Cretaceous – Paleogene extinction event

"What is the definition of the Cretaceous-Paleogene event in the geological clock of the earth? What caused it, and when did it happen?"

If you’ve ever read an article on geology or dinosaurs, I’m sure you would have seen the abbreviations K-T (Cretaceous-Tertiary) extinction or K-Pg (Cretaceous – Paleogene) extinction. This event is a critical moment in the earth’s geological clock as it eliminated 80 percent of animal species between the Cretaceous and Paleogene periods. 

Here we will be looking at what these two periods mean, what happened between and what came after this event.

cretaceous paleogene extinction

Defining the Cretaceous-Paleogene extinction

This mass extinction was called the Cretaceous-Paleogene extinction—also known as the K-T or K-Pg extinction.

This extinction event happened 66 million ago when the earth was hit by an asteroid that landed in the gulf of Mexico’s Yucatán Peninsula; as years passed, this created a prominent crater that was identified in the early 1990s. In the geological record, the K-T extinction event is marked by a thin layer of rocks called the K-Pg boundary; this is found worldwide in terrestrial and marine rocks. The clay within this boundary contains a large amount of metal called iridium. Iridium is much more likely to be found in asteroids than in the earth’s crust. 

In the early 1980s, scientist Luis Alvarez and his son Walter gave an impact theory suggesting that the K-T extinction was caused by an asteroid that hit the surface, leading to environmental changes, stopping photosynthesis, and ending all life forms; clearly, Dr Alvarez was spot on and the discovery of the crater years later confirms this.

The K-T extinction wiped away three-quarters of plants’ life and animal species, including the dinosaurs. The only survivors after this event were avian dinosaurs, a few reptiles, such as sea turtles and crocodiles, and other tetrapods that did not weigh more than 55 pounds (25 kilograms).

That is a summary of what the Cretaceous extinction event is. Still, further along in the article, I will discuss its significance by describing both these periods respectively, to give you another depth of understanding of what was happening before and after the K-T extinction. You’ll see that despite this tragic event which ended all life forms, one of these periods continued or adapted some traits from the previous and continued to evolve.

cretaceous paleogene extinction

The birth of planet earth

When thinking about a title for my introduction, I was leaning towards using the birth of our dear mother earth, but I started thinking about who birthed the earth. I’m sure this could have led to other exciting interpretations, but that is not the day’s topic, as we want to learn more about its existence rather than its gender or birth history. So… the earth has an age that we call a geological time scale, presented in four spans. Each of these times has its characteristics, species, evolution, and extinction events. For avid readers who love reading about geology, they know that the birth of another period came after a specific event that brought along extinction.

The first era of the earth was called the Precambrian, the longest era of all four; according to our calculations, it started 4.6 billion to 542 million years ago. For a very long time, there was no life on the earth until the end of the Precambrian era, when single-celled organisms began to exist. No one knows for sure how that came to be. 

Still, many theories have suggested some interesting theories, such as the Hydrothermal vent theory, Primordial soup theory, and the Panspermia theory. There was still no life on land at the end of the Precambrian era, but some beings, like jellyfish, started to evolve in the water. The atmosphere had also started to gather a good amount of oxygen for medium-sized animals to survive in the future. The small number of living organisms that existed at the time only started to multiply and diversify in the next era.

The next era that followed was the Paleozoic era, which came into existence after the Cambrian explosion. The Paleozoic era came into existence between 542 million to 250 million years ago, so far, it is the shortest period, but a lot of life forms from the sea had time to flourish and come out to land. Plants also started to develop, and some invertebrates came on land. Many species began to evolve and grow. The most significant extinction event marked the end of the Paleozoic era due to drastic climate changes as 95 percent of life forms from the waters and 70 percent from the land were wiped out. But this introduced a new life and a grander era.

The extinction event after the Paleozoic was named the Permian, and after this event, earth welcomed the glorious dinosaur age known as the Mesozoic era. Since the previous era ended because of extreme heat, the climate of the Mesozoic era was tropical and humid, so there was a lot of green and lush vegetation on the earth. This climate also favored the development of dinosaurs, who started small at the beginning of the era but got more prominent at the end. Some mammals emerged, and other herbivores and birds developed from dinosaurs.

