THE LATE EOCENE-OLIGOCENE EXTINCTIONS

The transition from the Eocene to the Oligocene Epochs, from about 40 to 30 Ma (million years ago), was the most significant interval in Earth history since the dinosaurs died out 65 Ma. From the warm, equable greenhouse climate of the early Eocene (a relict of the age of dinosaurs), the Earth experienced major climatic changes. Global temperature plum­ meted, and the first Antarctic sheets appeared. These climatic stresses triggered extinctions in plants and animals, both on land and in the oceans. By the early Oligocene (33 Ma), the Earth had a much cooler, more temperate climate, with a much lower diversity of organisms. Indeed, the Eocene-Oligocene transition marked the change from the global green­ house world of the Cretaceous and early Cenozoic to the glaciated ice house world of today. Despite the intense research interest in mass extinctions over the past two decades, the Eocene-Oligocene extinctions have been relatively under­ studied and misunderstood. While hundreds of papers have been published on the terminal Cretaceous extinction of dinosaurs and ammonites since the discovery of the iridium anomaly in 1980, only a few dozen articles have been published on the Eocene-Oligocene extinctions. Much of this work has now been invalidated by new data. In the enthusiasm to force the Eocene-Oligocene extinctions into the mold of the Cretaceous-Tertiary impact hypothesis and the periodic extinc­ tion hypothesis (Raup & Sepko ski 1984), a lot of misinformation has appeared. Typically, impact proponents treat the Eocene-Oligocene tran-