Acristavus is a genus of saurolophine dinosaur. Fossils have been found from the Campanian
Two Medicine Formation in Montana and Wahweap Formation in Utah. 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.
Discovery and occurrence 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. 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 Middle Mudstone Member of the
Wahweap Formation in Utah. It was collected by C. R. Nelson in 2000, in lithified, yellow
sandstone believed to be from the same time period as the type specimen. UMNHVP 16607
consists of a partial articulated skull, including both lacrimals, a complete braincase, and
a cervical vertebra. Classification
Acristavus was first assigned to the Brachylophosaurini, in a basal position, by Gates et al.. 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 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: Distinguishing anatomical features
A diagnosis is a statement of the anatomical features of an organism 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. According to Gates, Acristavus can be distinguished
based on the following characteristics: an enlarged caudodorsal brow of the postorbital
and the basioccipital, which extends caudally well beyond the dorsal border of the foremen
magnum to a greater extent than in other hadrosaurine species
a deep laterally squared frontonasal suture swelling of the interdigitate suture between
the prefrontal and frontal a deep depression is present on the lateral
surface of the postorbital jugal process Paleoecology
Habitat The Wahweap Formation has been radiometrically
dated as being between 81 and 76 million years old. During the time that Acristavus lived,
the Western Interior Seaway was at its widest extent, almost completely isolating southern
Laramidia off from the rest of North America. The area where dinosaurs lived included lakes,
floodplains, and east-flowing rivers. The Wahweap Formation is part of the Grand Staircase
region, an immense sequence of sedimentary rock layers that stretch south from Bryce
Canyon National Park through Zion National Park and into the Grand Canyon. The presence
of rapid sedimentation and other evidence suggests a wet, seasonal climate.
Paleofauna Acristavus shared its paleoenvironment with
other dinosaurs, such as the ceratopsian Diabloceratops eatoni, unnamed ankylosaurs and pachycephalosaurs,
and the theropod Lythronax argestes, which was likely the apex predator in its ecosystem.
Vertebrates present in the Wahweap Formation at the time of Acristavus included freshwater
fish, bowfins, abundant rays and sharks, turtles like Compsemys, crocodilians, and lungfish.
A fair number of mammals lived in this region, which included several genera of multituberculates,
cladotherians, marsupials, and placental insectivores. These mammals were more primitive than those
that lived in the area that is now the Kaiparowits Formation. Trace fossils are relatively abundant
in the Wahweap Formation, and suggest the presence of crocodylomorphs, as well as ornithischian
and theropod dinosaurs. In 2010 a unique trace fossil was discovered that suggests a predator-prey
relationship between dinosaurs and primitive mammals. The trace fossil includes at least
two fossilized mammalian den complexes as well as associated digging grooves presumably
caused by a maniraptoran dinosaur. The proximity indicates a case of probable active predation
of the burrow inhabitants by the animals that made the claw marks. Invertebrate activity
in this formation ranged from fossilized insect burrows in petrified logs to various mollusks,
large crabs, and a wide diversity of gastropods and ostracods.
References