DEVONIAN PERIOD: CHARACTERISTICS, GEOLOGY, FAUNA, FLORA - GEOGRAFÍA - 2025

The Devonian period was one of the five subdivisions of the Paleozoic Era. It lasted about 56 million years, in which the planet underwent a large number of changes at a geological level, but especially in biodiversity.

During this period, there was a broad development of some groups of animals, especially those that lived in marine environments. There were also important changes in terrestrial habitats, appearing large plants and the first terrestrial animals.

Representation of the environment in the Devonian. Source: Devonianscene.jpg: Eduard Riou (1838-1900) from The World Before the Deluge 1872, United Statesderivative work: Rursus, via Wikimedia Commons

Despite being a period in which life became so diversified, the Devonian also has the dubious reputation of being the time period when a large number of animal species (80%) became extinct. During this period, a mass extinction event occurred that permanently disappeared many species from the face of the Earth.

General characteristics

Duration

The Devonian period lasted approximately 56 million years. It began about 416 million years ago and ended about 359 million years ago.

Evolution of large groups of animals

During the Devonian period, existing animal groups underwent incredible evolution and diversification. Life in the seas flourished to a great extent.

Coral reefs became true ecosystems in which new species of sponges and corals appeared. Larger animals appeared and became predators.

The group of vertebrates that underwent the greatest development was that of fish, of which a large number of species appeared, some of which have managed to survive until today.

An important milestone of this period was the beginning of the conquest of the terrestrial habitat. In this period the first amphibians appeared and specialists believe that some fish began to approach the land to settle on it.

Divisions

The Devonian period is divided into three major subdivisions:

• Lower or Early Devonian: made up of three floors or ages (Lochkovian, Pragian and Emsian).
• Middle Devonian: spanning two ages (Eifelian and Givetian).
• Upper or Late Devonian: consisting of two ages (Frasni and Famenian).

There was a process of mass extinction

At the end of the Devonian period, a mass extinction event occurred in which a large number of species were lost, mainly those that inhabited the seas of the tropical part of the planet.

Among the species most affected by this event are: corals, fish (especially agnathates), mollusks (gastropods, ammonoids), crustaceans (especially ostracods), among others.

Fortunately, the species that developed in terrestrial ecosystems were not so affected by this phenomenon, so the conquest of terrestrial habitat could continue its course.

geology

The Devonian period was marked by intense plate tectonic activity. There was a clash of them, forming new supercontinents. Such is the case of the formation of Laurasia, a phenomenon that occurred at the beginning of this period when Laurentia and Baltica collided.

During this period, the extremely extensive supercontinent Gondwana, occupying a large space at the planet's south pole, also remained. The supercontinent Laurasia was also at the south pole.

The northern part of the planet was occupied by the supercontinent Siberia and the immense and deep Panthalassa Ocean. This ocean covered almost the entire northern hemisphere.

Apart from the Panthalassa ocean, there were still other smaller oceans such as:

• Ural: located between Siberia and Baltic. During this period it was decreasing in size until it ended up becoming a mere maritime channel, due to the fact that Baltica and Siberia were in constant proximity until they finally collided in the Carboniferous period.
• Proto - Tethys: between Laurasia and Gondwana. During the Devonian this ocean gradually closed. In the following period it disappeared completely.
• Paleo - Tethys: located between Laurasia and Gondwana.
• Rheico: also between Gondwana and Laurasia. During this period the ocean was narrowing, due to the displacement of Gondwana towards Laurasia.

From the point of view of orogeny, during this period the formation process of some mountain ranges began, such as the Appalachian Mountains of the United States.

Likewise, during this period the last events of the Caledonian orogeny occurred, which caused the formation of mountain ranges in the area where Great Britain and the Scandinavian countries (specifically Norway) settle today.

Weather

Climatic conditions during the Devonian period were relatively stable. In general, specialists suggest that the Devonian climate was hot and humid, with abundant rains. However, inside the great continental masses on the planet the climate was arid and dry.

