468 TERTIARY TO PRESENT/Eocene Marine Realm Figure Coastal section in Alum Bay, Isle of Wight, UK, show ing strata spanning almost the entire Eocene Oldest is to the right, youngest to the left The red and brown are Ypresian, the yellow with dark intercalations is Lutetian, the grey below the house is Bartonian (all vertical), and the white (mainly horizontal) is Priabonian anticlockwise rotation of Iberia resulted in initial uplift of the Pyrenees Further south, India completed its northward drift and docked with Asia in the Early Eocene, laying the foundations for the Himalayan uplift Rifting between Australasia and Antarctica widened and developed into an (at first) narrow Tasman Sea Australasia began its 50-million-year-long trek to lower latitudes at this time Subduction beneath the west side of the Antarctic Peninsula led to its uplift and shallowing of the back-arc basin to the east South America remained connected to the Antarctic Peninsula for most of the epoch However, development of the Scotia Arc and formation of the Scotia Sea (Drake Passage) meant that, by the end of the Eocene, all major elements of Gondwana had separated and the modern arrangement of the continents was essentially in place Two important impact-ejecta strewn fields have also been recognized within Late Eocene sediments It has been suggested that two large impact craters, Popigai in Siberia and Chesapeake Bay in the USA, were the sources for these (Figure 1) Biota The Eocene biota is characteristically diverse and abundant Most surviving groups had recovered from the Early Paleocene diversity low by this time and had undergone or were undergoing radiation Marine groups, such as molluscs, crustaceans, and echinoids, had a familiar modern appearance On land, the same was true of many reptile and amphibian groups, but Eocene mammals differed radically from their living relatives Key biotic events are discussed below Calcareous nannoplankton An important turnover in this group of microscopic algae occurred during the first million years of the Eocene with the origination of many new taxa The genera that had radiated and dominated in the Paleocene became extinct Survivors were the previously low-diversity genera of modern aspect This turnover is attributed to the PETM (see below) Later in the Eocene, diversity reduced regionally and deeper water habitats were vacated because of progressive cooling New taxa evolved in response to the temperature changes and increased eutrophication The rapid evolution and widespread occurrence of nannoplankton at this time have enabled the Eocene to be divided into 12 global biozones (Figure 1) Dinoflagellates The PETM resulted in a near-global spread of dinoflagellate species belonging to the genus Apectodinium through middle and high latitudes This event, known as the Apectodinium acme, is an important biostratigraphical marker for the Paleocene–Eocene boundary Continued radiation produced increasingly diverse cyst assemblages during the Early and Middle Eocene, especially of the genus Wetzeliella and its relatives Diversity remained high for most of the Eocene, despite the later cooling of sea surface temperatures The very end of the epoch, though, bears witness to a reduction in abundance of low-latitude taxa, as well as the invasion of these areas by formerly higher latitude groups Foraminifera The initial Eocene turnover in planktonic foraminifera at the PETM was minor Radiation continued from the Paleocene to reach a diversity maximum in the Middle Eocene Then, deteriorating temperatures caused long-term stepwise extinctions, of which the most intense was at the end of the Middle Eocene (Bartonian) Surface-dwelling species were gradually replaced by cold-tolerant, subsurface species Benthic foraminifera fared much worse at the PETM, suffering 30–50% extinctions for middle bathyal through abyssal forms This is known as the Benthic Foraminiferal Extinction (BFE) Shallower water taxa fared better The latter include the best-known Eocene calcareous foraminifera, some of which (e.g., the nummulites) became important rock formers and reached maximum diameters of 10 cm Benthic foraminifera, like the planktonics, underwent stepwise extinctions throughout the Middle and Late Eocene The evolutionary and cosmopolitan attributes of planktonic foraminifera, like those of the calcareous nannoplankton, have resulted in their use for dividing the Eocene into 13 global biozones (Figure 1)