Phanerozoic

Phanerozoic
Name	Phanerozoic
Color	Phanerozoic
Top boundary def	Appearance of the Ichnofossil Trichophycus pedum
Top gssp location	Fortune Head, Newfoundland, Canada
Top gssp accept date	1992
Bottom boundary def	Defined chronometrically
Bottom gssp accept date	N/A
Timeline	Phanerozoic
Caption	The Phanerozoic Eon, showing its constituent eras (Paleozoic, Mesozoic, Cenozoic) and periods.

Phanerozoic. The Phanerozoic is the current eon in the Geologic time scale, encompassing approximately the last 541 million years of Earth's history. It is defined by the proliferation of complex, multicellular life forms with hard body parts, which left an abundant fossil record. This eon witnessed the dramatic evolution of life from early marine invertebrates to the dominance of vertebrates on land and the rise of modern ecosystems.

Etymology and Definition

The term "Phanerozoic" derives from the Ancient Greek words *phanerós* (φανερός), meaning "visible" or "evident," and *zōḗ* (ζωή), meaning "life." It was coined by the American geologist George Halcott Chadwick in 1930 to describe the eon of "visible life," contrasting it with the earlier, largely microbial life of the Precambrian supereon. The base is formally defined by the first appearance of the trace fossil Trichophycus pedum at the Global Boundary Stratotype Section and Point (GSSP) in Fortune Head, Newfoundland. This boundary marks the beginning of the Cambrian Period and the Paleozoic Era.

Geologic Time Scale

The Phanerozoic Eon is the primary division of the Geologic time scale following the Proterozoic Eon. It is subdivided into three major eras: the Paleozoic, Mesozoic, and Cenozoic, which are further divided into 12 geologic periods. The time scale is calibrated using a combination of biostratigraphy, radiometric dating, and chemostratigraphy. Major boundaries often correspond to significant extinction events or rapid evolutionary radiations, such as the Permian–Triassic extinction event and the Cretaceous–Paleogene boundary.

Subdivisions

The primary subdivisions of this eon are its three eras, each characterized by distinct fossil assemblages and tectonic regimes. The Paleozoic Era (541–252 million years ago) is subdivided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Mesozoic Era (252–66 million years ago) consists of the Triassic, Jurassic, and Cretaceous periods. The Cenozoic Era (66 million years ago to present) includes the Paleogene, Neogene, and Quaternary periods. These periods are defined by specific GSSP locations, like the Dob's Linn section for the Ordovician–Silurian boundary.

Paleozoic, Mesozoic, and Cenozoic Eras

The Paleozoic Era began with the Cambrian explosion, which saw the rapid diversification of most major animal phyla in oceans, including trilobites and brachiopods. It concluded with the formation of the supercontinent Pangaea and the devastating Permian–Triassic extinction event. The Mesozoic Era, often called the "Age of Reptiles," was dominated by dinosaurs, pterosaurs, and marine reptiles like ichthyosaurs; it ended with the Cretaceous–Paleogene extinction event triggered by the Chicxulub impactor. The Cenozoic Era is known as the "Age of Mammals," featuring the radiation of mammals, birds, and flowering plants, along with the evolution of primates and the genus Homo.

Major Events and Transitions

This eon was shaped by profound geological and biological events. Tectonically, it saw the assembly and breakup of supercontinents like Pangaea, the opening of the Atlantic Ocean, and the ongoing Himalayan orogeny from the collision of India with Asia. Major climatic shifts included the Late Ordovician glaciation, the warm Cretaceous Thermal Maximum, and the Pleistocene ice ages. Key evolutionary transitions include the colonization of land by plants during the Silurian, vertebrates during the Devonian, and the subsequent development of complex terrestrial ecosystems.

Life Evolution and Diversity

Life underwent extraordinary diversification, from the first shelled animals in the Cambrian to the emergence of intelligence. Major evolutionary milestones include the development of jaws in gnathostomes, the evolution of seeds in gymnosperms, and the origin of flight in insects, pterosaurs, and later birds. The fossil record documents major adaptive radiations following extinctions, such as the rise of mammals after the demise of the non-avian dinosaurs. The eon culminates with the appearance and global impact of Homo sapiens, alongside a modern biota profoundly shaped by events like the Great American Interchange.

Category:Geologic time scales Category:Phanerozoic