Paleozoic Era

The history of the Earth can be divided into three great eras: Paleozoic, Mesozoic and Cenozoic. The Paleozoic Era is the first of them all, and it begins when life originated on Earth…


The Paleozoic Era is composed of several periods:

Cambrian Period

This Period that began approximately 542 Million years ago. It begins with the “Cambrian Explosion”, an event that would change the history of life. Before the Cambrian, in the Ediacaric, life was reduced to simple and uncomplicated organisms, but in this “explosion” the metazoans, that is to say, the animals, appeared. Trilobites, echinoderms, and some predators of the great seas such as the Anomalocaris appeared. The Anomalocaris can be considered the “first large predator”, being an aquatic invertebrate that could reach 2 meters in length and ate everything that was put in front of it.

Cambrian is the first Period of the Paleozoic Era, where animal life begins, but not plant life. It took a few million years before the first terrestrial plants appeared.

From Cambrian Period is also a small animal, the Haikouichthys. This small animal, which barely reached 2.5 cm in length, is the common ancestor of all current vertebrates.

Ordovician Period

A period that began approximately 488 Million years ago. The climate at first was still the same as in the Cambrian Period, it was still warm, but soon it would change as temperatures would drop drastically, giving the second glaciation occurred on the planet. The Ordovician period came to an end about 440 million years ago.

This glaciation caused the Second Great Extinction, greater than the one that occurred in the Cretaceous (which caused the extinction of non-avian dinosaurs), and only surpassed by the Great Extinction of the Permian.

There are two main types of animals in the Ordovician Period: scorpions and fish. The current scorpions have their origin in the Ordovician Period, but they reached a size of 50cm like the Megarachne up to 2.5m like the Megalograptus. In the case of fish, the well-known placoderms appear, fish that as their name indicates were composed of bony plaques (mainly in the cephalic zone).

Siluric Period

This Period began approximately 440 Million years ago, with the Second Glaciation and with a great extinction due to it.

The Silurian highlights the large number of fossil shells that have been found, mainly of the genera of Ammonites, but also bivalves and brachiopods. Although many of the trilobites became extinct at the end of the Ordovician, in the Silurian they expanded again in much more striking ways.

In the Silurian Sea the first terrestrial plants begin to form. These plants are nothing like today’s plants. They were small, globular-shaped plants. It has been hypothesized that these plants were made up of a symbiosis similar to that between fungi and current algae giving lichens. This type of symbiosis can be the origin and in turn explain why some fungi and bacteria in the roots of plants, allowing them to absorb nutrients more easily, or make them more resistant to stressful environments.

Devonian Period

This Period began approximately 415 Million years ago. It is the “Fish Period“, where the gnatostomed fish originated. These fish are the first animals to have jaws, which represented an evolutionary advance, although the first teeth have not yet been formed.

The gnatostomed fishes like the Dunkleosteus did not have teeth, but their jaws were serrated and molded, giving the appearance of teeth, but they were solid bones.

At this time the animals also began to go out to the mainland, giving rise to the first amphibians. Because they had no competitors on land (except for invertebrates, which had already been modified to live on land, reaching huge sizes), they evolved and radiated rapidly, gradually displacing the invertebrates.

Carboniferous Period

This period began approximately 360 Million years ago. It is a very humid and warm period, in which the first large prehistoric coniferous forests were originated at the end of the period, although they would soon occupy the entire Earth. A lot of the fossil coal is from this period, that’s why it has that name.

In the Carboniferous also originated the reptiles, and introduced two great advances to evolution: the amniotic egg and the scales. The scales were a breakthrough so that they became independent from the water, their life no longer needed a constantly moist skin or the need for a wet environment such as a pond or a river.

The amniotic egg was the same step, the independence of the water, but this time for the young. This allowed the offspring to be born very similar to adults, they did not have to go through a metamorphosis, only grow.

Permian Period

The Permian is the last period of the Paleozoic Era, starting almost 300 million years ago and ending 250 million years ago. It is a period with almost extreme temperature changes (very hot summers and very cold winters), but as it comes to an end it passes to a drier and warmer climate, with an elevated average temperature.

At this period a series of special reptilian animals, the reptiles-mamipheroids: the synapses (from which the mammals would radiate) originated. The first reptiles-mamipheroids that appeared and dominated the terrestrial environment were the pelicosaurus, some well known as the Dimetrodon and the Edaphosaurus, with an aspect that resembles more a dinosaur, but dinosaurs are later.

At the end of the Permian period, the reptiles-mamipheroids that dominated the terrestrial environment were the terapids. These animals have an appearance more reminiscent of today’s mammals, although still very distant from them, as can be seen in the Gorgonops.

At the end of the Permian period, 250Ma ago, a great cataclysm occurred, decimating almost all of the earthly life that was there. This is the Great Extinction of the Permian, the greatest extinction that has ever occurred on Earth, and which almost killed all life on the planet. With it, the Paleozoic Era is finished and the Mesozoic, the Age of the Dinosaurs, begins.