The first word of this story is Paleozoic, a name that translates from Greek as ancient life, yet it describes a time when the planet was barely recognizable to modern eyes. This era began 538.8 million years ago, marking the start of the Phanerozoic Eon and the first appearance of complex life in the rock record. Before this moment, the Earth was dominated by microscopic organisms and soft-bodied creatures that left few traces, but the Paleozoic Era witnessed a dramatic transformation. The boundary between the Proterozoic and Phanerozoic Eons is defined by the first appearance of trace fossils, specifically the Treptichnus pedum assemblage, found at Fortune Head in Newfoundland. This location serves as the Global Stratotype Section and Point, the official reference for the start of the Paleozoic. The era ended 251.9 million years ago with the Permian-Triassic extinction event, the largest mass extinction in Earth's history, which wiped out 95% of all life on Earth. The recovery of life took millions of years, with the Mesozoic Era only beginning to see a return to diversity after 30 million years of struggle. The name Paleozoic was first used by Adam Sedgwick in 1838 to describe the Cambrian and Ordovician periods, and later redefined by John Phillips in 1840 to include all periods from Cambrian to Permian. The era is subdivided into six geologic periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian, each representing a unique chapter in the history of life on Earth.
The Cambrian Explosion And Early Seas
The Cambrian Period, spanning from 539 to 485 million years ago, marked the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion. During this time, most modern animal phyla first appeared, including arthropods, molluscs, and early fish. The most ubiquitous creatures of the period were armored arthropods like trilobites, which dominated the marine environment. The supercontinent Pannotia began to break up during the Cambrian, with the opening of the Iapetus Ocean and other Cambrian seas, leading to a dramatic rise in sea level. The early Cambrian climate was probably moderate at first, becoming warmer over the course of the period, as the second-greatest sustained sea level rise in the Phanerozoic got underway. However, as if to offset this trend, Gondwana moved south, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over the South Pole. The early Paleozoic climate was strongly zonal, with the result that the living space of most organisms of the time , the continental shelf marine environment , became steadily colder. Despite these challenges, the Cambrian explosion set the stage for the evolution of complex life, with the first vertebrates appearing in the form of primitive fish. The Cambrian Period is the first period of the Paleozoic Era of the Phanerozoic, and it marked a boom in evolution in an event known as the Cambrian explosion in which the largest number of creatures evolved in any single period of the history of the Earth.