Biology Index
By Richard Bruce BA, MA, and PhC in Economics

A summary of on the Geologic Periods of the Paleozoic

The Paleozoic is the era before the Mesozoic. The Mesozoic is also called the age of dinosaurs. The Mesozoic was before the Cenezoic, also called the age of mammals.

At the beginning of the Paleozoic multicellular animals which clearly moved, had senses like sight, and preyed on one another appeared. Unlike mammals and birds they were cold blooded. Warm blooded animals first evolved in the last geologic period of the Paleozoic. So the Paleozoic might loosely be termed the age of cold bloods.

The Paleozoic was about 300 million years long. It started with the Cambrian explosion and ends with greatest mass extinction, the Great Dying, at the end of the Permian.

I have had some difficulty remembering the geologic periods of Paleozoic, which makes it difficult to read the literature. So I have tried to straighten the whole thing out in my mind, and now I am writing my thinking down on this page. That way both the general public and I will be able to refer to it. This page contains relatively little in the way of new insight, I am simply organizing information.

The Paleozoic has been split into six geologic periods.

Those six geologic periods are as follows:

  1. Cambrian
  2. Ordovician
  3. Silurian
  4. Devonian
  5. Carboniferous
  6. Permian

Before the Paleozoic and the Cambrian

Before the Paleozoic and the Cambrian multicellular life was soft bodied. There are only a few fossils that suggest the animals moved or had senses, for example sight. For the animals of this era there was probably little predation.

Cambrian 541 ~ 485 Million Years Ago

The Paleozoic begins with the Cambrian, and the Cambrian begins with the Cambrian explosion. The Cambrian explosion refers to the rapid appearance of many types of life at the beginning of the Cambrian.

Almost all phyla of multicellular animals appear in the geologic record at this time, including Mollusks: snails, Arthropods: trilobites, and Chordates: vertebrates.

For the first time organisms with hard body parts evolved, so for a long time scientists thought this was the beginning of the fossil record. Scientists gradually found fossils of the soft bodied animals of the earlier period. Animals in the Cambrian moved and preyed on one another.

Ordovician 485 ~ 445 Million Years Ago

The Ordovician period saw a second explosion of diversity, the Ordovician radiation, or the great Ordovician biodiversification event (GOBE). The modern phyla first appeared in the earlier Cambrian, in the Ordovician many of the classes and lower taxonomic categories evolved. The diversity of animals achieved the level that it would maintain through the rest of the Paleozoic.

Some particularly interesting marine groups evolved in the Ordovician: primitive fish, cephalopods, and coral evolved. Squid and octopuses are cephalopods.

Furthermore the Arthropods invaded the land. Insects and spiders are examples of arthropods that live on land.

The Ordovician ended with a massive ice age, which has been described as a snowball earth. It is considered the first mass extinction and the second most serious, so it was worse than the one that wiped out the dinosaurs.

Silurian 443 ~ 419 Million Years Ago

In the Silurian, many fish evolved, including the first jawed fish, and the first freshwater fish. Arachnids, centipedes, fungi, and vascular plants evolved on land. Vascular plants have vascular tissues allowing them to move resources around the plant and grow much larger. This was a crucial adaptation that made it possible for trees to evolve.

Devonian 419 ~ 359 Million Years Ago

The Devonian is known as the age of fish, for the first time fish, not invertebrates were the top predators in the sea. Fish moved onto land and amphibians evolved. The first trees evolved. The Devonian ended with the second mass extinction.

Carboniferous 359 ~ 299 Million Years Ago

The Carboniferous was a warm period with lots of swamps and trees. Much of our coal was formed in this period. The organisms that normally break down tree trunks had not evolved yet, so the tree trunks lasted long enough to be formed into coal.

Reptiles evolved from amphibians. The reptiles unlike the amphibians did not have to return to the water to lay eggs. They could spend their entire life on land.

The Carboniferous period ended with a giant glaciation, ice age, centered on the south pole. With all that coal being formed carbon was removed from the atmosphere, which reduced the green house effect, and brought on glaciations, or ice ages.

Permian 290 ~ 252 Million Years Ago

The Paleozoic ends with the Permian. The most famous animal of the Permian is undoubtedly Dimetrodon the large predatory reptile with a sail fin on its back. Dimetrodon was not a dinosaur or a lizard, but was part of a group of animals from which mammals ultimately evolved. Nevertheless, its life style was basically that of a large lizard, like the Komodo dragons.

In the second half of the Permian the Therapsids evolved. Therapsids were thought to be warm blooded and mammals evolved from them. So from this point on the large land animals were warm blooded animals like mammals including humans and dinosaurs including birds.

The Permian ended with the third and most severe mass extinction, the Great Dying. The Permian is followed by the Mesozoic, a summary of which is here.

More summaries of natural history, and other biology web pages can be found in the biology index.

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Biology Index