Cretaceous Period

The Cretaceous Period

In the previous article, we focused on dinosaurs and pterosaurs that ruled the Earth in Mesozoic. Now it is time to talk about the Cretaceous period, to discuss their last years on Earth and their extinction.

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Blossom of the Flowering Plants

Plants had a generally good time during the Cretaceous. Conifers continued spreading, new species of ferns developed, and only two taxa of gymnosperms went extinct before the end of the period. Moreover, one more major kind of plant developed. Flowering plants are the kind that produces seeds, like the gymnosperms. Their ancestors actually came from the Triassic gymnosperms. However, they develop flowers as organs for reproduction and their seeds are inside a fruit. That adaptation made them one of the most successful land plant groups today.

We know more than 300,000 species and still new is discovered! New kind started to spread fast, diversifying and soon taking over many ecosystems. In addition, their evolution comes with the appearance of the first bees. Some insects evolved together with plants, adapting to one another. We call that phenomena coevolution.

A.liaoningensis it the oldest known flowering plant from the Cretaceous period.
A.liaoningensis it the oldest known flowering plant
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The First Birds of the Cretaceous period

As we discussed in the previous article, pterosaurs were the first vertebrates to rule the Earth. However, in the Cretaceous, they were joined by the only surviving dinosaur group. The exact origin of birds remains unknown. One hypothesis suggests, that at first small lizards developed an ability to fly. Another one says it comes from the ability of very high jumping as a form of prey hunting. Feathers may have evolved from dinosaurs coverings, probably mostly for thermoregulation purposes. Like flowering plants, birds quickly became a very successful group.

Also Read:
The Triassic Period (The first dinosaurs)
The Jurassic Period (The age of the dinosaurs)

By the end of the period, they occupied many ecological niches, often as main predators. For example, Hesperornithiformes were marine flightless birds. Other dominating species in the ocean included modern cartilaginous fish, including rays and sharks. On the land, most dinosaur species did not extinct until the end of the period, however, some large sauropods went extinct approximately in the middle.

First birds evolved during the Cretaceous period
First birds evolved during the Cretaceous period
Image Courtesy

Changes in Earth

In the early Cretaceous Earth started cooling. Snow became quite common, and high attitudes became permanently frozen. There was a little bit of glaciation in Southern Australia. However, after a while, things started to warm up again. It was probably due to the volcanic activity, which raised the levels of carbon dioxide. Fossils of animals can be found all over the globe, so many species adapted also to a bit of cooler conditions. There were no major global winds, so oceans were calmer than today. Moreover, they were also a lot warmer, on average about 37 degrees Celcius (99 F), compared to today’s 20 degrees (68 F).

When it comes to continents, everything continued to breakup. Gondwana separated into Australia, South America, Australia, and India, forming also the Indian Ocean.

Continents during the Cretaceous period.
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The Cretaceous-Paleogene Extinction Event

It comes with no surprise, that the period ends with a mass extinction event. It is also well-known that it was caused by an asteroid. However, let’s stop here for a moment. A massive body, 10 to 15 kilometers wide, a comet or an asteroid hit the planet. The dust covered the Earth, resulting in a sudden impact winter. Due to that, plants and plankton stopped photosynthesizing and died. After them, other species died from hunger and lack of oxygen. Totally, 75% of species went extinct, including almost all vertebrates weighing more than 25 kg.

As you can see, when producents die out, the food chain breaks and heterotrophic species have no chance of survival. The same thing would happen to us if the conditions repeated. As a result of the event, all non-avian dinosaurs and ammonites went extinct forever, many mammals, sharks, and birds also suffered. However sad it sounds, without that event we would not be here. Many ecological niches emptied and the next era is marked with radiation of many mammals, ourselves included.

All dinosaurs except birds went extinct during the K-Pg
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The Cretaceous Period Evidence

There are many pieces of evidence from the Cretaceous period. First of all, the chalk, for which the period is famous for. It accumulated due to seawater circulation through the shore ridges. As a result, a lot of calcium was in the water. Of course, last fossils of dinosaurs and pterosaurs come from the Cretaceous, so do the last ever ammonites. There are also many seastars, mollusks, and more shell-covered organisms.

We can also find sedimentary rocks, . From the land come first flowering species and many new ferns. There are also fossils of first birds, like Confuciusornis. There are also very similar rocks we can find in India, Australia, South America, and Antarctica, proving they were once one landmass. Moreover, if you look closely at a world map you can see, that those continents (especially Africa and South America) fit together like puzzles.

The Cretaceous period evidence includes fossils of first birds.
The Cretaceous period evidence includes fossils of first birds.
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Read the previous in the series: Dinosaurs: discussion and controversy.

Author’s message

I hope that the article helped you learn about the last period of the Mesozoic Era. If you enjoyed it, please share it with family and friends. In the next one, we will be moving on to the current era- The Cenozoic. Until then, if you have any questions feel free to contact me. You can use a form below or find my email address in the team’s section of the page.

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