From Simple English Wikipedia, the free encyclopedia
The
Devonian
is the fourth period of the
Paleozoic
era
and the
Phanerozoic
eon
. It lasted from about 419 million years ago (mya) to about 359 mya.
[1]
It is named after
Devonshire
,
England
, where rocks from this period were first studied.
Sea levels were high, and there was a great variety of fish and other marine organisms.
[2]
The Devonian fish included the
Agnatha
(jawless fish), the
Acanthodii
(spiny fish), the
Placoderms
(armoured fish), the
Chondrichthyes
(cartilaginous fish), and the early
Osteichthyes
(bony fish). The Devonian
strata
are divided into lower, middle and upper subdivisions.
During the Devonian the
evolution
of
fish
into
tetrapods
occurred.
[3]
Various terrestrial
arthropods
also became established. The first
seed-bearing plants
spread across dry land, forming huge forests.
The supercontinent of
Gondwana
was to the south, and the continent of
Siberia
to the north. The early form of the small continent of
Euramerica
was in between. The supercontinent
Pangaea
was just starting to form.
By the late Devonian, the land had been colonized by plants and
insects
. In the oceans, massive reefs were built. Euramerica and Gondwana were beginning to converge into what would become
Pangaea
.
Tectonic
and
volcanic
activity was high. A series of
extinction events
occurred towards the end of the Devonian, including a major event at the Frasnian-Famennian boundary in the Late Devonian, about 364 mya.
A major extinction occurred at the start of the last phase of the Devonian period, the Frasnian-Famennian boundary, about 372.2 mya. Almost all the fossil
agnathan
fishes suddenly disappeared. A second strong extinction closed the Devonian period. The late Devonian extinction was one of five major extinction events in the history of the Earth. The cause or causes are not known.
[4]
- ↑
Gradstein, Felix M; Ogg J.G. & Smith A.G. 2004. A geologic time scale. Cambridge: Cambridge University Press.
ISBN
0-521-78673-8
.
- ↑
The Devonian is sometimes called the Age of Fish.
- ↑
see
Tetrapod#fishapods
for discussion and references.
- ↑
McGhee, George R. Jr, 1996.
The late Devonian mass extinction: the Frasnian/Famennian crisis
. Columbia University Press.
ISBN 0-231-07504-9