The splendor of the Mesozoic era lasted for 250 to 60 million years; it ended with a meteorite hit that wiped out all the dinosaurs and other animals. After this ordeal, the earth moved on to the Cenozoic era, the last period on the geological time scale. This era started 65 million ago and is still going as it is the era we are in right now (what a time to be alive!). Apart from avian dinosaurs, there were also smaller mammals that grew bigger. Climate-wise, the temperature came down over a short time, leading to the ice age. The cold and dry climate was the cause of the rapid evolution of all life forms, including humans.

cretaceous paleogene extinction

Cretaceous period

The Cretaceous period is the last of the Mesozoic era and comes after the Jurassic and Triassic periods. So this was the last period that dinosaurs ruled the earth for the remaining 79 million years of this era.

Cretaceous comes from the Latin word “creta”, which means chalk. This term was first used by Belgian geologist Omalius d’Halloy in 1882 when mentioning the strata encircling the Paris Basin. He noticed thick deposits of calcium carbonate and chert. Today you will find a widespread chalk deposit dating from the Cretaceous period in specific areas worldwide, such as both sides of the English channel. The Big Bend National Park in Texas also harbors some Cretaceous-age rocks that contain chalk and have been to be indicators of the change in sea level of the Cretaceous Interior Seaway. 

Highlights of the Cretaceous period

During this period, flowering plants (angiosperms) began to multiply and diversify; according to fossil records, this started 100 million ago during the Cretaceous period. The earliest forms of angiosperms started to develop from a particular group of ferns, and once they appeared, they quickly became the most popular plant life and still do today. Flowering plants adapted to different climates and habitats, as you’ll find them in the deserts, mountains, and even along the coast.

Nearing the end of the Cretaceous period, the western part of North America experienced rising from the sea, and it wasn’t the first time. This led to Laramide Orogeny, a series of repeated lifts, volcano eruptions, and erosions that continued during the Cenozoic era. The Laramide Orogeny disturbed a large portion of land already subject to substantial block uplifts, creating the Ancestral Rocky mountains. Apart from the mountain uplifts, many Laramide deformations took place in the adjacent basins, and each of these basins received rocks that came off from uplifted areas. These rocks compose the vital record of the events of the orogeny.

The Laramide uplift happened mainly in the center and southern Rocky mountains of Wyoming and Colorado. This also occurred north and south of the northern Rocky Mountains of Montana and Alberta, Canada. The result of the Laramide Orogeny gave us these national parks around the US: Florissant Fossil Beds National Monument, Rocky Mountain National Park, Bighorn Canyon National Recreation Area, Glacier National Park, and Great Sand Dunes National Monument.

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Paleogene period

The Paleogene period is the longest period of the Cenozoic era and lasted between 66 and 23 million years ago; it is also the first period of the Cenozoic era. There were no dinosaurs anymore or giant marine reptiles roaming the earth; instead, rodent-sized mammals became the popular species on the earth and grew bigger and more dominant with time.

Highlights of the Paleogene period

During the Paleogene period, continents drifted further away from each other, and oceans largened the gaps. Europe parted from North America, and Australia and Antarctica parted as well. The temperature began to drop, and so did sea levels; old Cretaceous levels drained most interior seaways.

For a period of a thousand years, there was another temperature drop, and sea levels came down to nine and fourteen degrees Fahrenheit ( five and eight degrees Celcius). This climate change killed many single-celled organisms in the water and some invertebrates. The climate was not suitable for many of the forests in the Northern hemisphere as the earth lost many pines and sequoias. But this new subtropical climate favored the growth of palm trees and fruits such as guavas.

Many mammals evolved from the middle to the end of the Paleogene period. Several large animals evolved on land and made the sea their habitat. Some reptiles (crocodiles, turtles, snakes, and lizards) had existed from the previous Cretaceous period, got used to the sub-tropical climate, and multiplied.

Meanwhile, sharks started to appear in the oceans along with other freshwater fish species like trout and bass. Many lakes were formed in the western part of the United States. The Green River Formation is an area filled with fossils created by these lakes; today, many fish fossils are found in this sediment.

Birds, survivors of the dinosaur age, started to evolve, diversify, and start flying. Some flightless birds, like the ‘terror birds,” went extinct; they were dominant for a while until mammals started to multiply and vary. The terror birds were predators, living in forests and plains. Over 23 million years, primates, dogs, cats, and horses all evolved into what we see today.

Picture of Elisha Hosea

Elisha Hosea


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