"The unusually large size of the humerus and the enormously long claws are so unlike any known Upper Cretaceous deinodont as to at once set the animal off as a new type of theropod dinosaur." While most of the skeletal material originally described by Gilmore belongs to a medium-sized tyrannosaur, the large clawed forelimbs that inspired the name come from an unidentified segnosaur. Later discoveries have shown that Alectrosaurus had small forelimbs like other tyrannosaurs.