EOCENE: CHARACTERISTICS, SUBDIVISIONS, GEOLOGY, SPECIES - GEOGRAFÍA - 2025

The Eocene was one of the eras that made up the Paleogene period of the Cenozoic Era. It was a time of great changes from the geological and biological point of view; great mountain ranges were formed as a result of the collision of the great continental masses, which moved thanks to continental drift.

Likewise and in a contradictory way, it was a time of separation, since the supercontinent Pangea, which until recently had been a single land mass, was almost completely separated.

Eocene fossils. Source: I, porshunta

From the biological point of view, there were several groups of animals that evolved and diversified at this time, including birds and some marine mammals.

General characteristics

Duration

The Eocene epoch lasted approximately 23 million years, divided into four ages.

Time of changes

The Eocene was a time when the planet underwent a large number of changes from the geological point of view, the most significant being the rupture of the supercontinent Pangea to originate the continents as they are known today.

Climatic events

At this time, two climatic events of great importance took place: the Paleocene - Eocene Thermal Maximum and the Azolla event. Both were opposite, since one meant an increase in environmental temperature, while the other consisted of a decrease in it. Both brought consequences for the living beings that populated the planet at that time.

Birds

One of the groups of animals that experienced greater diversification was that of birds. Many of those that inhabited the planet at this time were fearsome predators, some of considerable size.

geology

During the Eocene epoch, the Earth experienced intense geological activity that resulted in the total fragmentation of the supercontinent Pangea.

Total Fragmentation of Pangea

Pangea

Before this time began, the supercontinent Pangea had already begun to fragment. In the northern part, known as Laurasia, it was widely fragmented, leading to the separation of what is now known as Greenland, Europe and North America.

Each one began to move, thanks to continental drift, towards the positions they currently occupy. In such a way that Greenland moved north, North America west, and Europe east.

Likewise, a fragment of Africa, known as the Indian subcontinent (what is now India), collided with the Asian continent. Similarly, what is currently the Arabian peninsula also collided with Eurasia.

It is important to remember that at the beginning of this time, there were some fragments of Pangea that were still united, such as Australia and Antarctica. However, there came a time when due to continental drift, both pieces were separated. Antarctica shifted south to the position it occupies today, and Australia shifted slightly north.

Changes in bodies of water

The movement of the great masses of land resulted in a rearrangement of the oceans and seas that existed at that time. The Sea of ​​Tethys ended up disappearing, thanks to the rapprochement between the African continent and Eurasia.

On the contrary, it happened with the Atlantic Ocean, which was widening and gaining more and more ground with the displacement of North America towards the west. The Pacific Ocean remained the largest and deepest ocean on the planet, as it is today.

Orogeny

During this time the orogenic activity was quite intense, product of the displacement and collision of the different fragments that made up Pangea.

The Eocene was a geological time in which a large number of the mountain ranges that are observed today were formed. The collision of what is now India with the Asian continent originated the formation of the mountain chain that boasts the highest peaks in the world, the Himalayas.

Likewise, in what is North America there was also orogenic activity, forming mountain ranges such as the Appalachian Mountains.

Alpine Orogeny

It took place on the territory of the European continent. It originated the formation of several mountain ranges in three current continents: Europe, Asia and Africa.

On the African continent the Atlas Mountains were formed, while in Europe the Alps, the Pyrenees, the Balkan Mountains and the Caucasus were formed. Lastly, the mountain ranges that formed in Asia were the Elburz Mountains, the Himalayan mountain range, Karakoram and Pamir, among others.

This orogeny was the main consequence of the collision of the Eurasian tectonic plate with the plates of Africa, the Sub-Indian continent and Cimmeria.

This orogenic process was powerful and, taking into account that the continental drift has not stopped and that therefore the continental masses continue to move, it is still active.

Weather

Apparently the climatic conditions during the Eocene epoch were quite stable. However, at the beginning of this time, the ambient temperature experienced a sudden increase of approximately 7 - 8 degrees.

This became known as the Paleocene - Eocene Thermal Maximum. Likewise, at the end of the Eocene, another event occurred that greatly modified the prevailing environmental conditions; the Azolla event.

Paleocene - Eocene Thermal Maximum

In the opinion of specialists, this event took place 55 million years ago. During this process there was practically no ice on the planet. At the poles, which are naturally frozen sites, there was a temperate forest ecosystem.

