THE KIRTLANDIAN LAND-VERTEBRATE “AGE”—LATE CRETACEOUS OF WESTERN NORTH AMERICA

For more than 30 years, only five land-vertebrate “ages” (LVAs) have been recognized, based mostly on unique assemblages of fossil mammal taxa, in the Late Cretaceous of the Western Interior (ascending): Aquilan, Paluxian, Judithian, Edmontonian and Lancian. However, significant temporal gaps in this succession were recognized at their inception. We recently “filled-in” the youngest of these gaps (between the Judithian and Edmontonian) by naming the Kirtlandian LVA based on the vertebrate faunas (Hunter Wash and Willow Wash local faunas) from the Upper Cretaceous Fruitland and Kirtland formations, San Juan Basin, New Mexico. Radioisotopic dates confirm that the Kirtlandian spans ~ 2.9 my, and is equivalent to “Bear Paw time.” The Kirtlandian LVA is defined as the time between the first appearance of Pentaceratops sternbergii, (which is also designated as the index taxon for the Kirtlandian) and the first appearance of Pachyrhinosaurus canadensis.

The Kirtlandian vertebrate faunas (assemblages) are relatively unique in their taxonomic composition. Although characterized by all vertebrate groups, the dinosaur taxa are especially useful for correlation. Among the unique dinosaur taxa are: Pentaceratops sternbergii, Parasaurolophus tubicen and P. cyrtocristatus, Kritosaurus navajovius, Nodocephalosaurus kirtlandensis and Prenocephale goodwini. Because of these unique taxa, previous attempts to correlate the Kirtlandian fauna to other faunas have not been entirely satisfactory. However, a few dinosaur taxa, notably Pentaceratops sternbergii, Parasaurolophus cyrtocristatus and Kritosaurus navajovius, are now known to occur in other North American vertebrate faunas, thus allowing for biostratigraphic correlation of the Kirtlandian outside of the San Juan Basin, NM. This correlation identifies Kirtlandian-age vertebrate fossil assemblages in northern Mexico, SE Arizona-SW New Mexico, northwestern Colorado and south-central Utah. We also conclude that the Fruitland/Kirtland vertebrate faunas that define the Kirtlandian LVA do not represent an endemic paleobiogeographic assemblage, as advocated by some; instead, these taxa represent a single, temporally distinct vertebrate fossil assemblage.