Phanerozoic Eon

Phanerozoic Eon, most recent of the four eons of the geological timescale, the others being, in order of descending age, the Priscoan Eon, the Archaean Eon, and the Proterozoic Eon. An eon is the longest formal unit of geological time, incorporating a number of smaller units known as eras that, in turn, are subdivided into periods. The Phanerozoic runs from the end of the Proterozoic, 545 million years ago, to the present. It comprises three eras: the Palaeozoic (545 million to 248 million years ago), the Mesozoic (248 million to 65 million years ago), and the Cenozoic (65 million years ago to the present).

The term “Phanerozoic” derives from the Greek, meaning “clear evidence of life”, a reference to the fact that organisms with hard parts (such as skeletons and shells) appeared on Earth only at the start of the eon. Organisms with hard parts are more likely to be preserved in fossil form than are soft-bodied ones, and hence are preserved in far greater abundance. Although the Phanerozoic is much the shortest of the three eons in which life is seen—the others being the Proterozoic and Archaean—it nevertheless encompasses the most spectacular evolution of life forms, both plant (flora) and animal (fauna). The early Palaeozoic saw the appearance of animals such as trilobites, brachiopods (see Lampshell), corals, graptolites, fishes, bivalves, and gastropods. The basis of marine biodiversity had been established. By the middle of the era animals and plants had colonized the land. By the Carboniferous Period, amphibians and reptiles were firmly established, as were insects and arachnids. By this time trees (lycopods and ferns among them), horsetails, and a variety of seed-bearing plants had produced dense forests. Following a major extinction at the end of the Palaeozoic Era, life continued to diversify throughout the subsequent Mesozoic and Cenozoic Eras. The Mesozoic is often called the “Age of Reptiles”, largely because the dinosaurs appeared, lived, and, as the era ended, became extinct. In the seas, ammonites achieved extraordinary diversity, but they, too, did not survive the Mesozoic. The Cenozoic has been called “the age of mammals”, reflecting their rapid rise to dominance among terrestrial vertebrates after the demise of the dinosaurs.

Throughout the Phanerozoic there have been phases of mountain building (orogeny)—for example, the Caledonian and Variscan during the Palaeozoic Era, and the Alpine and Himalaya during the Cenozoic. Erosion of such mountain ranges resulted in vast regions of sedimentation in submontaine basins and on the ocean floor. Particularly at times of “greenhouse” climate, great spreads of limestones were derived from chemical precipitation and accumulations of dead shells—for example, during the Carboniferous Period of the middle Palaeozoic. It was during the Phanerozoic, too, that the supercontinent Pangaea formed from the continental masses of Gondwana and Laurasia, and then subsequently disintegrated, the fragments drifting apart to form the present continents.