A Triceratops, NSM PV 20379, excavated from the Upper Cretaceous Hell Creek Formation, Bowman County, North Dakota, USA, confirms the articulation of the right forearm. Detailed study of the forelimb anatomy presented here indicates the manus is in a semi-supinated orientation. This reconstruction is based upon the following morphology: the row of the metacarpals was arranged in an ‘L’-shape in proximal view, as is the distal articular surface of the forearm is in articulation; the second digit is directed parallel to the rotational plane of the elbow joint, reinforced by neighboring digits I and III, and these three inner digits are articulated with the broad articular surface of the radius. Arrangement and directions of robust inner digits seem to be suitably arranged for a powerful stroke exerted by the elbow joint extension. Cerapods, including ceratopsians and ornithopods, share following morphology of the manus: long and robust metacarpals II and III, ungual phalanges on digits I to III, reduced digits IV and V, and a divergent metacarpal V. A laterally oriented manus with reduced outer digits is present in the basal bipedal group of the Cerapoda. Although reversal to a quadrupedal stance and a evolution to a large, heavy body occurred independently in many lineages of Cerapoda, the basic features of the manus were retained throughout Neoceratopsia.
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