Cretaceous period
The Cretaceous period is the third and final period of the Mesozoic era, starting 145 million years ago and ending 66 million years ago. The duration of the period was 79 million years. The period got its name from the abundant chalk deposits found all over the world, which testify to the great diversity of planktonic organisms of that time (especially algae - Coccolithophores). The Cretaceous period was eventful and is one of the key stages in the history of the development of life on Earth.
During this period, the division of huge continents continued. A little earlier, at the end of the Jurassic period, the northern continent of Laurasia divided into North America and Eurasia. Throughout the Cretaceous, the ocean between them - the North Atlantic - gradually expands. The division of the southern continents also began - India, South America, and Africa are breaking away from the giant Gondwana. Australia and Antarctica still exist as a single continent. As a result, the map of the Earth begins to acquire modern outlines.
The sea level fluctuated greatly, but on the whole, it was still significantly higher than it is today, creating numerous shallow seas. These were ideal conditions for the formation of new and the development of previously existing groups of marine organisms. The climate became a bit cooler than in the previous period, but overall remained warm and humid, with slight temperature differences between the poles and the equator. This facilitated the development of abundant vegetation and the growth of reptile diversity.
In the middle of the Cretaceous period, one of the largest changes in the world of plants took place. If in the first half of the Cretaceous period the world was dominated by gymnosperms and ferns, then from the second half of the Cretaceous period, the world began to belong to flowering (angiosperm) plants. Following the development of flowering plants, significant changes took place in the world of insects. Ancient groups of insects were replaced by familiar ants, bees, and wasps.
In the seas of the Cretaceous period, ammonites and belemnites, bony fish, and many others were also diverse. Sharks and various marine reptiles were at the top of the food chain. In the first half of the period, one could encounter ichthyosaurs, pliosaurs, and plesiosaurs. In the middle of the period, sea monitors - mosasaurs, for example, mosasaur (Mosasaurus) and tylosaurus (Tylosaurus) - came to replace the ichthyosaurs and pliosaurs.
By the Cretaceous period, dinosaurs had reached the peak of their diversity. That's when the Tyrannosaurus, Velociraptors, Triceratops, Ankylosaurus, and Parasaurolophus lived. The sky was ruled by ancient birds and pterosaurs. Among the feathered ones, the mysterious Enantiornithes were especially diverse, although another, modern group of birds - Ornithurae, was forming, and gradually increasing the number of species. Most birds still retained teeth and claws on their wings, although the first toothless forms existed.
Pterosaurs reached their maximum sizes throughout the history of their existence, presenting the world with such giants as the Quetzalcoatlus and Hatzegopteryx.
The period ended with one of the largest mass extinctions in Earth's history, known as the Cretaceous-Paleogene extinction. The causes of this event are still disputed to this day. Various scenarios are proposed: an asteroid impact, volcanic eruptions on the Indian subcontinent, various biological causes, and much more. However, the asteroid hypothesis remains the most popular. According to this theory, about 66 million years ago, an asteroid fell onto the territory now occupied by the Yucatan Peninsula (North America). Clouds of dust and ash obscured the sun for more than a year, causing a sharp cooling, as well as the death of most plants due to lack of light. Catastrophic climate changes and ecosystem destruction resulted in the mass extinction of numerous animals, from marine unicellular organisms to gigantic dinosaurs, marine reptiles, and pterosaurs. However, some species of mammals, amphibians, reptiles, insects, and birds were able to survive and adapt to new conditions.
The Mesozoic era ends with the Cretaceous period. After the Cretaceous period, the Cenozoic era began, which continues to this day. The mass extinction at the border of the Cretaceous and Paleogene periods became one of the defining events in the history of life on Earth, marking the start of a new cycle of evolution.
Asteroids and meteorites constantly fall to Earth, but it's important to understand the difference between them. Asteroids are celestial bodies that orbit the Sun, primarily located in the asteroid belt between the orbits of Mars and Jupiter. They can be large, from a meter to hundreds of kilometers in diameter. There are also meteoroids - "space rocks", ranging in size from a fraction of a millimeter to 30 meters. Lastly, there is tiny "space dust". Meteorites, on the other hand, are any celestial bodies that have fallen to Earth. They can be of any size, and may originally be a comet, an asteroid, a meteoroid, or dust, the main point is that they have fallen onto the planet, then they can be called a "meteorite". Meteorites the size of a grain of sand or pebble fall every day, burning up in the atmosphere and not reaching the surface. Large asteroids capable of causing serious damage fall very rarely - for example, asteroids about 1 kilometer in size can hit Earth roughly once every 500,000 years. Asteroids capable of causing global catastrophes fall even more rarely, approximately once every several million years. Scientists actively observe asteroids to predict their approach to Earth in advance.
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