It is believed that the main cause of this abrupt increase in environmental temperature was the emission of huge amounts of carbon dioxide (CO2) into the atmosphere. The reason for this is not yet clear.

Now, apart from the increase in environmental carbon dioxide, some scientists agree that there was also an exaggerated elevation of methane (CH4). Naturally, on the seabed there is a large amount of methane stored in the form of methane hydrates under strict conditions of pressure and temperature.

Specialists suppose that, in one way or another, the temperature of the oceans increased, and therefore these methane reservoirs were disturbed, causing methane hydrates to be released into the atmosphere.

It is well known that both methane and carbon dioxide are two greenhouse gases, so their release into the atmosphere is a more than likely cause of the rise in environmental temperature.

All these changes caused that, at least at the beginning, the planet's climate was hot, with little rainfall. However, as time progressed, those conditions seemed to stabilize and rainfall began to abound.

Thanks to increased rainfall, the planet's climate became humid and warm, staying that way for much of the Eocene.

Azolla event

In the middle of the Eocene, another climatic event known as the Azolla event took place, which resulted in a decrease in atmospheric concentrations of carbon dioxide and a consequent decrease in environmental temperature.

The cause of this event was the uncontrolled proliferation of a species of ferns, Azolla filiculoides. This growth took place on the surface of the Arctic Ocean.

In those times this ocean was totally surrounded by continents that were just separating. Because of this, its waters did not flow regularly.

Likewise, it is pertinent to remember that at that time there was a large amount of precipitation, which caused large amounts of fresh water to fall into the Arctic Ocean.

Exemplary of Azolla. Source: Joydeep

In the same way, thanks to the high environmental temperatures, the surface of the ocean evaporated rapidly, increasing its salinity and of course its density.

All this resulted in the formation of a layer of fresh water on the surface of the Arctic ocean, creating favorable environmental conditions for the Azolla fern to develop and spread.

Along with this, the amount of oxygen at the bottom of the ocean was decreasing, which hindered the activity of organisms that decompose organic matter. Therefore, when fern plants died and descended to the seabed, they were not decomposed, but underwent a process of fossilization.

All this caused a considerable reduction in atmospheric carbon dioxide and of course, a decrease in ambient temperature. There are records that indicate that temperatures in the Arctic dropped from 13 ° C to -9 ° C (current). This stayed that way for about a million years.

Finally, with the continuous movement of the continents, channels were expanded that allowed the communication of the Arctic Ocean with other oceans, with which the entry of brackish water was possible, increasing the salinity of the waters of its waters. With this, the ideal conditions for the proliferation of the Azolla fern were over, causing its death.

Lifetime

During the Eocene epoch, the environmental conditions of the planet allowed the development of various species, both plants and animals. In general, it was a time when there was an abundance and diversity of living beings, thanks to the humid and warm climate.

-Flora

From the flora point of view, the change experienced during the Eocene was quite noticeable, which had to do with the change in the climatic conditions of the planet.

In the early days, when temperatures were warm and humid, the planet had an abundance of jungles and forests. There is even evidence that there were forests at the poles at this time. The only sites that remained with a scarcity of plants were those desert ecosystems in the interior of the continents.

Among the plants that dominated the planet at that time, we can mention:

Metasequoia

It is a genus of plants that are characterized by being deciduous, that is, they lose their leaves at certain times of the year. Its leaves are bright green, except when they fall, which lose that color to a brown color.

They belong to the group of gymnosperms (plants with bare seeds).

These plants were found in the northern hemisphere of the planet, distributed throughout its extension, including in the Arctic area. Determining this has been possible thanks to the fossil records that have been recovered, mainly from nearby Canadian territory and even within the Arctic Circle.

Cupresaceae

They are plants that belong to the group of gymnosperms, specifically conifers. This group of plants is quite versatile, as they can be as small as shrubs or large trees. In addition, its leaves are similar to scales, arranged closely together with each other. Sometimes they release certain pleasant aromas.

-Fauna

During this time the fauna diversified widely, being the groups of birds and mammals those that dominated the scene.