At the beginning of this period, the average ambient temperature was around 30 ° C. As time progressed, there was a decrease until it reached approximately 25 ° C.

Later, at the end of the period, temperatures dropped so much that there was a glaciation or glaciations (specialists have not agreed on this point).

In short, specialists have suggested that during the Devonian period there was an extremely cold area at the South Pole, while around the Equatorial area the climate was humid.

Lifetime

During the Devonian period there were significant changes in relation to the living beings that populated the planet. The most important of these changes was the definitive conquest of terrestrial habitats.

-Flora

During the previous Silurian period, small vascular plants such as ferns had already begun to develop. During the Devonian period, these small ferns acquired a greater development in various aspects, the most representative being their size.

Similarly, other plant forms appeared on the surface of existing continents. These types of plants include lycopodiophytes and others that did not survive and became extinct, such as trimerophytes and progymnosperms.

In this period the first forests began to appear, thanks to the fact that plants were able to develop resistant structures that allowed them to support tall leaves and branches. Even through fossil records it has been established that there were trees that reached 30 meters in height.

The proliferation of plants in terrestrial environments brought as a direct consequence an increase in atmospheric oxygen, since these plants carried out the photosynthesis process. Thanks to this, the diversification of animals in terrestrial habitats was possible.

-Fauna

In the Devonian period life continued to diversify on the seas in unimaginable ways.

Fishes

One of the groups that underwent the greatest evolution were fish. So much so that this period is called “the Age of Fish”. Among the groups of fish that originated during this period can be mentioned:

• Sarcopterygii: like all fish, they belong to the group of vertebrates. They also have as a distinctive feature the presence of a jaw. Those from this period had lobed and paired fins. Likewise, the scales were mainly sheets of bone covered by keratin. Despite the fact that most of the species in this group are extinct, dipnoa and coelacanth still persist today.
• Actinopterygians: they are called bony fish. They are made up mainly of bone and very little cartilage. Thanks to the fossils that have been found, it is known that their scales were not superimposed and they had asymmetric tails. They managed to survive the adverse conditions of the different geological periods of the planet and today they cover most of the fish that exist.
• Ostracoderms: despite being extinct, they had the honor of being considered the first known vertebrates. They were characterized because their body was covered with scales and a kind of bone shell. They also had no jaw. Some specimens could reach 60cm in length.
• Selacios: is the group to which sharks belong. There were some small species. Few fossils of these fish have been found, but specialists suggest that they were an important part of the food chain in the sea.

Coccosteus (Placoderm type of fish). Source: I,

Reefs

Reefs, which were made up of sponges, corals and some types of algae, prospered successfully at the bottom of the seas. Siliceous sponges appeared. There were large coral reefs, some of which disappeared over time.

Arthropods

The first representatives of the animal kingdom that began to colonize the terrestrial habitat were arthropods. Among the arthropods that could be found in terrestrial environments, centipedes, mites, spiders and scorpions can be mentioned.

Likewise, in the seas there were also representatives of the arthropod phylum, which also experienced great diversification and massification. They even developed an air breathing system

Mollusks

During the Devonian period, the group of mollusks also experienced great diversification. One change during this period was that some specimens began to invade freshwater habitats. An example of these were the lamellibranchs, similar to today's mussels.

Terrestrial vertebrates

The first vertebrates to appear in the terrestrial environment are believed to be amphibians that, despite needing to live near bodies of water, could subsist on dry land. They did it at the end of the Devonian.

Likewise, there are hypotheses that some fish were leaving the marine environment to enter the terrestrial environment and colonize it. Of course, for this they had to evolve and develop certain structures to adapt.

Massive expanse of the Devonian

At the end of the Devonian Period a process of mass extinction occurred. Scientists have not yet fully agreed on whether it was one big event or several small events.

In any case, it greatly affected the living beings of the moment, since it caused the disappearance of more than 80% of the living species.

