ORDOVICIAN PERIOD: CHARACTERISTICS, GEOLOGY, FLORA, FAUNA - GEOGRAFÍA - 2024

The Ordovician period was one of the six periods that made up the Paleozoic Era. It was located immediately after the Cambrian and before the Silurian. It was a period characterized by high sea levels, the proliferation of life in marine ecosystems and a drastic reduction in biodiversity at the end of the period resulting from an extinction event.

The animals that dominated the fauna were mainly arthropods, cnidarians, mollusks and fish. Despite the fact that important events took place during this period, it is one of the least known geological periods.

Representation of the seabed during the Ordovician. Source: Charles R. Knight

However, this is changing, as more and more specialists decide to delve into this interesting and momentous period of Earth's geological history.

General characteristics

Duration

The Ordovician period lasted approximately 21 million years, extending from about 485 million years ago to about 443 million years ago.

Climatic variations

It was a period in which there were important climatic variations between its beginnings and the end. At the beginning of the period the temperatures were rather high, but as time passed and thanks to a series of environmental transformations, the temperature dropped significantly, even reaching an ice age.

Mass extinction

At the end of the period an extinction took place that wiped out 85% of the species of living beings that existed at that time, essentially in marine ecosystems.

Divisions

The Ordovician period was divided into three epochs: Lower, Middle, and Upper Ordovician. Between these three epochs there were a total of seven ages.

geology

One of the essential characteristics of this period is that for most of its duration, sea levels were the highest the planet has ever had. During this period, there were four supercontinents: Gondwana (the largest of all), Siberia, Laurentia, and Baltica.

The northern hemisphere of the planet was mostly occupied by the great Panthalasa Ocean and only contained the supercontinent Siberia and a very small part of Laurentia.

In the southern hemisphere was the supercontinent Gondwana, which occupied almost all of the space. Likewise, here there were also Baltica and part of Laurentia.

Similarly, Gondwana began to experience fragmentation. A small piece began to come off. Today that piece of land corresponds to China.

The oceans that existed at that time were:

• Paleo Thetis: circling the supercontinent Siberia
• Panthalasa: also surrounding Siberia and almost entirely occupying the northern hemisphere of the planet.
• Lapetus: also known as Iapetus. It was located between the supercontinents Laurentia and Baltica. At the end of the Ordovician period it decreased in size thanks to the fact that these two land masses approached each other.
• Rheico: located between Gondwana and other supercontinents such as Laurentia and Baltica, which would later merge to form the supercontinent Laurasia.

The rock fossils recovered from the Ordovician mainly present rocks of the sedimentary type.

During this period, one of the most recognized geological phenomena took place: the Taconic Orogeny.

Taconic Orogeny

The Taconic orogeny was produced by the collision of two supercontinents and lasted for 10 million years, from about 460 million years ago to about 450 million years ago.

It was a geological process that resulted in the formation of the Appalachian Mountains, a mountain range that stretches across eastern North America, from part of Canada (Island of Newfoundland) to the state of Alabama in the United States.

This geological phenomenon owes its name to the Taconic Mountains, which belong to the aforementioned mountain range.

Weather

In general, the climate during the Ordovician period was warm and tropical. According to specialists on the subject, the temperatures that were registered on the planet were much higher than the current ones. There are even indications that there were places where a temperature of 60 ° C was recorded.

However, at the end of the period, temperatures decreased in such a way that there was an important glaciation that mainly affected the supercontinent Gondwana, which at that time was in the southern hemisphere of the planet, close to the south pole. It lasted approximately 0.5 to 1.5 million years.

Due to this process, a large number of animal species became extinct that could not adapt to the new environmental conditions.

Recent studies suggest that the glaciation even extended to the Iberian Peninsula. This is contrary to the belief that the ice was limited to areas near the south pole.

The causes of this glaciation remain unknown. Many talk about the decrease in carbon dioxide (CO2) concentrations as a possible cause, since in that period its levels fell.

However, studies on the subject are still being carried out to answer questions about the causes.

Lifetime

During this period there was a great diversification of life, especially that which took place at sea. In the Ordovician a large number of genera appeared that gave rise to new species.

Flora

Taking into account that in this period life on Earth developed mainly in the marine habitat, it is logical that most of the exponents of the Plantae kingdom were also there. However, it is important to make a clarification; in this period there were also representatives of the Fungi (mushroom) kingdom.

Green algae proliferated in the seas. Likewise, certain species of fungi were also present, which fulfilled the function they fulfill in every ecosystem: decompose and disintegrate dead organic matter.

The story in terrestrial ecosystems was different; it was practically non-existent. However, there were small plants that began to colonize the mainland.

These plants were primitive and very basic plants. They were non-vascular, which means that they did not have conducting vessels (xylem and phloem). Due to this they had to stay very close to the water in order to have a good availability of this resource.

These types of plants resembled today's liverworts, so named because their shape is reminiscent of the human liver.

Fauna

During the Ordovician period the fauna was really abundant in the oceans. There was a great diversity of animals, from the smallest and most primitive, to other more evolved and complex ones.

Arthropods

This was a fairly abundant edge in the Ordovician. Among the representatives of this phylum we can mention: trilobites, brachiopods and marine scorpions.

