|
Paleozoic
("Ancient Life") Era
This is the first of three geologic eras squeezed into the last
10% of Earth's whole geologic history. But things are really starting
to happen! This last 10% of Earth's history is called the "Phanerozoic
Eon" or eon of "evident life." This means that rocks
from the Phanerozoic contain fossils, lots of fossils!
The Paleozoic period
lasted about 325 million years, from about 570 million years ago
to about 245 million years ago. So much happened during the Paleozoic
that it is divided into seven geologic time periods, shown on the
red steps of the "Staircase of Time." Many different things
happened during each period, but we can only give a summary of them
here. You can learn more by going to your library or searching the
Internet for words like "Paleozoic" or the names of each
of the periods.
Here in the Paleozoic,
Earth's interior has cooled down to something like modern levels,
so that volcanic activity is usually about as humanity experiences
it: a few minor eruptions like Mount St. Helens each year, and major
ones like Krakatoa every century or so. However, gigantic "hot-spot"
type eruptions still occur every hundred million years or so. Plate
tectonics continues to push land masses across Earth's surface.
At this particular time--the middle of the Silurian Period--most
of the land is still locked in two supercontinents called Gondwanaland,
which happens to be wandering over the South Pole in our view, and
Laurasia, which is on the other side of the globe. Huge glaciers
cover the interior of Gondwanaland, and Earth is experiencing one
of its ice ages.
Over the next hundred million years, Gondwanaland will move north
over the equator and begin to break up, and the climate will warm
up substantially (See the Mesozoic.).
The composition of the
atmosphere has continued to slowly change, mostly due to the increase
of oxygen produced by photosynthetic algae floating on the ocean.
By the Paleozoic, the composition of the air has reached something
like what we breathe now: about 4/5 nitrogen, 1/5 oxygen, and small
amounts of carbon dioxide, water vapor, and other gases. At long
last the air is capable of supporting large animals, and almost
in response, life explodes into the Paleozoic!
At
the beginning of the Paleozoic, life existed
only in or near the ocean. Trilobites,
shellfish, corals, and sponges appeared,
followed by the first fish. Land plants
appeared near the end of the Ordovician
and for the first time we see the green
of land plants in our global view. Huge
forests and swamplands formed during the
warm climate of the Mississippian and
Pennsylvanian periods that later fossilized
into the giant coal beds of the eastern
United States. Animal life also moved
onto the land, first the arthropods (spiders
and insects to you), then the amphibians,
and later the reptiles. The most abundant
animals on land and sea during the Paleozoic
were those like shellfish and insects
that lacked backbones, so the Paleozoic
is often called "The Age of Invertebrates."
The
Paleozoic was also marked by several mass
extinctions: geologically short periods
of time during which large numbers of
life forms died out. Mass extinctions
occurred at the end of the Ordovician,
the Devonian, and the worst one of all
at the end of the Permian, when about
95% of all life on Earth died! These mass
dyings were probably caused by climate
changes and periods of giant volcanic
eruptions.
We
can follow the development of life in
detail during the Paleozoic, because at
the beginning of that Era, life forms
developed hard parts like shells, teeth,
bones, and woody parts that were easily
preserved as fossils. Earlier life forms
were single-celled and soft-bodied, so
older rocks contain few fossils.
Back
| Next
|
