Paleozoic
Adam Sedgwick first used the name Paleozoic in 1835 to describe rock layers containing Cambrian and Ordovician fossils. He defined the base of this era by the appearance of complex life, specifically pointing to trilobite-dominated fauna found in the rock record. John Phillips redefined the term two years later in 1840 to include all periods from the Cambrian through the Permian. The word itself comes from Greek roots meaning ancient life. Geologists now date the start of this era at 538.8 million years ago. This specific number marks the boundary between the Neoproterozoic Eon and the Phanerozoic Eon. International Commission on Stratigraphy members spent decades debating where exactly to draw this line. They eventually chose a site called Fortune Head on the Burin Peninsula in Newfoundland as the Global Stratotype Section and Point. This location sits below the first appearance of both trilobites and small shelly fauna. Instead, they selected trace fossils known as Treptichnus pedum as the true marker for the beginning of time.
The breakup of the supercontinent Pannotia began with the opening of the Iapetus Ocean during the early Cambrian period. Sea levels rose dramatically as these new oceans formed. Central Africa sat directly over the South Pole during the Ordovician period, creating massive glaciers. Gondwana assembled after Pannotia broke apart. By the mid-Paleozoic, North America collided with Europe to create the Acadian-Caledonian uplifts. Eastern Australia experienced uplift from subducting plates. Continental collisions later formed the supercontinent Pangaea by the late Paleozoic. These collisions created great mountain chains including the Appalachians, Caledonides, Ural Mountains, and mountains of Tasmania. The land mass became very dry during the Permian period when all continents joined together. Harsh seasons developed because the interior of Pangaea was not regulated by large bodies of water. Panthalassa circled this single massive continent.
Most modern animal phyla first appeared during the Cambrian period between 539 and 485 million years ago. Armored arthropods like trilobites were the most ubiquitous creatures of that time. Algae evolved alongside these animals but did not dominate the seas. Almost all marine phyla emerged within this short geological window. The event is known as the Cambrian explosion due to its rapid pace. Predators made up only about four percent of fauna in Palaeozoic assemblages compared to seventeen percent in temperate Cenozoic times. Non-motile animals untethered to the substrate were abundant then but extremely rare today. Infaunal animals composed just four percent of soft substrate communities back then versus forty-seven percent now. Microbiota phytoplankton overall were nutrient-poor themselves and adapted to nutrient-poor environmental conditions. This scarcity has been cited as an explanation for the Paleozoic's relatively low biodiversity.
Plants mostly remained aquatic until the Silurian Period around 420 million years ago when they began transitioning onto dry land. Early vascular plants called Cooksonia allowed vegetation to gain a foothold on continents. Fully terrestrial life included early arachnids, fungi, and centipedes appearing during the Silurian. Arthropods became the first animals to venture onto dry land. Some fish developed lungs and powerful bony fins by 367.5 million years ago that allowed them to crawl ashore. These bones eventually evolved into legs creating the first tetrapods. Amphibians dominated until the mid-Carboniferous when climate change reduced their diversity. Amniotes took over after splitting into two clades: synapsids and sauropsids. Synapsids continued to prosper and increase in number until the end of the Permian period. The evolution of amniotic eggs allowed amphibians to move farther inland and remain dominant vertebrates throughout much of this era.
The Ordovician, Silurian extinction event killed off sixty percent of marine invertebrates and twenty-five percent of families. Glaciation of Africa caused a major drop in sea level killing all life along coastal Gondwana. This cold spell marked the second-greatest mass extinction of the Phanerozoic Eon. The Late Devonian extinction ended seventy percent of existing species through several smaller turnover pulses. The final catastrophe occurred at the start of the Mesozoic Era 251.9 million years ago. Ninety-five percent of all life on Earth disappeared during the Permian-Triassic extinction event known as The Great Dying. Recovery of life on land took thirty million years into the next era. Sea recovery may have been much faster than terrestrial recovery. Radiometric dating of volcanic clay layers near Meishan, Zhejiang Province in southern China confines the boundary age to a narrow range of 251.902 plus or minus 0.024 million years.
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Common questions
Who named the Paleozoic era and when was it first used?
Adam Sedgwick first used the name Paleozoic in 1835 to describe rock layers containing Cambrian and Ordovician fossils. John Phillips redefined the term two years later in 1840 to include all periods from the Cambrian through the Permian.
When did the Paleozoic era begin according to current geological dating?
Geologists now date the start of this era at 538.8 million years ago. This specific number marks the boundary between the Neoproterozoic Eon and the Phanerozoic Eon.
Where is the Global Stratotype Section and Point for the Paleozoic era located?
International Commission on Stratigraphy members eventually chose a site called Fortune Head on the Burin Peninsula in Newfoundland as the Global Stratotype Section and Point. This location sits below the first appearance of both trilobites and small shelly fauna.
What major geological events occurred during the Paleozoic era regarding continents?
The breakup of the supercontinent Pannotia began with the opening of the Iapetus Ocean during the early Cambrian period. Continental collisions later formed the supercontinent Pangaea by the late Paleozoic.
Which extinction event ended the Paleozoic era and when did it occur?
The final catastrophe occurred at the start of the Mesozoic Era 251.9 million years ago. Ninety-five percent of all life on Earth disappeared during the Permian-Triassic extinction event known as The Great Dying.