Paleozoic Era: From the Cambrian Explosion to the Great Dying

The Paleozoic Era is the span of Earth’s history from about 538.8 to 251.9 million years ago, the first era in which animals became abundant in the fossil record. It began with the Cambrian Explosion of marine life and ended with the Permian–Triassic extinction, the worst mass extinction known. In between, fish, land plants, insects, amphibians and reptiles all appeared, and life moved from the sea onto the continents.

Naming and divisions

Geologists divide Earth’s history into eons, eras and periods. The Paleozoic is the first of three eras in the Phanerozoic Eon, followed by the Mesozoic and Cenozoic. The name, from the Greek for “ancient life”, was introduced as Palaeozoic by the English geologist Adam Sedgwick in 1838, and John Phillips gave the three-era scheme its modern form around 1840.

The six periods of the Paleozoic were defined mainly in the 19th century:

The dates are those of the International Commission on Stratigraphy and are refined as new measurements appear.

A quarrel that created a period

The Paleozoic timescale grew out of fieldwork in Wales and the Welsh borders. In the 1830s Sedgwick worked on the older rocks of north Wales, which he called Cambrian, while his friend Roderick Murchison studied younger rocks to the south-east, which he called Silurian. As both men extended their systems, they claimed the same layers, and the dispute turned into a long and bitter feud.

The problem was solved only after both had died. In 1879 Charles Lapworth proposed that the disputed rocks form a separate period, the Ordovician, named after another ancient Welsh people. It was not formally adopted internationally until 1960.

A changing planet

The geography of the Paleozoic was very different from today’s. At the start of the era, the large southern continent of Gondwana lay mainly in the Southern Hemisphere, while smaller continents, including Laurentia (the core of North America), Baltica and Siberia, were scattered nearer the equator. Oceans such as the Iapetus separated them.

Over hundreds of millions of years these landmasses converged. The closing of the Iapetus Ocean in the Silurian and Devonian joined Laurentia and Baltica and raised the Caledonian mountains, whose worn-down remains still form parts of Scotland, Scandinavia and the northern Appalachians. Later collisions built Pangaea by the end of the era.

Climate swung between warm “greenhouse” times with high sea levels, when shallow seas flooded the continents, and ice ages. Major glaciations struck at the end of the Ordovician and again from the late Devonian to the Permian, when ice sheets spread over southern Gondwana.

Life in the early Paleozoic seas

The era opened with the Cambrian Explosion, when most major animal groups appear in the fossil record. Trilobites, brachiopods, sponges and early molluscs spread across shallow seas, and the first chordates, the group that includes vertebrates, appeared.

In the Ordovician, marine life diversified further in what is called the Great Ordovician Biodiversification Event. Reefs, straight-shelled nautiloids, sea lilies (crinoids), corals and bryozoans flourished. The period ended with the Late Ordovician mass extinction about 445 to 443 million years ago, linked to a short ice age centred on Gondwana, which removed an estimated 85 per cent of marine species.

The Silurian saw reefs recover and jawed fishes become common. Sea scorpions, or eurypterids, some reaching around two metres in length, hunted in coastal waters.

The rise of fishes is one of the era’s great stories. The earliest vertebrates, small jawless fishes, appear in the Cambrian, and armoured jawless forms were common in the Ordovician and Silurian. Jaws evolved from the supporting bars of the gills, an innovation that let vertebrates take larger prey and eventually dominate the seas.

The conquest of land

Plants were the first complex organisms to colonise the land. Spores of simple land plants appear in Ordovician rocks around 470 million years ago, and small upright plants such as Cooksonia are known from the Silurian, about 430 million years ago. Arthropods such as millipedes followed onto land at around the same time.

The Devonian, from about 419 to 359 million years ago, is often called the Age of Fishes. Armoured placoderms, early sharks and lobe-finned fishes thrived. Some lobe-finned fishes developed sturdy fins and lungs; Tiktaalik, from about 375 million years ago, shows a mix of fish and limbed-animal features. The first four-limbed vertebrates, or tetrapods, appeared in the Late Devonian. The first forests also grew in this period, with tree-like plants such as Archaeopteris.

The Late Devonian was marked by a series of extinction pulses, especially around 372 and 359 million years ago, which badly affected reef ecosystems and armoured fishes.

Coal forests and the first reptiles

In the Carboniferous, vast swamp forests of giant clubmosses, horsetails and tree ferns covered low-lying land near the equator. Their buried remains became much of the world’s coal, as described in the article on Carboniferous coal forests. Oxygen levels were high, and insects such as the dragonfly-like Meganeura reached wingspans of about 70 centimetres.

The amniotes, animals whose eggs can develop on land, appeared around 320 to 310 million years ago. They split early into two lines: the synapsids, which would eventually lead to mammals, and the sauropsids, which include reptiles and later dinosaurs and birds.

By the late Carboniferous and Permian, the continents had collided to form the supercontinent Pangaea. Its huge interior became drier, and the coal swamps declined. In the Permian, synapsids such as the sail-backed Dimetrodon and later the therapsids became the dominant land animals.

The end of the era

The Paleozoic ended about 251.9 million years ago with the Permian–Triassic extinction, often called the Great Dying. An estimated 80 to 90 per cent or more of marine species and around 70 per cent of land vertebrate species disappeared, although exact figures vary between studies.

The leading explanation is the eruption of the Siberian Traps, enormous volcanic outpourings in what is now Russia. They released vast amounts of carbon dioxide, causing rapid global warming, ocean acidification and widespread loss of oxygen in the seas. Trilobites, which had survived for nearly 270 million years, vanished for good.

Timeline

• c. 538.8 million years ago: The Cambrian Period and Paleozoic Era begin.
• c. 485.4 million years ago: The Ordovician begins; marine diversity rises sharply.
• c. 445–443 million years ago: The Late Ordovician mass extinction strikes.
• c. 430 million years ago: Small vascular plants such as Cooksonia grow on land.
• c. 375 million years ago: Tiktaalik lives in Devonian streams in what is now Arctic Canada.
• c. 372–359 million years ago: Late Devonian extinction pulses affect reefs and fishes.
• c. 320–310 million years ago: The first amniotes appear amid Carboniferous coal forests.
• c. 251.9 million years ago: The Permian–Triassic extinction ends the Paleozoic.

Why it matters

The Paleozoic is when the living world took on its familiar structure: the main animal groups, forests, insects and backboned animals on land all date from this era. Its mass extinctions also show how changes in climate and ocean chemistry can reshape life on a planetary scale.