A docodont ulna from the Upper Jurassic in North America and its implications for the forelimb morphology in Docodonta (Mammaliaformes)

An isolated ulna referable to Docodon, from the Upper Jurassic Morrison Formation of Wyoming, resembles the ulna of Borealestes in the presence of a small facet for the radial condyle of the humerus that is confined to the middle of the lateral side of the facet for the ulnar condyle of the humerus and a prominent tubercle on the latter. These taxa and Haldanodon are characterized by the presence of a deep, slit-like lateral fossa for one of the ligaments of the elbow joint. These features are currently only known for Docodonta. However, the ulna of Docodon differs from those of Borealestes and Haldanodon in the lack of an anterior curvature at its proximal end. It shows the most plesiomorphic morphology currently known for docodontans, suggesting that this taxon lacks specialized fossorial or aquatic adaptations.