The Eocene epoch began 10 million years after the extinction of the dinosaurs, 65 million years ago, and continued for another 22 million years. As with the preceding Paleocene epoch, the Eocene was characterized by the continuing adaptation and spread of prehistoric mammals, which filled the ecological niches left open by the dinosaurs' demise. The Eocene constitutes the middle part of the Paleogene period (65-23 million years ago), preceded by the Paleocene and succeeded by the Oligocene epoch (34-23 million years ago); all of these periods and epochs were part of the Cenozoic Era (65 million years ago to the present).
Climate and geography. The Eocene epoch picked up where the Paleocene left off, with a continuing rise in global temperatures to near-Mesozoic levels. The later part of the Eocene saw a pronounced cooling trend, probably related to decreasing levels of carbon dioxide in the atmosphere, which culminated in the re-formation of ice caps at both the north and south poles. The earth's continents continued to drift toward their present positions, having broken apart from the northern supercontinent Laurasia and the southern supercontinent Gondwana, though Australia and Antarctica were still connected. The Eocene epoch also witnessed the rise of North America's western mountain ranges.
Terrestrial Life During the Eocene Epoch
Mammals. Perissodactyls (odd-toed ungulates, such as horses and tapirs) and artiodactyls (even-toed ungulates, such as deer and pigs) can all trace their ancestry back to the primitive genera of the Eocene epoch. Phenacodus, a small, generic-looking ancestor of hoofed mammals, lived during the early Eocene, while the late Eocene witnessed much bigger "thunder beasts" like Brontotherium and Embolotherium. Carnivorous predators evolved in synch with these plant-munching mammals: the early Eocene Mesonyx only weighed as much as a large dog, while the late Eocene Andrewsarchus was the largest terrestrial meat-eating mammal that ever lived. The first recognizable bats (such as Palaeochiropteryx), elephants (such as Phiomia), and primates (such as Eosimias) also evolved during the course of the Eocene epoch.
Birds. As is the case with mammals, many modern orders of birds can trace their roots to the Eocene epoch. The most notable birds of the Eocene were giant penguins, as typified by the 100-pound Inkayacu of South America and the 200-pound Anthropornis of Australia. Another important Eocene bird was Presbyornis, a toddler-sized prehistoric duck.
Reptiles. Crocodiles (such as the weirdly hooved Pristichampsus), turtles (such as the big-eyed Puppigerus) and snakes (such as the 33-foot long Gigantophis) all continued to flourish during the Eocene epoch. Much tinier lizards, like the three-inch-long Cryptolacerta, were also a common sight (and food source).
Marine Life During the Eocene Epoch
The Eocene epoch was when the first prehistoric whales left dry land and opted for a life in the sea, a trend that culminated in the middle Eocene Basilosaurus, which attained lengths of up to 60 feet and weighed in the neighborhood of 50 to 75 tons. Sharks continued to evolve as well, but few fossils are known from this epoch. In fact, the most common marine fossils of the Eocene epoch are of tiny fish, like Knightia and Enchodus, that plied lakes and rivers in vast schools.
Plant Life During the Eocene Epoch
The heat and humidity of the early Eocene epoch made it a heavenly time for dense jungles and rainforests, which stretched almost to the North and South Poles (The coast of Antarctica was lined with tropical rainforests about 50 million years ago!) Later in the Eocene, global cooling produced a dramatic change: the jungles of the northern hemisphere gradually disappeared, to be replaced by deciduous forests that could better cope with seasonal temperature swings. One important development had only just begun: the earliest grasses evolved during the late Eocene epoch, but didn't spread worldwide (providing sustenance for horses and ruminants) until millions of years later.
Next: the Oligocene Epoch