A redescription and revised diagnosis of the syntypes of the Mongolian tyrannosaurAlectrosaurus olseni

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Abstract
The two syntypes of Alectrosaurus olseni Gilmore, 1933, are redescribed and found to belong to two theropod families. One syntype, which includes a virtually complete hind limb, was correctly diagnosed by Gilmore as a tyrannosaur and is recognized here as the lectotype of the species. Catalogued under the same number are two small manual unguals that were associated with the lectotype in the field and originally presumed to belong to the same individual. These are probably not the unguals of a tyrannosaur, however, and are only provisionally retained among the lectotype materials. The second syntype of Alectrosaurus olseni consists of three associated forelimb elements that were found approximately 30 m away from the lectotype and may, or may not, pertain to a single individual. None of these materials is tyrannosaurid as originally thought, and they are all tentatively referred to the Segnosauridae Perle, 1979. Four small vertebrae were associated with the forelimb elements in the field and are described for the first time. These vertebrae are tentatively regarded as belonging to a single individual that is distinct from the individual or individuals represented by the forelimb elements. We provisionally identify them as the caudal vertebrae of a small theropod dinosaur that is not referable to either the Tyrannosauridae or Segnosauridae. They bear some resemblance to the proximal caudal vertebrae of Deinonychus, but also to the middle caudal vertebrae of the sauropodomorph Plateosaurus.

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