Dinosaurs, the age of the Fruitland and Kirtland Formations, and the Cretaceous-Tertiary boundary in the San Juan Basin, New Mexico

Dinosaurs from the Upper Cretaceous Fruitland Formation and Kirtland Shale in the San Juan Basin, New Mexico, pertain to the following taxa: Ornithomimidae, cf. Ornithomimus edmonticus, cf. Struthiomimus altus, Dromaeosauridae, Albertosaurus sp., cf. Tyrannosaurus rex, Alamosaurus sanjuanensis, ?Pachycephalosauridae, Ankylosauria, Ankylosauridae, Nodosauridae, ?Euoplocephalus sp., ?Panoplosaurus sp., Hadrosauridae, Kritosaurus navajovius, Parasaurolophus tubicen, P. cyrtocristatus, Ceratopsidae, cf. Chasmosaurus sp., Pentaceratops sternbergii, P. fenestratus, and Torosaurus cf. T. utahensis. The dinosaur fauna of the Fruitland Formation is temporally equivalent to the dinosaur faunas of the Judith River (Montana) and Oldman (Alberta) Formations and is of late Campanian (Judithian) age. This correlation is based primarily on the absence in the Fruitland Formation of dinosaurs typical of post-Judithian dinosaur faunas elsewhere in western North America. The dinosaur fauna of the Kirtland Shale below the Naashoibito Member is virtually identical to that of the Fruitland Formation. Based on stratigraphic relationships, the Kirtland Shale must be younger than the Fruitland Formation and may be as young as Edmontonian (latest Campanian-early Maastrichtian) below the Naashoibito Member. The Naashoibito Member of the Kirtland Shale contains cf. Tyrannosaurus rex, Alamosaurus sanjuanensis, and Torosaurus cf. T. utahensis, taxa indicative of a Lancian (middle-late Maastrichtian) age. Therefore, Kritosaurus from the Naashoibito Member represents the youngest known occurrence of this genus. The Lancian age of the Naashoibito Member indicates that the unconformity at the base of the Ojo Alamo Sandstone is not of as great a temporal magnitude as most previous workers had believed. Thus, there is a nearly complete record of the Cretaceous-Tertiary transition in the west-central San Juan Basin, indicated by dinosaur-based correlation of the Fruitland and Kirtland Formations. This correlation is consistent with most other evidence, except magnetostratigraphy, that has been used to determine the age of the Fruitland and Kirtland Formations.