medial side of the metacarpus and manus, may account for bone pathologies observed in some ceratopsid specimens. This stance, in combination with the parasagittal orientation of the rear limbs, has no precise modern analogue.
KEY WORDS: limb stance, Chasmosaurus, step cycle, neoceratopsid, locomotion, dinosaur; gait, parasagittal
Copyright: Society of Vertebrate Paleontology,