[D] Acristavus gagslarsoni [sG] [T]
Gates, Horner, Hanna and Nelson 2011
Cretaceous Late Campanian
Ornithischia Ornithopoda Hadrosauridae Hadrosaurinae Brachylophosaurini
Two Medicine Formation, Museum of Rockies locality number TM-281, approximately 14 km southwest of Choteau, Teton County, Montana, US
Abstract: A new hadrosaurid dinosaur, Acristavus gagslarsoni, is here named on the basis of several autapomorphic characteristics of the frontal, postorbital, and dentary. Acristavus is a member of the newly erected clade Brachylophosaurini, which along with its other members, Brachylophosaurus and Maiasaura, constitutes the earliest hadrosaurine hadrosaurid clade.
The new taxon occurred approximately 79 million years ago and has been recovered from the Two Medicine Formation of western Montana and nearly simultaneously in the Wahweap Formation of southern Utah. Corresponding with its age and relationship to the other members of the Brachylophosaurini, it is not surprising that Acristavus possesses traits seen in both Brachylophosaurus and Maiasaura, but not necessarily shared between them.
One of the most interesting morphological features of Acristavus is the lack of cranial osteological ornamentation, which is in stark contrast to every other hadrosaurid dinosaur except Edmontosaurus. Combining stratigraphic and phylogenetic data from Acristavus yields support for the hypothesis that the hadrosaurid ancestor did not possess cranial ornamentation, and that the subfamilies Hadrosaurinae and Lambeosaurinae each independently developed display structures.
Etymology: A (Latin)-non, crist (Latin)-crest, avus (Latin)-grandfather, in reference to the absence of an osteological nasal crest, its stratigraphic position relative to other hadrosaurid taxa in the Two Medicine Formation, and the primitive nature of the skull. gagslarsoni from Gags Larson, nickname of landowner Russell Ellsworth Larson, on whose property the specimen was discovered.
Holotype: MOR 1155, nearly complete partially articulated skull and associated articulated postcrania, including 11 cervical vertebrae, three fragmentary dorsal vertebrae, one anterior caudal vertebra, dorsal ribs, the 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, and IV-4, and right pedal phalanx II-1.
Referred Specimen: UMNHVP 16607 (Figs. 5, 6B, 9), a partial articulated skull roof including lacrimals and entire braincase, and a single cervical vertebra.
Horizon and Locality: MOR1155 was collected from the base of a fluvial channel deposit (i.e., a tan-colored, moderately sorted, subangular to subrounded, calcareously cemented sublitharenite with a basal lag deposit containing macrovertebrate, microvertebrate, and plant remains) in the lower part of the Upper Cretaceous Two Medicine Formation, Museum of Rockies locality number TM-281, approximately 14 km southwest of Choteau, Teton County, Montana, U.S.A. The locality was discovered in 1999 by a group of ‘Junior Paleontologists’ from the University of Chicago while on a field expedition with the Old Trail Museum of Choteau. Based on its stratigraphic position, ∼40 m above the 80 Ma dated crystal tuff of Rogers et al. (1993), its estimated age is ∼79.43 Ma.
Remarks: UMNHVP 16607 was collected from wellindurated tan-colored sandstone within the upper portion of the Middle Mudstone Member of the Wahweap Formation in Grand Staircase-Escalante National Monument, Utah. The specimen was located on the top of a tall butte by one of us (C.R.N.) in July of 2000 near the junction of Smokey Mountain Road and Right Hand Collet Canyon. It was found at a stratigraphic level approximately 170 m from the base of the Wahweap Formation. Using published sedimentation rates and radiometric ages (Jinnah et al., 2009), its estimated age range is 79.34–78.91 Ma.
Diagnosis: Acristavus is a hadrosaurine hadrosaurid diagnosed by the following unique features: enlarged posterodorsal brow of postorbital and basioccipital extending posteriorly well beyond the dorsal border of the foramen magnum to a greater extent than in other hadrosaurine species (these two are preserved on the holotype MOR 1155), as well as the following best observed in the referred specimen UMNHVP 16607: deep laterally squared frontonasal suture; swelling of the interdigitate suture between the prefrontal and frontal; deep depression on the lateral surface of the postorbital jugal process. Acristavus also possesses a unique combination of the following features: widely expanded alar process; deeply developed basipterygoid ridge; deep, flattened supratemporal bar; absence of nasal ornamentation; lacrimal wide in posterior view; anterior nasal process terminating posteriorly to anterior margin of external nares; dentary with nearly flat ventral margin.