Acristavus
Acristavus (meaning "non-crested grandfather") is a genus of saurolophine dinosaur. Fossils have been found from the Campanian Two Medicine Formation in Montana and Wahweap Formation in Utah, United States. The type species A. gagslarsoni was named in 2011. Unlike nearly all hadrosaurids except Edmontosaurus, Acristavus lacked ornamentation on its skull. The discovery of Acristavus is paleontologically significant because it supports the position that the ancestor of all hadrosaurids did not possess cranial ornamentation, and that ornamentation was an adaptation that later arose interdependently in the subfamilies Saurolophinae and Lambeosaurinae. It is closely related to Brachylophosaurus and Maiasaura, and was assigned to a new clade called Brachylophosaurini.[1]
The holotype specimen of Acristavus, MOR 1155, was recovered at the Two Medicine Formation, in Teton County, Montana. The specimen was collected in 1999 by C. Riley Nelson in well-indurated tan colored calcareous sandstone that was deposited during the Campanian stage of the Cretaceous period, approximately 79 million years ago.[2] MOR 1155 consists of an almost complete skull with associated postcrania including eleven cervical vertebrae, three incomplete dorsal vertebrae, a proximal caudal vertebra, several dorsal ribs, the left humerus, the left ulna, the right sternal, the left pubis, the left femur, the left tibia, two left metatarsals, five left pedal phalanges and one right pedal phalanx. A second specimen UMNHVP 16607 assigned to Acristavus in 2011, was excavated from the Smokey Mountain Road locality in the Reynolds Point Member of the Wahweap Formation in Utah. It was collected by C. R. Nelson in 2000, in lithified, yellow sandstone and its age was estimated to be 80.69 Ma, with a range of uncertainty of 81.05-80.19 Ma.[3]UMNHVP 16607 consists of a partial articulated skull, including both lacrimals, a complete braincase, and a cervical vertebra.
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism.
Acristavus was first assigned to the Brachylophosaurini, in a basal position, by Gates et al. (2011). All subsequent phylogenetic analyses have confirmed this assignment. Brachylophosaurines are derived members of the group Saurolophinae. Other brachylophosaurins include Maiasaura, Brachylophosaurus, and potentially Wulagasaurus. Gates concluded that Acristavus and Maiasaura shared a sister-taxon relationship but more recent analysis by Prieto-Márquez (2013) shows that Maiasaura is more closely related to the more derived Brachylophosaurus.
The following cladogram is based on the 2013 phylogenetic analysis by Prieto-Márquez (the relationships within Lambeosaurinae and between basal hadrosauroids are not shown):[4]
