Geological Time Scale

The 4.5-billion-year history of the Earth is organised into a geological time scale made of three broad eras — Paleozoic, Mesozoic and Cenozoic — themselves divided into periods, and the most recent periods further split into epochs (see Table 6.1). Reading from oldest to youngest, the Paleozoic era is dominated by marine invertebrate fossils in its early periods and, later, by early vertebrates other than birds and mammals; its seven periods run Cambrian (age of invertebrates) → Ordovician (freshwater fishes, molluscs) → Silurian (origin of fishes) → Devonian (age of fishes) → Mississippian (earliest amphibians) → Pennsylvanian (earliest reptiles) → Permian (mammal-like reptiles). The Mesozoic era, the 'Golden Age of Reptiles', runs Triassic (origin of egg-laying mammals) → Jurassic (dominance of dinosaurs, the fossil bird Archaeopteryx) → Cretaceous (extinction of toothed birds and dinosaurs, rise of modern birds). The Cenozoic era, the age of mammals, splits into the Tertiary (Paleocene → Eocene → Oligocene → Miocene → Pliocene, tracing placental mammals through to the first man-like apes and eventually man from ape-like ancestors) and the Quaternary, which saw the decline of many mammal lines and the beginnin …