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ABSTRACT: Stratigraphic Succession of Dinosaur Faunas in the North American Western Interior

Spencer G. Lucas

Dinosaurs can be used to recognize a minimum of eight, distinct dinosaur-bearing vertebrate faunas in Late Triassic-Late Cretaceous strata of the North American Western Interior: (l) Late Carnian-early Norian fauna of the Dockum Group and Chinle Formation older than the Rock Point Member and its correlatives. Dinosaurs include Coelophysis s.s., Revueltosaurus, Technosaurus, and fragmentary material of ceratosaurs, staurikosaurids, and anchisaurids. (2) Middle Norian fauna of the Chinle Rock Point Member and the Redonda Formation. Dinosaurs include the mass-death assemblage of Coelophysis s.1. at Ghost Ranch, NM, and abundant Grallator footprints. (3) Sinemurian-Pliensbachian fauna of the Kayenta Formation including the dinosaurs cf. Scelidosaurus, Dilophosaurus, Syntarsus Scutellosaurus, and Massospondylus. (4) Tithonian-Neocomian Morrison Formation with its classic dinosaur fauna dominated by Camarasaurus, diplodocids, Stegosaurus, Allosaurus, and Camptosaurus. (5) Aptian-Albian fauna of the Cloverly Formation. Dinosaurs include Tenontosaurus, Deinonychus, Sauropelta, and fragmentary sauropods. Extensive sauropod-dominated ichnofaunas of the Trinity Group in the Gulf Coast region are correlative. (6) Late Campanian (Judithian) faunas from Alberta to New Mexico that include Albertosaurus, ornithomimids, and abundant hadrosaurids and ceratopsians. (7) The broadly similar early Maastrichtian Albertosaurus co-occurs with Edmontosaurus. (8) Late Maastrichtian (Lancian) faunas from Montana to Texas with dinosaurs similar to those of the Judithian but that als contain distinctive taxa including Tyrannosaurus, Torosaurus, Triceratops, and Alamosaurus.

Some significant observations are: (l) The oldest North American dinosaurs are late Carnian, and there is no reason to believe they are younger than South American Triassic dinosaurs long claimed to be older. (2) No succession of dinosaur faunas can yet be recognized in the Morrison Formation, and thus no significant dinosaur-faunal change corresponds to the Jurassic-Cretaceous boundary. (3) Poorly known dinosaur body-fossils and some extensive dinosaur ichnofaunas from late Albian-Santonian strata suggest the sudden appearance at the beginning of the Late Cretaceous of the hadrosaur-dominated fauna typical of the Campanian-Maastrichtian.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990