The Paleozoic Era is the first of the three geologic eras that make up the Phanerozoic Eon.
The term “Paleozoic” means “ancient life, ” and it was coined in 1838 by British geologist Adam Sedgwick.
The era began about 541 million years ago and ended about 251.9 million years ago, making it the longest era of the Phanerozoic Eon.
The Paleozoic Era was characterized by significant changes in the earth’s geology, climate, and the evolution of living organisms.
It began with the breakup of a supercontinent and also saw the formation of a new one before the era ended.
The shifting continents caused major changes in the Earth’s climate throughout the era.
The era is also known for the most rapid diversification of life on Earth in geologic history.
This is known as the Cambrian explosion, an event that is characterized by the dramatic explosion in the diversity of living animals in Earth’s oceans over a few million years.
Plants became widespread during the Paleozoic, and animal species took diverse forms, especially in the oceans.
The first vertebrate species to colonize land habitats also appeared during this period, marking the transition from an exclusively aquatic fauna to a terrestrial one.
Ironically, the Paleozoic also ended with one of Earth’s largest mass extinction events, which wiped out up to 90% of the marine species, a group that was otherwise very successful during the period.
In this post, we’ll explore the Paleozoic Era in greater detail, including the changes that occurred during this period, climate, geography, and the key events of the Paleozoic Era.
Timeline of the Paleozoic Era
The Paleozoic Era is the first era of the Paleozoic Eon (the current geologic eon of the geologic time scale).
It began 538.8 million years ago, just after the Neoproterozoic Era, the last era of the Proterozoic Eon.
The Paleozoic Era is followed by the Mesozoic Era. It is the longest of the three eras that make up the Phanerozoic.
The Paleozoic Era lasted from 538.8 to 251.902 million years ago, taking up more than half (approximately 300 million years) of the Phanerozoic Eon.
It is subdivided into 6 sub-eras (periods): Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
Sub-Eras of the Paleozoic Era
The Paleozoic Era is divided into six geologic periods or sub-eras of varying lengths.
The sub-eras of the Paleozoic began with a burst of life characterized by the emergence of new animal phyla.
This is known as the Cambrian explosion. The era also ended with one of the biggest mass extinctions in the planet’s geologic history.
Each of the Paleozoic sub-eras is characterized by various geologic and evolutionary events, as described in detail below.
Cambrian Period (539–485 Million Years Ago)
The Cambrian Period was the first sub-era of the Paleozoic.
It began about 539 million years ago and spanned until the beginning of the Ordovician Period about 485 million years ago.
The start of the Cambrian Period is marked by a major evolutionary event known as the Cambrian explosion.
The Cambrian explosion is often referred to as the biological big bang because it led to a rapid diversification of life forms in the Earth’s oceans.
Before the Cambrian explosion, most organisms on Earth were relatively simple single-celled or multi-celled individuals and colonies.
Within the first 13-25 years of the Cambrian Period, the Earth’s life forms evolved rapidly, and more sophisticated multicellular organisms began to appear.
The Cambrian Period is also noted as the time when geologic records began.
This is due to the emergence of animals with protective shells or exoskeletons, such as armored arthropods, that could be preserved as fossils.
Several marine phyla evolved during this period of biological explosion.
By the end of the Cambrian, the oceans were teeming with various marine creatures, and complex ecosystems were beginning to form.
The supercontinent began to break up during the Cambrian Period.
The ancient landmass, also known as the Greater Gondwana, was formed towards the end of the Precambrian Period and began to break apart during the Cambrian.
This breakup was accompanied by changes in sea level and other dramatic changes in the Earth’s climate.
Some scientists have theorized that changes in oceanic chemistry as a result of the continent’s breakup contributed to the rapid diversification of lifeforms that occurred in the Cambrian Sea.
Ordovician Period (485–444 Million Years Ago)
The Ordovician Period follows the Cambrian Period.
It began about 485 million years ago and ended 444 million years ago.
The period was characterized by further diversification of the marine life forms that emerged during the Cambrian.
Many of the marine animal classes still living today (mainly the fishes and arthropods) emerged during this time.
The most dominant marine organisms during the Ordovician were the trilobites, brachiopods, and corals.
Primitive jawless fish also appeared during the Ordovician and became more abundant as the period progressed.
The first evidence of life colonizing land is seen during the Ordovician Period.
Primitive plants and fungi appeared on Earth’s continent, which formed a supercontinent known as Gondwana.
The empty landmass moved steadily towards the south pole, triggering a global ice age at the end of the Ordovician Period.
