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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 197 impoverished record. If one examines the distribution of first- and second-order episodes throughout the Tertiary, one notes a tendency for clustering of several episodes, notably in the Early Miocene, when the Arikareean associates with the two Hemingfordian episodes, and in the Late Miocene where a string of three second-order episodes is associated with the final pair of first-order episodes. Mammal Age Order Genera Hemingfordian 2 (ca. 18 Ma) Rodentia Blackia Copemys Eomys Petauristodon Carnivora Miomustela Mionictis Plithocyon Pseudaelurus Sthenictis Perissodactyla Aphelops Hemphillian 3 (ca. 5 Ma) Rodentia Mimomys Nebraskomys Promimomys Lagomorpha Ochotona Carnivora Agriotherium Chasmaporthetes Lynx Megantereon Parailurus Ursus Artiodactyla Bretzia Odocoileini Blancan 2 (ca. 2.5 Ma) Edentata Dasypus Holmesina Glyptotherium Glossotherium Eremotherium Rodentia Erethizon Neochoerus Mictomys Pliopotomys Synaptomys Carnivora Canis Tremarctos Artiodactyla Bovinae DISCUSSION During the past decade, mammalian paleontologists have discovered, somewhat to their collective surprise, that the pulse of the Cenozoic succession in North America is strongly syncopated. In particular, publication of a detailed mammalian biochronology for North America (Woodburne, 1987) underlined the unevenness of faunal turnover. The rhythm of long- stable chronofaunal intervals punctuated by rapid turnover episodes has emerged as a clear pattern (Vrba, 1985a; Webb, 1989). At the level of a continental ecosystem this pattern may be referred to as "syncopated equilibrium." The need now is for closer analysis of rapid turnover episodes to gather new insights into the mechanisms and modalities that translate environmental change into radical reorganization of terrestrial ecosystems.
