A detailed description ofthe holotype specimen of Mononykus olecranus, a basal flightless bird from the Late Cretaceous Nemegt Formation of Mongolia, is presented. The holotype comprises a fragmentary skull, most precaudal vertebrae, fore and hindlimbs, the thoracic girdle, and fragments of the pelvis and synsacrum. In the skull, caudal, dorsal, and rostral tympanic recesses are well developed. The maxilla is toothless, and the anterior margin of the antorbital fossa lacks accessory fenestrae. The only dental element found is a tiny isolated tooth that lacks serrations and has a constricted base. The axial skeleton is remarkable in having a biconvex posterior dorsal vertebra, and keeled posterior synsacral vertebrae. The forelimb is short and extremely robust. The humerus bears a prominent deltopectoral crest. The olecranon process of the ulna is hypertrophied. The carpometacarpus is very short, subquadrangular, and massive. The alular digit is extremely robust, bearing a robust ungual phalanx. The sternum is stout and carinate. In the pelvis, the ilium bears a strong antitrochanter and the pubis is retroverted. The hindlimb is gracile. In the femur the trochanteric crest is undivided, and the popliteal fossa is bounded distally by projections from both condyles. The tibia and proximal tarsals are partially fused. Two cnemial crests are present on the tibio tarsus. The metatarsals are unfused; metatarsal III ity of the transition between nonavialian therois completely reduced proximally and does not pods and modem birds, witnessing the differenreach the tarsus. tiation of a totally new group ofbasal birds during Mononykus olecranus emphasizes the complexity of the transition between nonavialian theropods and modem birds, witnessing the differenreach the tarsus. tiation of a totally new group of basal birds during the Cretaceous.
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