The holotype specimen of "A. atacis" is a group of three connected dorsal vertebrae. Cervical vertebrae are uncommon from Bellevue and most of them are poorly preserved. The vertebrae are opisthocoelous, with centra that lengthen toward the rear. Neural spines have been preserved on the vertebrae, and they face toward the rear. Two dorsal vertebrae were preserved with a complete, but very crushed neural arch, a neural spine with a characteristic shape: it is very widened distally and narrows downwards. The vertebrae also have an internal structure that is spongy, with very large cells. The caudal vertebrae are all strongly procoelous and are compressed on the sides. The neural spines of the caudal vertebrae are very narrow and very tall and point toward the rear. The middle caudal vertebrae are more compressed on the sides that the others. Also, the centrum of middle caudal vertebrae is longer proportionally. Two sacral vertebrae were known from Bellevue. They are poorly preserved, and are thought to belong to a juvenile. A complete sacrum was also found in 2002.