The global glaciations that occurred due to the plunging temperatures caused a major drop in the sea level, killing off the majority of life forms in the seas surrounding the Earth’s continents.
At least 60% of all marine organisms died during the Ordovician extinction.
This event is considered the second deadliest mass extinction of the Paleozoic Era.
Silurian Period (444–419 Million Years Ago.)
The Silurian Period lasted between 444 and 419 million years ago.
During the Silurian, the Earth started recovering from the previous extinction event that killed off the majority of primitive lifeforms in the ocean.
Earth’s temperature warmed up as Gondwana drifted away from the South Pole.
The continents closer to the South Pole still had extensive glaciations, while the ones near the equator were warmer.
The warm temperature allowed the evolution and diversification of animal species.
The jawless fishes became more abundant, and new animal families like the jawed fishes emerged.
Arthropods, like the giant sea scorpions, were still the apex predators of the Silurian seas.
Some of these aquatic lifeforms (in the form of both plants and animals) also began to venture on land.
Silurian plant life was mainly fungi, with no vascular tissue or leaves. The earliest land animals were arachnids and centipedes.
By the end of the Silurian, the supercontinent started breaking up into four subcontinents, namely Gondwana (which included present-day Africa, South America, Siberia, and Australia), Avalonia (Western Europe), Baltica (Northern Europe), and Laurentia (North America).
Devonian Period (419–359 Million Years Ago)
The Devonian period which started 419 million years ago and ended 359 million years ago, is also known as the “The Age of the Fish” because it was characterized by a huge diversification of fish species.
The jawed fishes of the Silurian evolved into armored species like the Dunkleosteus and the lobe-finned fishes.
The sea level was quite high during the Devonian Period, so most of the continents were covered by a shallow sea.
However, there were still some dry areas in the interiors of the continents.
Earth’s continents were closer to the equator, so the climate was generally warm and humid.
On land, plant life exploded during the Devonian with the emergence of ferns and other primitive vascular plants.
The first plants with seeds also emerged during this period.
The explosion of plant life on the continents allowed the diversification of terrestrial arthropods, especially insects and large centipedes, that moved in to fill the new habitats.
The first amphibians evolved from lobe-finned fish during the Devonian Period and soon became the dominant animal group of the period.
Carboniferous Period (359–299 Million Years Ago)
The Carboniferous Period started 359 million years ago and lasted till about 299 million years ago.
The period has been further divided into two sub-periods: the Mississippian Period (359 and 323.2 million years ago) and the Pennsylvanian Period (323.2 million years ago–299 million years ago).
During the Carboniferous Period, the ferns and seed-producing plants that emerged during the Devonian formed the planet’s first forests.
The plants of Carboniferous were characterized by vascular tissues made up of a structural polymer called lignin.
Most of the earth’s coal deposits were formed during the Carboniferous Period.
These deposits were formed by debris left behind by dead plants.
The absence of microorganisms to decompose these plant debris means they were left buried until they formed coal deposits.
Earth’s terrestrial landscape changed during the Carboniferous Period due to changing sea levels as the continents moved towards each other.
A glaciation event also caused global temperatures to drop during the Middle Carboniferous Period.
These changes in the terrestrial ecosystem paved the way for the evolutionary development of new species on land.
However, receding seas meant many shallow marine organisms disappeared.
As amphibians moved further inland due to receding seas, the first organisms to produce amniotic eggs emerged.
This also paved the way for the first reptiles and synapsids (ancestors of modern mammals).
The reptilian group that evolved during the Carboniferous includes the archosaurs. These were the ancestors of crocodiles, dinosaurs, and birds.
Evolution was also ongoing in the invertebrate animal world.
The first land snails emerged during the Carboniferous, and the insect group became even more diverse with the emergence of winged insects like dragonflies and cockroaches.
The flying insects of the Carboniferous Period were large, with wingspans of up to three feet in some species.
The Carboniferous Period ended with the glaciation of Gondwana as it drifted towards the South Pole. This led to the breakdown of the Carboniferous rainforest.
Permian Period (299–252 Million Years Ago)
The Permian Period lasted for 47 million years from 299 to 252 million years ago.
It was the last period of the Paleozoic Era, characterized by the formation of a single landmass known as Pangea.
Pangea was formed by the collision of the Gondwana and Euramerica supercontinents which began during the Carboniferous.
The single landmass was encircled by a worldwide ocean known as Panthalassa.
The Permian seas were warm and teeming with animal life.
The warm condition supported the formation of coral reefs that provided shelter for various marine creatures.
