What the dinosaur record says about extinction scenarios

The record of dinosaurs over the last 10 m.y. of the Cretaceous, as well as surrounding the Cretaceous-Paleogene boundary, helps to defi ne extinction scenarios. Although Late Cretaceous dinosaur fossils occur on all present-day continents, only in North America do we fi nd a terrestrial vertebrate fossil record spanning the Cretaceous-Paleogene boundary, although promising work may yield comparable records in South America, India, China, and Europe. For the present then, the North American record represents the proxy for our knowledge of dinosaur extinction. Over the last 10 m.y. of the Cretaceous (late Campanian to late Maastrichtian) in the northern part of the western interior of North America, the number of nonavian dinosaur species dropped from 49 to 25, almost a 50% reduction, even though a 16% greater extent of fossil-bearing exposures record the last dinosaurs in the latest Cretaceous in the western interior. Important, but less-well-exposed, nonavian-dinosaur–bearing units suggest this drop occurred around, or at least commenced by, the CampanianMaastrichtian boundary. These losses began during climatic fl uctuations, occurring during and possibly in part caused by the last major regressive cycle of the Cretaceous, which also reduced the expanse of the low coastal plains inhabited by nonavian dinosaurs. The pulse of Deccan Trap emplacement that began some time later in the latest Cretaceous was also likely a major driver of climatic change. As for the dinosaur record near the Cretaceous-Paleogene boundary, even the best-known records from North America remain enigmatic and open to interpretation. Newer studies suggest some decline in at least relative abundance approaching the CretaceousPaleogene boundary, but the cause (or causes) for the fi nal extinction (if it was the case) of nonavian dinosaurs remains unresolved, although the Chicxulub impact undoubtedly played a major role. *[email protected] Archibald, J.D., 2014, What the dinosaur record says about extinction scenarios, in Keller, G., and Kerr, A.C., eds., Volcanism, Impacts, and Mass Extinctions: Causes and Effects: Geological Society of America Special Paper 505, p. 213–224, doi:10.1130/2014.2505(10). For permission to copy, contact editing@geosociety. org. © 2014 The Geological Society of America. All rights reserved. INTRODUCTION The best-preserved and most-studied sequences of Late Cretaceous and Paleocene fossil vertebrates occur in western North America. From the latest Cretaceous of western North America alone, over 100 vertebrate species are known, representing all major vertebrate clades. Although nonavian dinosaurs represent at most only 20% of this taxonomic diversity, interest in the possible causes and patterns of their extinction far outweighs the interest for the extinction of other species, vertebrate or not (Archibald, 1996). The study of dinosaur extinction is certainly not the most conducive for general understanding of the processes on August 25, 2014 specialpapers.gsapubs.org Downloaded from