Acristavus

Life restoration

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.

According to Gates (2011), 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

The Wahweap Formation has been radiometrically dated as being between 81 and 76 million years old.[5] 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.[6]

Acristavus shared its paleoenvironment with other dinosaurs, such as the lambeosaur Adelolophus hutchisoni,[7] the ceratopsian Diabloceratops eatoni,[8][9] unnamed ankylosaurs and pachycephalosaurs, and the theropod Lythronax argestes, which was likely the apex predator in its ecosystem.[10] Vertebrates present in the Wahweap Formation at the time of Acristavus included freshwater fish, bowfins, abundant rays and sharks, turtles like Compsemys, crocodilians,[11] and lungfish.[12] A fair number of mammals lived in this region, which included several genera of multituberculates, cladotherians, marsupials, and placental insectivores.[13] 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.[14] 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.[15] Invertebrate activity in this formation ranged from fossilized insect burrows in petrified logs[16] to various mollusks, large crabs,[17] and a wide diversity of gastropods and ostracods.[18]