CRETACEOUS: CHARACTERISTICS, SUBDIVISIONS, FLORA, FAUNA, CLIMATE - GEOGRAFÍA - 2025

The Cretaceous or Cretaceous is the last of the three divisions or periods that make up the Mesozoic Era. It had an approximate extension of 79 million years, distributed in two epochs. Likewise, it was the longest period of this era.

During this period, a flourishing of existing life forms could be seen, both in the seas and on the land surface. In this period a great diversification of the group of dinosaurs was observed and the first flowering plants appeared.

Representation of a typical Cretaceous scene. Source: Pavel.Riha.CB

However, despite all the biological prosperity that was lived in almost the entire length of this period, in the end one of the most devastating events in the geological history of history occurred: the mass extinction of the Cretaceous - Palogenous, which ended with dinosaurs almost entirely.

The Cretaceous is one of the periods best known and studied by specialists in the area, although it still has certain secrets to discover.

General characteristics

Duration

The Cretaceous period lasted 79 million years.

Presence of dinosaurs

During this period a great proliferation of dinosaur species was appreciated, which populated both terrestrial and marine ecosystems. There were herbivores and carnivores, of various sizes and with very varied morphologies.

Mass extinction process

At the end of the Cretaceous period, one of the best known processes of mass extinction took place and studied by specialists. This process has drawn the attention of specialists in the area powerfully because it meant the extinction of the dinosaurs.

Regarding its causes, only possible hypotheses are known, but there are none reliably accepted. The consequence was the extinction of 70% of the species of living beings that existed at that time.

Subdivisions

The Cretaceous period consisted of two epochs: early Cretaceous and late Cretaceous. The first lasted 45 million years, while the second lasted 34 million years.

geology

The most notable feature of this period is the separation of a large continental mass known as Pangea, which was formed by the collision of all supercontinents that existed separately in earlier eras. The fragmentation of the Pangea began during the Triassic period, at the beginning of the Mesozoic Era.

Pangea

Specifically in the Cretaceous, there were two supercontinents: Gondwana, which was located to the south, and Laurasia, in the north.

During this period the intense activity of the continental plates continued, and consequently, the disintegration of that supercontinent that once occupied the planet, Pangea.

What is now South America began to separate from the African continent, while the Asian and European continents still remained united. Australia, which had been linked to Antarctica, began its separation process to move to the place it occupies today.

What is today India, which was once linked to Madagascar, separated and began its slow movement north, to later collide with Asia, a process that gave rise to the Himalayas.

At the end of the period, the planet was made up of several land masses that were separated by bodies of water. This was decisive in the development and evolution of the various species, both animals and plants that were considered endemic to one region or another.

Oceans

Likewise, during the Cretaceous period the sea reached the highest levels reached up to that moment. The oceans that existed in this period were:

• Sea of ​​Thetis: it was in the space that separated Gondwana and Laurasia. It preceded the appearance of the Pacific Ocean.
• Atlantic Ocean: began its formation process with the separation of South America and Africa, as well as with the movement of India to the north.
• Pacific Ocean: largest and deepest ocean on the planet. It occupied all the space surrounding the land masses that were in the process of separation.

It is important to note that the separation of the Pangea caused the formation of some bodies of water, apart from the Atlantic Ocean. These include the Indian Ocean and the Arctic, as well as the Caribbean Sea and the Gulf of Mexico, among others.

In this period there was a great geological activity, which gave rise to the formation of large mountain ranges. Here continued the Nevadian Orogeny (which had started in the previous period) and the Laramide Orogeny.

Nevadian Orogeny

It was an orogenic process that took place along the west coast of North America. It began in the middle of the Jurassic period and ended in the Cretaceous period.

Thanks to the geological events that developed in this orogeny, two mountain ranges were formed that are located in the current state of California in the United States: Sierra Nevada and the Klamath Mountains (these include part of the southern state of Oregon as well).

The Nevadian Orogeny took place approximately 155 - 145 million years ago.

Laramide Orogeny

The Laramide Orogeny was a fairly violent and intense geological process that occurred about 70 - 60 million years ago. It spread along the entire west coast of the North American continent.

This process resulted in the formation of some mountain ranges such as the Rocky Mountains. Also known as the Rockies, they extend from British Columbia in Canadian territory, to the state of New Mexico in the United States.

Descending a little further down the west coast, in Mexico this orogeny gave rise to the mountain chain known as Sierra Madre Oriental, which is so extensive that it crosses several states of the Aztec nation: Coahuila, Nuevo León, Tamaulipas, San Luis Potosí and Puebla, among others.