Both trilobites and brachiopods had a large number of specimens and species circulating in the Ordovician seas. Likewise, there were some species of crustaceans.

Mollusks

The phylum of mollusks also underwent a great evolutionary enlargement. In the seas there were nautiloid cephalopods, bivalves and gastropods. The latter traveled to the seashore, but having gill respiration, they could not stay in the terrestrial habitat.

Graphic representation of an Orthoceras. Source: Nobu Tamura (http://spinops.blogspot.com), from Wikimedia Commons

Fishes

While it is true that fish existed since the Cambrian, in the Ordovician the jawed fish began to appear, among which the best known was the coccosteus.

Corals

In the Ordovician period solitary corals are no longer appreciated, but these began to group together to form the first coral reefs of which we have news.

These were made up of coral specimens, as well as various varieties of sponges, which had already been diversifying since the previous period, the Cambrian.

Ordovician mass extinction - Silurian

It was known as the first great extinction of which there are fossil records. It occurred about 444 million years ago, that is, on the border between the Ordovician and Silurian periods.

As with many other prehistoric processes, specialists can only make conjectures and establish theories about the reasons why they happened.

In the case of this massive extinction process, the main causes have to do with the modification of the prevailing environmental conditions at that time.

Decrease in atmospheric carbon dioxide

Many specialists agree that the decrease in this greenhouse gas resulted in a decrease in environmental temperature, which in the long run triggered a long ice age in which only a low percentage of species survived.

Decrease in sea level

This seems to be another of the causes that caused the definitive extinction of many genera and species of living beings. This process occurred due to the approximation of the great land masses (supercontinents) that existed at that time.

In this case, due to the action of continental drift, the supercontinents Laurentia and Baltica were getting closer, until they collided.

This caused the Lapetus (Iapetus) ocean to close in its entirety, causing a decrease in sea level and of course, the death of all living species that had thrived on its shores.

Glaciation

This is the primary cause par excellence that specialists wield when talking about the extinction of the Ordovician. It is believed that it was related to the decrease in atmospheric carbon dioxide.

The most affected continent was Gondwana, whose surface was covered in a large percentage of ice. Of course, this affected the living beings that inhabited its shores. Those who survived, it was because they managed to adapt to this new variation in environmental conditions.

Explosion of a supernova

This is another of the theories raised about this extinction. It was developed during the first decade of the 21st century and states that a supernova explosion occurred in space at that time. This resulted in the Earth being flooded with gamma rays from the explosion.

These gamma rays caused a weakening of the ozone layer, as well as the loss of life forms that were found in coastal areas, where there is little depth.

Consequences

Regardless of the causes that led to the Ordovician mass extinction, the consequences of this were truly catastrophic for the planet's biodiversity.

It is to be expected that the organisms most affected were those that inhabited the waters, since in the terrestrial habitat there were very few, if not none.

It is known that approximately 85% of the species that were on the planet at the time disappeared. Among those that are almost entirely extinct are the brachiopods and bryozoans, as well as the trilobites and conodonts.

Likewise, large predators that swarmed in the waters became extinct, such as those of the order Eurypterida, which belonged to the arthropod phylum and were large.

Another example is the Orthoceras, a genus belonging to the phylum of mollusks. Both were fearsome predators of smaller organisms.

Their disappearance constituted a positive change for the life forms that were their prey, which were able to prosper and begin to diversify (of course, those that survived the extinction).

Divisions

The Ordovician period was divided into three epochs or series: Lower Ordovician (early), Middle Ordovician, and Upper Ordovician (Late).

Lower Ordovician (Early)

It is the first subdivision of the Ordovician period. It lasted approximately 15 million years, since it extended from about 485 million years ago to about 470 million years ago.

In turn, it was found divided into two ages:

• Tremadocian: with a duration of 8 million years.
• Floiense: it lasted approximately 7 million years.

Middle Ordovician

It lasted for approximately 12 million years. It extended from about 470 million years ago to about 458 million years ago. It was divided into two ages:

• Dapingian: happened about 470 million years ago to about 467 million years ago.
• Darriwillian: happened about 467 million years ago to about 458 million years ago.

Upper Ordovician (Late)

It was the last epoch of the Ordovician period. It lasted approximately 15 million years. It spanned from about 458 million years ago to about 443 million years ago.

The Upper Ordovician was in turn made up of three ages:

• Sandbiense: that lasted about 5 million years.
• Katiense: spanned about 8 million years.
• Hirnantian: with a duration of 2 million years.

References

1. Benedetto, J. (2018). The continent of Gondwana through time. National Academy of Sciences (Argentina). 3rd edition.
2. Cooper, John D.; Miller, Richard H.; Patterson, Jacqueline (1986). A Trip Through Time: Principles of Historical Geology. Columbus: Merrill Publishing Company. pp. 247, 255-259.
3. Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004
4. Sepkoski, J. (1995). The ordovician radiations: diversification and extinction shown by global genus - level taxonomic data. Society for sedimentary geology.
5. VVAA (2000). Dictionary of Earth Sciences. Editorial Complutense.
6. Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).