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SUMMARYVery few well-preserved, articulated, and complete pectoral girdle and forelimbs of ceratopsids are known. The specimen described here provides a rare opportunity to document these aspects of ceratopsid anatomy. In addition, because all elements were completely freed from the matrix during preparation, it was possible to fabricate an accurate replica of all elements that could then be conveniently manipulated without risk to the fragile bones. This permitted us to assemble a working model of the forelimb skeleton to test current hypotheses of ceratopsid limb stance that had been generated using incomplete or composite material, or based on qualitative assessments of museum skeletal mounts. Manipulation of this model suggests that neither the classic sprawling stance (Figure 9.1, 9.4), nor the completely upright stance (Figure 9.2, 9.5) was possible. Rather, the humerus maintained a relatively low angle to the frontal plane throughout most of the step cycle. The elbow was moderately everted. Such a forelimb stance is not incompatible with a moderately well-developed cursorial gait, but hardly implies that they approached extant quadrapedal or graviportal mammals in their range of locomotor abilities. Compression on the medial side of the metacarpus and manus that occurred during the propulsive phase as a result of this unique humeral orientation may explain the occurrence of bone abnormalities in this region in some ceratopsids. |
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