Weather

During the Cretaceous period, the climate was warm, according to the fossil records collected by specialists.

As mentioned above, the sea level was quite high, much higher than in previous periods. Therefore, it was common for the water to have reached the innermost part of the great land masses that existed at that time. Thanks to this, the climate in the interior of the continents softened a bit.

Likewise, during this period it is estimated that the poles were not covered with ice. Similarly, another of the climatic characteristics of this period is that the climatic difference between the poles and the equatorial zone was not as drastic as it is today, but a little more gradual.

According to specialists, average temperatures in the oceanic area were, on average, about 13 ° C warmer than today, while in the depths of the seabed they were even warmer (20 ° C more, approximately).

These climatic characteristics allowed a great variety of life forms to proliferate on the continents, both in terms of fauna and flora. This was so because the climate contributed to the ideal conditions for its development.

Lifetime

During the Cretaceous period life was quite diverse. However, the end of the period was marked by a massive extinction event, during which approximately 75% of the plant and animal species that inhabited the planet perished.

-Flora

One of the most important and significant milestones of this period in relation to the botanical area was the appearance and dissemination of flowering plants, whose scientific name is angiosperms.

It should be remembered that from previous periods, the type of plants that dominated the earth's surface were gymnosperms, which are plants whose seeds are not enclosed in a specialized structure, but are exposed and also have no fruit.

Angiosperms have an evolutionary advantage over gymnosperms: having the seed enclosed in a structure (ovary) allows it to keep it protected from hostile environmental conditions or from attack by pathogens and insects.

It is important to mention that the development and diversification of angiosperms was largely due to the action of insects such as bees. As is known, flowers can reproduce thanks to the pollination process in which bees are an important factor, since they transport pollen from one plant to another.

Among the most representative species that existed in terrestrial ecosystems are conifers, which formed extensive forests.

Likewise, in this period some families of plants began to appear, such as palm trees, birch, magnolia, willow, walnut and oak, among others.

-Fauna

The fauna of the Cretaceous period was dominated mainly by dinosaurs, of which there was a great variety, both terrestrial and aerial and marine. There were also some fish and invertebrates. Mammals were a smaller group that began to proliferate in the later period.

Invertebrates

Among the invertebrates that were present in this period, we can mention mollusks. Among these were the cephalopods, among which the ammonoid stood out. Likewise, we must also mention coleoids and nautiloids.

On the other hand, the phylum of the echinoderms was also represented by the starfish, the echinoids and the ophiuroids.

Finally, most of the fossils that have been recovered in the so-called amber deposits are arthropods. In these deposits, specimens of bees, spiders, wasps, dragonflies, butterflies, grasshoppers and ants have been found, among others.

Vertebrates

Within the group of vertebrates, the most prominent were reptiles, among which dinosaurs dominated. Likewise, in the seas, coexisting with marine reptiles, there were also fish.

In terrestrial habitats the group of mammals began to develop and to experience an incipient diversification. The same happened with the group of birds.

Land dinosaurs

Dinosaurs were the most diverse group during this period. There were two large groups, the herbivorous dinosaurs and the carnivores.

Herbivorous dinosaurs

Also known by the name of ornithopods. As can be seen, their diet consisted of a plant-based diet. In the Cretaceous there were several species of this type of dinosaur:

• Ankylosaurs: they were large animals, even reaching a length of 7 meters and a height of almost 2 meters. Its average weight was approximately 4 tons. His body was covered by bone plates that functioned as a cuirass. According to the fossils found, specialists have determined that the front limbs were shorter than the rear ones. The head was similar to a triangle, as its width was greater than the length.
• Hadrosaurs: also known as the "duck-billed" dinosaurs. They were large in size, measuring approximately 4-15 meters long. These dinosaurs had a large number of teeth (up to 2000), arranged in rows, all of the molar type. Likewise, they had a long and flattened tail that served to maintain balance when they moved on two legs (especially to flee from predators).
• Pachycephalosaurs: it was a large dinosaur, whose main characteristic was the presence of a bony protrusion that simulated a kind of helmet. This served as protection, as it could even be up to 25 cm thick. In terms of displacement, this dinosaur was bipedal. It could reach a length of up to 5 meters and a weight of up to 2 tons.
• Ceratopsids: These dinosaurs were quadrupeds. On the facial surface they had horns. Likewise, they had an enlargement in the back of the head that extended to the neck. As for its dimensions, it could measure 8 meters and reach a weight of 12 tons.

