The
Paleozoic or
Palaeozoic Era (from the Greek
palaios (παλαιός), "old" and
zoe (ζωή), "life", meaning "ancient life") is the earliest of three
geologicGeology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed...
eras of the
PhanerozoicThe Phanerozoic eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared...
eon. The Paleozoic spanned from roughly (ICS, 2004), and is subdivided into six geologic periods; from oldest to youngest they are: the
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
,
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
,
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, Devonian,
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
, and
PermianThe Permian
[The term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named...]
Paleozoic life
The Paleozoic covers the time from the first appearance of abundant, soft-shelled
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
s to the time when the
continentA continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criterion, with seven regions commonly regarded as continents – they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s were beginning to be dominated by large, relatively sophisticated reptiles and modern
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s. The lower (oldest) boundary was classically set at the first appearance of creatures known as
trilobiteTrilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the...
s and archeocyathids. The upper (youngest) boundary is set at a major
extinction eventAn extinction event is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms...
300 million years later, known as the Permian extinction. Modern practice sets the older boundary at the first appearance of a distinctive
trace fossilTrace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , footprints and feeding marks, and root cavities...
called
Trichophycus pedumTrichophycus pedum is regarded as the earliest widespread complex trace fossil...
.
At the start of the era, all life was confined to bacteria,
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
,
spongeSponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells that can transform into...
s and a variety of somewhat enigmatic forms known collectively as the Ediacaran fauna. A large number of body plans appeared nearly simultaneously at the start of the era -- a phenomenon known as the
Cambrian ExplosionThe Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record. This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. There is some evidence that simple life may already have invaded the land at the start of the Paleozoic, but substantial plants and animals did not take to the land until the Silurian and did not thrive until the Devonian. Although primitive
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s are known near the start of the Paleozoic, animal forms were dominated by
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s until the mid-Paleozoic.
FishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
populations exploded in the Devonian. During the late Paleozoic, great
forestA forest is an area with a high density of trees. There are many definitions of a forest, based on the various criteria. These plant communities presently cover approximately 9.4% of the Earth's surface in many different regions and function as habitats for organisms, hydrologic flow modulators,...
s of primitive plants thrived on land forming the great
coalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
beds of
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
and eastern North America. By the end of the era, the first large, sophisticated reptiles and the first modern plants (conifers) had developed.
Tectonics
Geologically, the Paleozoic starts shortly after the breakup of a
supercontinentIn geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
called
PannotiaPannotia, first described by Ian W. D. Dalziel in 1997, is a hypothetical supercontinent that existed from the Pan-African orogeny about 600 million years ago to the end of the Precambrian about 540 million years ago...
and at the end of a global
ice ageThe general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual...
. (See
Varanger glaciationThe Cryogenian is a geologic period that lasted from . The Cryogenian forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian period and followed by the Ediacaran...
and
Snowball EarthSnowball Earth refers to hypotheses regarding paleoclimatic global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past. The occurrence of Snowball Earths remains controversial...
). Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the era, the continents gathered together into a
supercontinentIn geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
called
PangaeaPangaea, Pangæa, or Pangea was the supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration.The name was first used by the German originator of the continental...
, which included most of the Earth's land area.
Climate
The Early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustained sea level rise in the Phanerozoic got underway. However, as if to offset this trend,
GondwanaGondwana , originally Gondwanaland, is the name given to a southern precursor-supercontinent. Its final joining occurred between ca. 570 and 510 Ma ago, joining East Gondwana to West Gondwana. It later separated from Laurasia 180- during the breakup of the Pangaea supercontinent that existed about...
moved south with considerable speed, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over the
South PoleThe South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects the surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
. The Early Paleozoic climate was also strongly zonal, with the result that the "climate", in an abstract sense became warmer, but the living space of most organisms of the time -- the continental shelf marine environment -- became steadily colder. However,
BalticaBaltica is a name applied by geologists to a late-Proterozoic, early-Palaeozoic continent that now includes the East European craton of northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 billion years ago. Before this time, the three segments/continents that now comprise...
(Northern Europe and Russia) and
LaurentiaLaurentia , like all craton land, was created as continents moved about the surface of the Earth, bumping into other continents and drifting away....
(eastern North America and Greenland) remained in the tropical zone, while China and Australia lay in waters which were at least temperate. The Early Paleozoic ended, rather abruptly, with the short, but apparently severe, Late Ordovician Ice Age. This cold spell caused the second-greatest mass extinction of Phanerozoic time. Over time, the warmer weather moved into the Paleozoic era.
The Middle Paleozoic was a time of considerable stability. Sea levels had dropped coincident with the Ice Age, but slowly recovered over the course of the Silurian and Devonian. The slow merger of Baltica and Laurentia, and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow sea floor. As plants took hold on the continental margins,
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
levels increased and
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
dropped, although much less dramatically. The north-south temperature gradient also seems to have moderated, or metazoan life simply became hardier, or both. At any event, the far southern continental margins of Antarctica and West Gondwana became increasingly less barren. The Devonian ended with a series of turnover pulses which killed off much of Middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.
The Late Paleozoic was a time which has left us a good many unanswered questions. The Mississippian epoch began with a spike in atmospheric oxygen, while carbon dioxide plummeted to unheard-of lows. This destabilized the climate and led to one, and perhaps two, ice ages during the
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
. These were far more severe than the brief Late Ordovician Ice; but, this time, the effects on world biota were inconsequential. By the Cisuralian, both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangea created huge arid inland areas subject to temperature extremes. The Lopingian is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction.