DEFINITION OF CARBONIFEROUS

A late Paleozoic geologic period in which

the Carboniferous System of rocks was deposited, between

355 million and 285 million years (Ma) ago. The system was

named after coal-bearing strata in Wales and has the distinc-

tion of being the first formally established stratigraphic system.

In the United States, it is customary to use the divisions

Mississippian Period (355–320 Ma) and Pennsylvania Period

(320–285 Ma), whereas Europeans and the rest of the world

refer to the entire interval of time as the Carboniferous Period

and divide the rocks deposited in the period into two subsystems,

the Upper and Lower, and five series.

The Carboniferous is known as the age of amphibians,

or the age of coal. The supercontinent Pangea straddled the

equator in the early Carboniferous, with warm climates dominating

the southern (Gondwana) and northern (Laurasia)

landmasses. In the Lower Carboniferous giant seed ferns and

great coal forests spread across much of Gondwana and

Laurasia, and most marine fauna that developed in the Lower

Paleozoic flourished. Brachiopods, however, declined in number

and species. Fusulinid foraminifera appeared for the first

time. Primitive amphibians roamed the Lower Carboniferous

swamps, along with swarms of insects including giant dragonflies

and cockroaches.

In the Early Carboniferous (Mississippian), Gondwana

was rotating northward toward the northern Laurentian continent,

closing the Rheic Ocean. Continental fragments that

now make up much of Asia were rifting from Gondwana,

and the west coasts of North and South America were subduction-

type convergent margins open to the Panthallassic

Ocean. Several arc and other collisions with North America

were under way, including the Antler orogeny in the western

United States. The Hercynian orogeny in Europe marked the

collision between Baltica, southern Europe, and Africa. In the

Late Carboniferous (Pennsylvanian), Laurentia and Gondwana

had finally collided, forming the single large landmass

of Pangea. This collision resulted in the Alleghenian orogeny

in the Appalachians of the eastern United States, the Ouachita

orogeny in southern United States and South America, and

formed the ancestral Rocky Mountains. In Asia, Kazakhstan

collided with Siberia, forming the Altai Mountains. Several

microcontinents were rifted off the Gondwana continents to

be accreted to form much of present-day Asia.

Global climates in the Carboniferous ranged from tropical

around much of Laurentia and northern Gondwana, to

polar on southern Gondwana, which experienced glaciation

in the Pennsylvanian. This widespread glaciation formed in

response to Gondwana migrating across the South Pole and

is characterized by several advances and retreats and glacial

deposits on Africa, Australia, South America, and India. Coal

formed at both high and low latitudes in the Pennsylvanian,

reflecting the warm climates from easterly trade winds

around the closing Rheic Ocean and future opening of the

Tethys Ocean. Most of the coal deposits formed in foreland

basins associated with continental collisions.

Many sedimentary deposits of Carboniferous age worldwide

show development in a repetitive cycle, including accumulation

of organic material (vegetation), deposition of

carbonates, deposition of clastic sands, and erosion to sea

level and soil development. These types of sedimentary

deposits have become known as cyclothems and reflect a uniform

fluctuation of sea level by 500–650 feet (150–200 m).

Analysis of the ages of each cyclothem have led to the recognition

that each cycle represents 300,000 years, but the cause of

the repetitive cycles remains a mystery. They may be related to

cyclical variations in orbital parameters (Milankovitch cycles),

or to variations in the intensity of the southern glaciation.

Extinctions in the Late Devonian paved the way for rapid

expansion of new marine invertebrate forms in many ecological

niches. Radiations in the brachiopods, ammonoids, bryozoans,

crinoids, foraminifera, gastropods, pelcypods, and

calcareous algae became widespread. Crinoids were particularly

abundant in the Mississippian forming dense submarine

gardens, along with reefs made of bryozoans and calcareous

algae. Fusulinid foraminifera with distinctive coiled forms

evolved in the beginning of the Pennsylvanian and serve as a

useful index fossil since they evolved so quickly and are abundant

in many environments.

Land plants originated in the Devonian and saw additional

diversification in the Carboniferous. Chordates, a prominent

gymnosperm with long thin leaves, flourished in the Mississippian,

whereas conifers appeared in the Late Pennsylvanian.

The tropical coal forests of the Pennsylvanian had trees that

were more than 98 feet (30 m) tall, including the prominent

Lepidodendron and Calamites trees and the seed-bearing Glossopteris

shrub that covered much of the cooler parts of Gondwana.

Warm climates in the low-latitude coal swamps led to a

flourishing fungi flora. The dense vegetation of the Carboniferous

led to high levels of atmospheric oxygen, estimated to have

comprised about 35 percent of the gases in the atmosphere,

compared to present-day levels of 21 percent.

The insects radiated in the Early Pennsylvanian and

included the wingless hexapods and the primitive Paleoptera,

ancestors of the modern dragonfly and mayfly. A giant Pennsylvanian

dragonfly had a wingspan of 24 inches (60 cm) and

preyed largely on other insects. Exopterygota, primitive crickets

and cockroaches, appeared in the Pennsylvanian.

Endopterygota, the folding wing insects including flies and

beetles, did not appear until the Permian.

The Carboniferous is famous for the radiation of

amphibians. Ten different amphibian families appeared by

the end of the Mississippian, living mostly in water and feeding

on fish. Eryops and other amphibians of this time resembled

crocodiles and include relatives of modern frogs and

salamanders. Embolomeres evolved into large (up to 13 feet,

or 4 m) eel-like forms with small legs, some living on land

and eating insects. Leopospondyls remained in the water, eating

mollusks and insects. The earliest known reptile, Westlothiana,

evolved from the amphibians in the Late

Mississippian by 338 million years ago. The transition from

amphibians to reptiles occurred quickly, within a few tens of

millions of years after the origin of amphibians. Amniotes are

four-legged animals (tetrapods) that produced eggs similar to

the modern bird egg and include reptiles with scales. The rise

of amniotes represents a major evolutionary step, since the

older amphibians went through an early tadpole stage in

which the young are vulnerable to prey. In contrast, the eggs

of the amniotes and later reptiles had enough food to provide

for the growth of the embryo in a safer environment. Descendants

of the amniotes include mammals and birds.

The evolutionary transition between reptiles and mammals

is gradual, with more intermediate evolutionary steps

known than for any other high-order taxa. Like many other

major evolutionary periods in Earth history, this evolutionary

step occurred during a supercontinental amalgamation,

enabling many species to compete. Many species were intermediate

between reptiles and mammals (the so-called mammal-

like reptiles), and these dominated the land fauna for

about 100 million years until the period of the dinosaurs

began in the Permian. Mammal-like reptiles include two

orders, the Pelycpsaurs and the Therasids. The mammal-like

reptiles had evolved into true mammals by this time, but they

did not become dominant until the dinosaurs were killed off

at the end of the Cretaceous.

See also MILANKOVITCH CYCLES; PALEOCLIMATOLOGY;

PALEOZOIC.

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