 A. fragilis is the type species and was named by Marsh in 1877 . It is known from the remains of at least sixty individuals , all found in the Kimmeridgian – Tithonian Upper Jurassic @-@ age Morrison Formation of the United States , spread across the states of Colorado , Montana , New Mexico , Oklahoma , South Dakota , Utah , and Wyoming . Details of the humerus ( upper arm ) of A. fragilis have been used as diagnostic among Morrison theropods , but the discovery of " A. jimmadseni " indicates that this will no longer be the case at the species level . 