Reconstruction of the skeleton of a Ceratopsid. Source: Ryan Somma from Occoquan, USA, via Wikimedia Commons

Carnivorous dinosaurs

Within this group theropods are included. These were carnivorous dinosaurs, most of the time large. They represented the dominant predators.

They were bipedal, with highly developed and strong hind limbs. The forelimbs were small and underdeveloped.

Its essential characteristic is that in its extremities they had three fingers oriented towards the front and one towards the back. They had large claws. Of this group, perhaps the most recognized dinosaur is the Tyrannosaurus rex.

Flying reptiles

Known by the name of Pterosaurs. Many mistakenly include them within the group of dinosaurs, but they are not. These were the first vertebrates to acquire the ability to fly.

Their size was variable, they could even measure 12 meters of wingspan. The largest Pterosaur known to date is the Quetzalcoatlus.

Marine reptiles

The marine reptiles were large in size, with an average size of between 12 and 17 meters in length. Among these, the best known were the mosasaurs and the elasmosaurids.

Elasmosaurids were characterized by having a very long neck, since they had a large number of vertebrae (between 32 and 70). They were known predators of some fish and mollusks.

On the other hand, mosasaurs were reptiles that were adapted to marine life. Among these adaptations they had fins (rather than limbs) and featured a long tail with a vertical fin.

Despite the fact that both sight and smell were underdeveloped, the mosasaur was considered one of the most fearsome predators, feeding on a wide variety of marine animals and even others of the same species.

Graphic representation of a mosasaur. Source: Heinrich Harder (1858–1935), via Wikimedia Commons

Mass extinction of the Cretaceous - Paleogene

It was one of the many extinction processes that planet Earth experienced. It occurred approximately 65 million years ago on the border between the Cretaceous and the Paleogene (first period of the Cenozoic Era).

It had a momentous impact, since it caused the total disappearance of 70% of the species of plants and animals that inhabited the planet at that time. The group of dinosaurs was perhaps the most affected, since 98% of the species that existed were extinct.

-Causes

Impact of a meteorite

This is one of the most widely accepted hypotheses that explain why this mass extinction occurred. It was postulated by the physicist and Nobel Prize winner Luis Álvarez, who was based on the analysis of various samples collected in which a high level of iridium was observed.

Likewise, this hypothesis is supported by the finding, in the area of ​​the Yucatan Peninsula, of a crater that has a diameter of 180 km and that could well be the footprint of the impact of a large meteorite on the earth's crust.

Intense volcanic activity

During the Cretaceous period, intense volcanic activity was recorded in the geographic area where India is located. As a result of this, a large amount of gases were expelled into the Earth's atmosphere.

Marine acidification

It is believed that as a result of the impact of the meteorite on the planet, the Earth's atmosphere overheated, generating the oxidation of nitrogen, producing nitric acid.

In addition, through other chemical processes sulfuric acid was also produced. Both compounds caused a drop in the pH of the oceans, greatly affecting the species that coexisted in this habitat.

Subdivisions

The Cretaceous period was divided into two epochs or series: Lower Cretaceous (early) and Upper Cretaceous (late), which in turn comprised a total of 12 ages or floors.

Lower cretaceous

It was the first epoch of the Cretaceous period. It lasted approximately 45 million years. This in turn was subdivided into 6 ages or floors:

• Berriasiense: it lasted about 6 million years on average.
• Valanginian: with a duration of 7 million years.
• Hauterivian: that spanned 3 million years.
• Barremian: 4 million years old.
• Aptian: lasted 12 million years.
• Albiense: about 13 million years.

Upper Cretaceous

It was the last time of the Cretaceous. It preceded the first period of the Cenozoic (Paleogene) era. It had an estimated duration of 34 million years. Its end was marked by a process of mass extinction in which the dinosaurs became extinct. It was subdivided into 6 ages:

• Cenomanian: lasting about 7 million years.
• Turonian: with a duration of 4 million years.
• Coniacian: it spanned 3 million years.
• Santonian: it also lasted 3 million years.
• Campanian: it was the age that lasted the longest: 11 million years.
• Maastrichtian: which lasted 6 million years.

References

1. Alvarez, LW et al. (1980). Extraterrestial cause for the Cretaceous-Tertiary extinction. Science 208, 1095-1108.
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3. Benton (1995). Paleontology and evolution of vertebrates. Lleida: Editorial Perfils. 369 pp.
4. González, V. Causes of the Great Cretaceous Extinction. Obtained from: muyinteresante.es
5. Lane, Gary, A., and William Ausich. Life of the Past. 4th ed. Englewood, NJ: Prentice Hall, 1999
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