Invertebrates

This group continued to diversify at this time, especially in the marine environment. Here, according to the scientists and the records collected, there were essentially mollusks, among which the gastropods, bivalves, echinoderms and cnidarians (corals) stood out.

Similarly, arthropods also evolved during this time, the ants being the most representative group.

Birds

In the Eocene and thanks to favorable environmental conditions, birds were a group that became quite diverse. Some species were even fierce predators of other groups of living beings.

Among the species of birds that existed on earth at that time, we can mention: Phorusrhacidae, Gastornis and penguins, among others.

Phorusrhacidae

This is a group of birds that were characterized by their large size (they reached up to 3 meters in height), which has been verified thanks to the fossil records. For example, in the Patagonia region, a skull of a specimen measuring 71 centimeters was recently found, from the occipital crest to the beak.

Another of its distinguishing characteristics was the inability to fly and its speed. It is believed that they could reach a speed of 50 km / h. Regarding its food preferences, this bird was an agile predator of small animals, including some mammals.

Gastornis

The specialists have dubbed it the "bird of terror", because of the appearance they must have had.

Among its most notable characteristics we can mention its size (up to 2 meters and more than 100 Kg) and its large head. His body was short and robust. Its beak was very similar to that of parrots, with an impressive force, which served to capture its prey.

It has been suggested that it was very fast and also did not fly.

Representative model of Gastornis. Source: Ghedoghedo, from Wikimedia Commons

Penguins

This is a group of flightless birds that have even survived to the present day. Today they are located in Antarctica at the south pole. However, at this time it is believed that they inhabited the South American continent, taking into account some fossils recovered from this site.

Regarding their size, the recovered records allow us to infer that there were specimens up to 1.5 meters, as well as other smaller ones.

Reptiles

Regarding the group of reptiles, it is known that large snakes existed at this time (more than 10 meters in length).

Mammals

This group continued to diversify, especially ungulates, cetaceans (marine mammals), and some large carnivores.

Ungulates

They are animals that are characterized by moving supported on the end of their fingers, which are sometimes covered by a hoof. During the Eocene, the suborders that are represented by pigs and camels, as well as cows, sheep and goats, had their origin.

Cetaceans

The Eocene was the golden age when it comes to the evolution of this group of mammals. The first cetaceans that existed were the archaeocetes, the first to begin to develop characteristics that allowed them to gradually adapt to aquatic life. Some exponents of this group were the ambuloketids, the protoketids and the remingtonoketids.

Ambulocetids

They are known as the first existing whales. This cetacean was large in length (more than three meters), although not in height (Approximately 50 centimeters). Its weight could be around 120 kilograms.

Physically it had a certain resemblance to crocodiles, with long limbs, which could function as flippers to move in the sea. They were carnivores. Its fossils have been found in India.

Protocetids

They were similar to today's dolphins, with an elongated snout and large eyes. It had short limbs that had the function of flippers. Specialists believe that they lived in seas with warm temperatures.

Remingtonoketids

They were large. They also resembled a crocodile or lizard, with an elongated snout and long limbs that ended in fingers. His eyes were small and his nostrils were located in the area of ​​the forehead.

Subdivisions

This epoch is divided into four ages:

• Ypresience: duration of 7 million years. It integrated what is known as the Lower Eocene.
• Lutetian: lasted about 8 million years. Together with the following age, it formed the Middle Eocene.
• Bartonian: lasted 3 million years.
• Priabonian: began 37 million years ago and ended 33 million years ago. It made up the Upper Eocene.

References

1. Berta A, Sumich J & Kovacs KM. (20119. Marine mammals. Evolutionary Biology. 2nd ed. Califòrnia: Academic Press
2. Donald R. Prothero (1993). The Eocene-Oligocene Transition: Paradise Lost. Columbia University Press
3. Keller, G. (1986) Eocene-Oligocene Boundary Reference Sections in the Pacific. Developments in Palaeontology and Stratigraphy. 9, 1986. 209-212.
4. Marie-Pierre Aubry, William A. Berggren, Marie-Pierre Aubry, Spencer G. Lucas (1998). Late Paleocene-Early Eocene Biotic and Climatic Events in the Marine and Terrestrial Records. Columbia University Press
5. Strauss, B. (2017). The Eocene Epoch (56-34 Million Years Ago). Extracted from: com / the-eocene-epoch-1091365