Phanerozoic

The Phanerozoic is the most recent and current eon in Earth’s geological timescale, extending from approximately 538.8 million years ago to the present. It encompasses the entire span of geological time during which abundant, diverse, and complex animal and plant life has flourished on Earth. This eon opened with the Cambrian Explosion, a period of rapid diversification in multicellular life forms, and includes the subsequent colonisation of land, the evolution of major animal and plant groups, and the development of modern ecosystems. The preceding interval, the Precambrian, is subdivided into the Hadean, Archaean, and Proterozoic eons.

Etymology

The term Phanerozoic was introduced in 1930 by the American geologist George Halcott Chadwick. It is derived from the Ancient Greek words for visible (phaneros) and life (zoe). Early geological thinking held that fossil-bearing, complex life began in the Cambrian because the Precambrian fossil record was poorly understood. The later discovery of Ediacaran organisms, however, demonstrated that large and complex life existed before the Cambrian, and further research has confirmed that primitive multicellular life dates back well into the Proterozoic, with simple prokaryotic communities present from the early Archaean.

The Proterozoic–Phanerozoic Boundary

The formal boundary between the Proterozoic and Phanerozoic eons is placed at 538.8 million years ago. Nineteenth-century geologists initially defined it by the appearance of abundant animal fossils, but systematic work in the twentieth century revealed diverse soft-bodied organisms in the Ediacaran Period. This understanding shifted interpretations of early life and placed the Cambrian Explosion as a major evolutionary transition rather than the absolute beginning of complex organisms.

Major Subdivisions of the Phanerozoic

The Phanerozoic is subdivided into three eras: the Palaeozoic, Mesozoic, and Cenozoic, which together contain twelve geological periods. These eras chart the progression of life from exclusively marine ecosystems to the complex terrestrial biomes familiar today.

Palaeozoic Era

The Palaeozoic was a time of sweeping biological innovation and ecological expansion. It witnessed the evolution of all major animal phyla, the emergence of vertebrates, the greening of the continents, and the establishment of early forests and terrestrial food webs. The era contains six geological periods.

Cambrian Period

Spanning 539 to 485 million years ago, the Cambrian is characterised by the Cambrian Explosion, a rapid diversification of animal body plans. Trilobites, radiodonts, and early molluscs were abundant, and nearly all major marine animal phyla appeared. The supercontinent Pannotia broke apart at the start of the period, with most of its fragments later converging into Gondwana.

Ordovician Period

From 485 to 444 million years ago, the Ordovician saw the Great Ordovician Biodiversification Event, which greatly expanded marine life. Nautiloids, jawless vertebrates, and reef-building organisms diversified, while primitive arthropods moved onto land. Early green algae also colonised terrestrial environments, giving rise to the first land plants. By the period’s end, Gondwana had shifted to polar latitudes, triggering glaciation and the Ordovician–Silurian extinction event that eliminated a large portion of marine life.

Silurian Period

The Silurian, covering 444 to 419 million years ago, marked a global warming trend and continued recovery from earlier extinctions. Jawless fish diversified further, and early jawed fish appeared. Land ecosystems expanded as vascular plants such as Cooksonia grew inland. Arthropods, including arachnids and myriapods, established significant terrestrial niches. Continental arrangements included Gondwana, Laurentia, Baltica, and Siberia.

Devonian Period

Extending from 419 to 359 million years ago, the Devonian is often called the Age of Fish due to the proliferation of jawless and jawed fish, including placoderms, acanthodians, and early sharks. Bony fish diversified into ray-finned and lobe-finned lineages, the latter giving rise to tetrapod ancestors. On land, extensive forests developed and plant groups such as ferns, lycophytes, and early seed plants flourished. Arthropods expanded into new ecological roles. The period ended with a series of extinctions collectively known as the Late Devonian extinction.

Carboniferous Period

The Carboniferous (359–299 million years ago) was dominated by dense tropical rainforests and extensive swamps that generated vast coal deposits. Elevated atmospheric oxygen enabled giant arthropods to evolve. Amphibians diversified, and amniotes—possessing eggs adapted to drier environments—emerged, giving rise to early sauropsids and synapsids. Cooling trends culminated in the Permo-Carboniferous glaciation, which triggered the collapse of widespread rainforests.

Permian Period

The Permian (299–251 million years ago) was the final period of the Palaeozoic. The formation of Pangaea created a vast supercontinent with extreme climatic contrasts and widespread aridity. Early synapsids, including pelycosaurs and later therapsids, diversified, as did various reptilian lineages. Several extinction events punctuated the period, including the Capitanian crisis and the Permian–Triassic extinction, the most severe biotic crisis recorded. Triggered by phenomena such as the Siberian Traps eruptions, it resulted in the loss of most marine species and large numbers of terrestrial taxa. Recovery extended well into the succeeding Triassic.

Mesozoic Era

The Mesozoic, known as the Age of Reptiles, saw the dominance of dinosaurs, the evolution of birds from theropod ancestors, the early diversification of mammals, and the rise of major modern reptile groups such as crocodilians, turtles, and lizards. Marine ecosystems included ichthyosaurs and plesiosaurs, while flowering plants emerged during the Cretaceous. The era ended with the Cretaceous–Palaeogene extinction event.

Cenozoic Era

Beginning 66 million years ago, the Cenozoic features the evolutionary radiation of mammals and birds following the extinction of non-avian dinosaurs. Modern plant communities expanded, climates fluctuated considerably through ice ages, and the genus Homo evolved during the most recent part of this era. The Cenozoic continues to the present day and defines current terrestrial biodiversity.