Section

The Greening of the Paleozoic World

Paleozoic Terrestrial Environments

The 291 million-year span of the Paleozoic Era began with the radiation of life in the oceans during the Cambrian and Ordovician Periods. After about 99 million years, just before the beginning of the Silurian Period, conditions began to take shape that could support plant and animal life on land. Until then, the terrestrial environments of the Earth had been barren wastes that consisted primarily of soil-less rock incapable of absorbing water. The only signs of life on land were found in near-shore regions where single-celled marine algae and the most rugged lichenlike fungi began to cling to dry land. This was the beginning of a revolution in the transformation of world habitats that led to colonization of the continents.

The rise of land organisms was triggered by dramatic changes to the geology and climate of the Earth. The struggle for survival and the ability to adapt were played out against a backdrop of radical flux in Earth's habitats marked by several devastating mass extinctions. This chapter explores the changing geology and climate of the Paleozoic Earth that created environments suitable for life on land.

the changing face of the earth

The evolving Paleozoic Earth experienced widespread tectonic disruptions, abrupt swings in global temperature, fluctuating sea levels, and fundamental changes to the makeup of the planet's atmosphere. The era was also marked by long spans of temperate warmth that contrasted with massive glaciations and their chilling effects.

EVOLUTIONARY MILESTONES OF THE PALEOZOIC ERA

Period

Span (Millions of Years Ago)

Duration (Millions of Years)

Organismal Milestones

Cambrian

542 to 488

54

Shelly marine invertebrates; trilobites; nautiloids; archaic mollusks, brachiopods, echinoderms; marine plants; ancestral chordates and vertebrates

Mass extinction

Casualties: brachiopods, conodonts, and trilobites

Ordovician

488 to 443

45

Radiation of marine invertebrates, corals;

jawless fishes; first land plants

Mass extinction

Casualties: trilobites, echinoderms, and nautiloids

Silurian

443 to 416

27

Marine invertebrates renew radiation, including reef colonies; diversification of land plants; diversification of arthropods (land and sea)

Devonian

416 to 359

57

Jawed fishes and ammonoids radiate; first insects and land vertebrates; first seeded plants, first large trees, and forests

Mass extinction

Casualties: ammonoids, trilobites, gastropods, reefs, armored jawless fish, placoderms

Carboniferous

359 to 299

60

Radiation of sharks, bony fishes, and lobe--finned fishes; crinoids, blastoids, and bryozoans; amphibians and insects flourish; coal swamps, early reptiles; ferns, seed ferns, and lycopod trees

Permian

299 to 251

48

Bony fishes and sharks diversify; finbacked synapsids and advanced synapsid reptiles;

early gymnosperms (conifers)

Mass extinction

Casualties: trilobites, crinoids, bryozoans, brachiopods, ammonoids, and other marine invertebrates; many early reptiles and mammal--like reptiles

The Earth's oceans were its first livable habitats. During the Cambrian and Ordovician Periods, shallow waters, warm seas, and increasingly oxygenated water led to the rapid and spectacular evolution of marine organisms. The Middle Paleozoic is known as the age of fishes, and not surprisingly—it was during that span of rapid diversification of marine vertebrates that the roots of today's cartilaginous and bony fishes were established.

Some of the key components that make up a successful terrestrial ecosystem are a stable ground cover of soil and plants, an abundance of water, breathable air, and shelter from ultraviolet solar radiation. During the Cambrian and Ordovician Periods, the world beyond the oceans was not inviting to life. This was because there was no land ecosystem to speak of, only barren rocks on the interior portions of the exposed continents. Complicating and governing all of the factors that combine to make up a terrestrial ecosystem were radical changes taking place in the configuration of the Earth's crust. During the Paleozoic Era, tectonic-plate movements dramatically changed the face of Earth's crust.

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