Analysis of forelimb function in basal ceratopsians

Here, I present the first study of forelimb function in basal ceratopsians [Correction added after publication 28 July 2007: in the preceding sentence ‘Here, I present the first study of forelimb function in basal ceratopsiansDinosauria Orthischia’ was corrected to ‘Here, I present the first study of forelimb function in basal ceratopsians’]. I examined forelimb bones and casts ofPsittacosaurus neimongoliensis,Psittacosaurus mongoliensis,Leptoceratops gracilisandProtoceratops andrewsi. ForPs. neimongoliensisandL. gracilis, I used manual manipulations of bones and casts to determine the range of motion at available forelimb joints. I then used range of motion and morphology to test the predictions of several hypotheses of forelimb function. Forelimb morphology and range of motion indicate thatPsittacosauruswas an obligate biped and thatLeptoceratopsandProtoceratopswere capable of quadrupedal locomotion. Forelimb mobility was too limited inPsittacosaurusfor the hands to reach the mouth.LeptoceratopsandProtoceratopsare members of an evolutionary radiation in which an extension of the glenoid enabled the forelimbs to sprawl laterally for transverse pivoting, perhaps for display, but quadrupedal locomotion was accomplished with the elbows tucked in. InProtoceratops, the radius crosses over the ulna, causing the palms to face caudally. InLeptoceratops, the radius does not cross over the ulna; the palms face largely medially and the fingers have been reoriented so that flexion produces a caudal, propulsive force, even without caudally facing palms.