It mainly affected the living forms of the seas. It seems that the living things that were on land did not suffer a great negative impact.

During this process, the trilobites, agnate fish, a large number of corals, among others, were almost completely lost.

This extinction lasted approximately 3 million years.

Causes

There are multiple causes that try to explain the Devonian mass extinction process. These include:

Meteors

For some years now, specialists who have been studying geological eras have suggested that the Devonian mass extinction happened thanks to the collision of meteors in the earth's crust.

Critical decline in oxygen levels in the seas

It is known that during this period the oxygen concentration in the seas decreased drastically, even talking about oceanic anoxia, although the causes are not known.

Certain specialists coincide in pointing out that the large terrestrial vascular plants are responsible. According to them, these plants had large and powerful roots that by burrowing deep into the earth were able to remove certain nutrients that ended up in the ocean.

This resulted in an unusual proliferation of algae, which could absorb a large percentage of oxygen in the water, thus depriving marine animals of it.

Despite not knowing the exact cause, it is reliably known that the oxygen level in the seas decreased, thus condemning a large number of species to extinction.

Global warming

Specialists believe that at that time the atmosphere had a high content of carbon dioxide. This caused a greenhouse effect to be generated, which caused the earth's temperature to increase.

This increase in temperature had repercussions in other aspects, such as the decrease in oxygen in the waters.

Plant growth

As mentioned above, during this period very tall vascular plants (30m) developed on the surface of the continents.

This resulted in an imbalance in environmental conditions, since these plants began to absorb a large amount of water and nutrients from the soil, which could have been used by other living beings.

Intense volcanic activity

-Numerous specialists have suggested that intense volcanic activity was recorded during the Devonian period that released a large amount of rocks and gases into the atmosphere.

This had the consequence that the atmospheric temperature increased, thus affecting living beings that were not used to high temperatures.

Divisions

The Devonian period was divided or separated into three epochs: Lower (Early), Middle, and Upper (Late).

Lower Devonian (Early)

It is the early Devonian period. It lasted approximately 26 million years, since it extended from about 419 million years ago to about 393 million years ago.

It was made up of three ages:

• Lochkoviense: with an approximate duration of 9 million years.
• Pragian: lasted on average about 3 million years
• Emsian: It was the longest, lasting approximately 14 million years.

Middle Devonian

It was the intermediate epoch, between the Lower and Upper Devonian. It extended from about 393 million years ago to about 382 million years ago, so it lasted approximately 11 million years.

It was made up of two ages:

• Eifelian: with a duration of 6 million years.
• Givetian: lasted approximately 5 million years.

Upper Devonian (Late)

Last epoch of those that integrated the Devonian period, immediately prior to the Carboniferous period. It had an average duration of 26 million years.

It spanned from about 385 million years ago to about 359 million years ago. During this time the Devonian mass extinction took place.

It was made up of two ages:

• Frasniense: that lasted approximately 13 million years.
• Fameniense: with a duration of 13 million years.

References

1. Campbell, NA and Reece, JB (2007). "The Evolutionary History of Biological Diversity". Biology (7th edition). Editorial Médica Panamericana. p. 1532
2. Ellwood, BB, SL Benoist, A El Hassani, C Wheeler, RE Crick (2003), Impact ejecta layer from the Mid-Devonian: possible connection to global mass extinctions. Science 300: 1734-1737.
3. Gradstein, FM; Ogg, JG; Smith, AG (2004). A Geologic Time Scale 2004. Cambridge university press
4. Sandberg, CA, JR Morrow & W Ziegler (2002), Late Devonian sea-level changes, catastrophic events, and mass extinctions in C Koeberl & KG MacLeod, Catastrophic Events and Mass Extinctions: Impacts and Beyond, Geol. Soc. Amer. Spec. Paper # 356, pp. 473-487.
5. Vargas P., Zardoya R. (2012) The tree of life: systematics and evolution of living beings. 3rd Edition.