But life on land was not progressing great.
The interior of the continents was dry and harsh due to the absence of nearby seas to moderate their climate.
New terrestrial groups flourished despite these conditions, notably the diapsids and synapsids.
The first conifers and ginkgo trees evolved on land, and herbivorous species evolved to feed on these plants.
The Permian continent grew drier and hotter towards the end of the Permian.
This eventually led to the collapse of the planet’s terrestrial and aquatic ecosystems.
Up to 95% of plant and animal life on earth disappeared in an extinction event known as the “Great Dying.”
This was the most severe extinction event of the entire Phanerozoic Eon.
Climate and Geography of the Paleozoic Era
During the Paleozoic Era, the Earth’s climate and geography underwent significant changes, mainly influenced by the distribution of the Earth’s landmasses at the time.
At the start of the Era, all of Earth’s landmasses were organized into a single continent which was just starting to break up.
The Cambrian Period was characterized by a relatively stable climate with warm temperatures on all continents.
However, by the beginning of the Ordovician, the southern continents were beginning to rearrange to form a new supercontinent known as Gondwana.
The smaller continents of the Northern Hemisphere also started drifting towards each other.
The continents aggregated near the South Pole, leading to a global glaciation characterized by the formation of icecaps and a drop in sea level.
This cooling trend continued into the Silurian Period. As the period progressed, the Earth’s climate began to warm up again.
The extensive icecaps formed during the Ordovician Period began to melt, leading to a rise in sea level.
The landmasses continued to converge throughout the Silurian, consolidating the Gondwana supercontinent.
The planet was warm for most of the Devonian Period but later cooled down as the period progressed.
The Early Devonian Period was characterized by high atmospheric carbon dioxide.
This greenhouse effect caused a warm tropical climate to form on Gondwana (which was closer to the equator).
Towards the end of the Devonian, the climate gradually became cooler and drier again, leading to the development of arid conditions in some regions.
The planet was restored to a warm and humid climate during the Carboniferous Period.
This climate supported the growth of extensive swampy forests that covered large areas.
The continents remained warm and humid during the Early Permian but soon grew arid again as the supercontinent Pangaea began to form.
The consolidation of all of earth’s landmasses into a single continent led to the reduction of coastal areas.
As a result, the interior of the newly formed supercontinent experienced extreme continental climates, with hot and dry conditions in most places.
Key Events and Developments of the Paleozoic Era
The Paleozoic Era was a pivotal time in Earth’s history, marked by numerous key events and developments.
Most significantly, the planet underwent major biological and evolutionary changes beginning with the Cambrian explosion, a transition of life from fully aquatic to terrestrial, and the formation of new biological ecosystems over the course of several million years.
Some of the most notable events of the Paleozoic Era are highlighted below.
The Cambrian explosion, which took place at the beginning of the Paleozoic Era, is characterized by the remarkable diversification of life forms on the planet.
The Cambrian oceans saw the rapid emergence of various complex organisms within a relatively short period.
Arthropods, brachiopods, mollusks, and chordates were the main groups that emerged during this period.
The Cambrian explosion is also called the “biological big bang” because it laid the foundation for future evolutionary developments.
Evolution of Hard Shells and Exoskeletons
Among the main groups of organisms that emerged during the Early Paleozoic Era were hard-shelled marine organisms.
Before the Cambrian, living organisms were soft-bodied with no exoskeletons.
The evolution of protective structures provided advantages, such as defence against predators, support for larger body sizes, and improved mobility for the animals of the Cambrian Period.
It also made it possible to construct a fossil record since dead organisms now left hard parts that could be preserved in rocks.
Colonization of Land
The transition of life from fully aquatic to semi-aquatic and terrestrial was one of the most significant developments that took place during the Paleozoic Era.
Primitive plants such as mosses and liverworts emerged on land for the first time during the Ordovician Period.
By the Silurian, vascular plants, such as ferns and early seed plants, had spread across the land.
This led to the formation of extensive terrestrial habitats that could support the incursion of animal species beyond the oceans.
Evolution of Fish and Tetrapods
The Devonian Period, often called the “Age of Fishes,” was characterized by the widespread diversification of fish.
Jawed fish, including armored and lobe-finned fishes, became prominent during this period.
The evolution of the jawed fish paved the way for the emergence of the tetrapods, the first four-limbed vertebrates that would eventually transition from aquatic to semi-terrestrial life towards the end of the Devonian Period.
Formation of Carboniferous Forests
The Carboniferous Period is renowned for the development of extensive swampy forests that eventually formed vast coal deposits that are still on Earth today.
