Earth and Moon

The Paleozoic: Cambrian, Ordovician, Siluric

The Paleozoic: Cambrian, Ordovician, Siluric

This ancient era, the Paleozoic, lasted about 290 million years. The planet was very different from today. The Paleozoic era or Primary era belongs to the Phanerozoic eon.

With the emergence of multicellular organisms, the Precambrian ended and began, about 541 million years ago, the Fanerozoic eon, which ended about 251 million years ago.

The emerged lands looked like more or less scattered islands around the Earth's equator. Some of these islands were South America, Laurentia and Gondwana. During the Paleozoic there were numerous folds that originated mountains.

Life in water and on earth

This era is characterized by a large number of fossils that demonstrate the presence of multicellular life on a habitable planet. The weather was still warm and humid. This favored the proliferation of increasingly complex organisms and their subsequent evolution.

In the beginning, life at sea became very rich. The fossils of the first half of the Paleozoic are some invertebrates such as trilobites, graptolites, and crinoids. Those corresponding to the second half comprise some fossils of plants and vertebrates, such as fish, amphibians and reptiles. Animal life came out of the water and began to colonize the mainland.

The Cambrian, the explosion of multicellular life

In the Cambrian period there was a real explosion of life, with about fifty groups of organisms (edges) of which no precursors are known and that, for the most part, did not evolve towards any current edge or species. This amazing phenomenon, unique in the history of the Earth, is known as the Cambrian explosion.

Plant and animal life was confined to the seas. The first snails and cephalopod mollusks appeared; also the first ropes, precursors of vertebrates, and ours. Algae in the oceans and lichens on earth predominated in the plant kingdom. Its enormous proliferation contributed to the increase of oxygen in the Earth's atmosphere.

Another fundamental evolutionary advance in the Cambrian was the formation of the first skeletons and exoskeletons. On the one hand, there was more raw material available, more carbonates in marine waters; on the other, new predators appeared, so having protection for soft tissues and greater movement capacity greatly increased the chances of survival and, with it, reproduction. This is how natural selection works.

Ordovician, vertebrates arrive

In the Ordovician, initiated 485 million years ago, there were two mass extinctions, at the beginning and end of the period. Animals emerged with a precursor anatomical structure of the spine, the first vertebrates, primitive fish without jaws called ostracoderms. There were no land animals because oxygen in the atmosphere was still scarce.

The fossils of the Ordivic abound trilobites. The plants of this period were similar to those of the previous period. The first coral reefs also appeared. The abundance of life, especially vegetable, caused that in some areas oil and gas deposits were formed.

Earth's rotation continued to slow down due to the influence of the Moon, which was closer than it is today. The day already lasted 21 hours. Sea levels were the highest in the entire history of the Earth.

Silurian, vascular plants and animals that breathe air

443 million years ago the Silurian period began, which ended 420 million years ago. The most important evolutionary advance was the appearance of the first animal of aerial respiration, a scorpion that wanted to leave the sea.

The first fossil of a vascular plant (land plant with tissues that transport the food), although the stems and leaves were not yet well differentiated. The appearance of these organisms suggests that the composition of the atmosphere began to resemble the current one, with slightly less oxygen.

The level of the oceans at the beginning of the Silúrico remained very high, and only descended towards the end of the period. In these seas the cartilaginous fish, the spiny sharks and the placodermos, which were the first fish with jaws and teeth.

Earth entered a long phase of very warm stable weather, to the point that both polar caps almost disappeared.

Discover more:
• Charles Darwin and the evolution of species
• Trilobites and the Cambrian explosion
• Placodermos, some already extinct fish that were fertilized with penetration


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Geological history: the PrecambrianThe Paleozoic: Devonian, Carboniferous, Permian