Genus: Acristavus GATES, HORNER, HANA & NELSON, 2011
Etymology: Latin, A, 'non', Latin, crist, "crest," and Latin, avus, "grandfather" in refernce to the absence of an osteological nasal crest, its stratigraphic position relative to other hadrosuarid taxa in the Two Medicine Formation, and the primitive nature of the skull.

Species: gagslarsoni GATES, HORNER, HANA & NELSON, 2011
Etymology: In honor of Gas Larson, nickname of the landowner Russell Ellsworth Larson, on whos property the specimen was discovered.

Holotype: MOR 1155

Locality: Museum of Rockies locality number TM-281, approximately 14 km southwest of Choteau, Teton County, Montana.

Horizon: Lower part of the Two Medicine Formation.

Biostratigraphy:

Age: Judithian age, Campanian Stage, Senonian subepoch, Upper Gulf epoch, Late Cretaceous.

Material: Nearly complete partially articulated skull and associated articulated poscrania, including 11 cervical vertebrae, 3 fragmentary dorsal vertebrae, 1 anterior caudal vertebra, dorsal ribs, left humerus, left ulna, right sternal, left pubis, left femur, left tibia, left fibula, left metatarsals II and III, left pedal phalanges II-1, II-3, IV-1, IV-3, IV-4, and right pedal phalanx II-1.

Referred material:

Locality: Top of a tall butte near the junction of Smokey Mountain Road and Right Hand Collet Canyon, Staircase-Escalante National Mounument, Garfield County, south-cental Utah.

Horizon: Upper portion of the Middle Mudstone Member, Wahweap Formation.

Biostratigraphy:

Age: Judithian age, Campanian Stage, Senonian subepoch, Upper Gulf epoch, Late Cretaceous.

Material:

UMNHVP 16607: Partial articulated skull roof including lacrimals and entire raincase and a single cervical vertebra.