These forests were dominated by primitive plants like ferns and early seed plants.
They played a crucial role in shaping the Earth’s climate, geology, and evolution of terrestrial animals.
Rise of Reptiles
Until the Carboniferous Period, life was locked to the sea because the only terrestrial animals of the time still had to return to lay eggs.
The emergence of animals that could lay amniotic eggs with shells changed that.
The Permian Period witnessed the rise and diversification of reptiles that could not reproduce on land.
This laid the foundation for the eventual dominance of reptiles in the subsequent Mesozoic Era.
Major Groups of Organisms in the Paleozoic Era
The Paleozoic Era saw the emergence and diversification of various groups of organisms.
Although many of them went extinct due to the two major extinction events of the Paleozoic, most of the Earth’s major phyla that are still living today evolved during the Paleozoic.
Here are some of the major groups that flourished during the Paleozoic Era.
Arthropods were among the most dominant groups during the Paleozoic Era.
Marine arthropods with hard exoskeletons and segmented bodies were among the first groups of multicellular organisms to dominate Earth’s ocean during the Cambrian.
Some of them, like the trilobites, occupied various ecological niches and are considered important index fossils for dating rock layers.
These were marine invertebrates with hinged shells composed of two valves and a fleshy stalk for attachment.
Brachiopods were filter feeders and played a crucial role in Paleozoic marine ecosystems.
Mollusks, such as snails, clams, and cephalopods, experienced significant diversification during the Paleozoic Era.
They had a variety of shell types and evolved different feeding strategies, which allowed them to inhabit diverse marine environments.
Mollusks species were also able to move terrestrial habitats later in the Paleozoic.
This was among the most successful animal groups that underwent significant diversification during the Paleozoic Era.
Some of the most prominent fish groups of the Paleozoic include jawless fish (agnathans), armored fish (placoderms), and lobe-finned fish (sarcopterygians).
During the Paleozoic Era, living organisms transitioned from an aquatic lifestyle to colonizing the land.
This began with the early amphibians, such as labyrinthodonts, that evolved from lobe-finned fish.
The amphibians diversified further throughout the Paleozoic, beginning from the Devonian Period.
Insects first appeared during the Paleozoic Era. Early insects included beetles, dragonflies, and cockroaches that occupied various ecological niches in Earth’s newly formed terrestrial habitats.
The Middle to Late Paleozoic Era was characterized by the widespread colonization of land habitats by plants.
This is a major milestone in Earth’s history. Early terrestrial plants included mosses, liverworts, and ferns.
The seed plants and conifers followed later during the era.
By the Carboniferous Period, forests dominated by these primitive plants covered vast areas, contributing to the formation of coal deposits.
The rise of reptiles was among the last significant developments that occurred during the Paleozoic Era.
Reptiles, with their amniotic eggs, became adapted to terrestrial environments during the Permian and played a crucial role in shaping future ecosystems.
Extinction Events of the Paleozoic Era
Throughout Earth’s geologic history, extinction events that wiped out numerous plant and animal groups helped shape the diversity and composition of the planet.
During the Paleozoic Era, at least three extinction events have been noted that had a profound impact on the progression of life over the course of the era. They include:
Late Ordovician Extinction (445 Million Years Ago)
This was one of the first major mass extinction events recorded in Earth’s history.
It primarily affected marine life since they were the major life form present during this period.
A combination of factors, including climate change and fluctuating sea levels, may have triggered this event.
Approximately 85% of marine species, including many trilobites and brachiopods, became extinct at the end of the Ordovician Period due to this event.
Late Devonian Extinction (375–359 Million Years Ago)
This extinction event occurred in two pulses between 375 and 359 million years ago.
The first of these two was more severe.
The Late Devonian extinction affected marine life, particularly shallow-water species such as coral reefs.
Although scientists are not certain of the causes of this event, factors such as climate change, anoxic events, and volcanic activity may have contributed to the disappearance of roughly 75% of marine species during the Devonian.
Permian-Triassic Extinction (252 Million Years Ago)
Also known as the “Great Dying,” the Permian-Triassic extinction event was the most severe extinction event of the Paleozoic Era and all of Earth’s history.
Approximately 96% of marine species and 70% of terrestrial vertebrate species died off at the end of the Permian, bringing the period to a dramatic end.
Likely causes for this extinct event include volcanic eruptions, global warming, ocean acidification, and oxygen depletion.
The Permian-Triassic extinction event marked the end of the Paleozoic Era and the beginning of the Mesozoic Era.