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CRETACEOUS

CRETACEOUS

Paleogeography

Pangea’s remaining continental spaces fell apart during the Cretaceous. Its southern section Gondwana split into separate continents: South America, Africa and India. Antarctica and Australia remained a single continent. When South America detached itself from Africa, the southern Atlantic Ocean opened. At the beginning of the Cretaceous, Pangea’s northern section Laurasia was still a single mass consisting of North America, Greenland and Eurasia. At the end of the Cretaceous, rifting and ocean formation detached North America from Laurasia.

Africa’s accelerated northeast movement also initiated subduction processes at the northern edge of the Tethys, in front of the Eurasian continental space, causing the Africa-Eurasia convergence.

Paleoclimate

The Lower Cretaceous saw a rise in average air and ocean temperatures to the highest levels in Earth history, to about 30°C (compared to present-day 15°C). This was partly due to the rising levels of carbon dioxide in the seas, resulting from intensive volcanic eruptions caused by the separation of continents and spreading of oceans. The carbon dioxide dissolved in the seas evaporated into the atmosphere, creating a greenhouse effect. The warmer climate raised the sea temperatures and increased the volume of water. Coupled with the elevated mid-ocean ridges caused by ocean spreading, this resulted in higher sea levels. Deep ocean water temperature was 13°-19°C (compared to present-day 3-8°C). The cold polar waters disappeared. Their higher density caused them to sink to the sea floor and supply it with oxygen. Only the shallow waters near the surface had oxygen, and the conditions below were anoxic.

Terrestrial life

Fauna – All dinosaur groups were present during the Cretaceous. They mostly lived in herds. The Albertosaurus (1), the Tyrannosaurus (2) and the Velociraptor were the most common among carnivorous Saurischia, and the Argentinosaurus (3), the largest dinosaur in history, was the most common herbivore. Some new members of the Ornithischia order appeared, such as the horned ceratops, the most notable representative of which is the Triceratops (4), and the duck-billed hadrosaurids, such as the cute Parasaurolophus (5), and the armoured ankylosaurs, such as the Ankylosaurus (6). The flying reptiles, known as the Pterosauria, ruled the skies. Most were scavengers, and some had wing spans up to 15 metres, such as the Quetzalcoatlus. The Pteranodon (7) was somewhat smaller, with a wing span of 8 metres and a long crest that it used for steering, since it had no tail.

Frogs, turtles and snakes, whose close relatives survive to this day, were common inhabitants of water environments. Mammals diversified, even though they were still small, approximately the size of present-day rodents. Marsupials and placental mammals were the most important new mammal groups. 

Flora – Angiosperms (8) were the greatest novelty in the Cretaceous ecosystem. Angiosperm seeds are contained in round ovaries, a modification of pteridophytes’ leaves. They first appeared in the Middle Cretaceous, and by the Upper Cretaceous they already boasted a greater diversity of species than the gymnosperms. Angiosperms owed their dominance to more effective seed nutrition in the ovary and to faster seed maturation. The scent of their flowers attracted insects, who disseminated pollen, pollinating other plants and helping the angiosperms spread.

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Život u moru

Oceans teemed with plankton, which played an important role in building lagoon sediments. Diatoms (9) appeared in the Middle Cretaceous and became a constituent part of the deep-sea sediments along with planktic foraminifera from the zooplankton group. Calcareous algae, in particular green calcareous algae (10) from the Dasycladaceae family, also played an important part in the oceans. Corals (11) and bryozoans were important reef-builders in the Cretaceous. Bivalves inhabiting reef environments, especially rudists (12), were at the height of their development in this period. Ammonites (13), the marine predators, were still the dominant cephalopods. Brachiopods, Echinodermata and crustaceans were also present. The first seaweeds appeared in the Cretaceous seas, carpeting parts of the sea floor and stabilising them with their roots. Osteichthyes were the dominant fish class, although Chondrichthyans (sharks) were also common. The development of reptiles continued. Some grew to a length of 15 metres, such as the Tylosaurus (14).

Pror-Velebit sedimentary basin

The closure of the Tethys caused powerful tectonic disturbances in the Dinaric Tethys, accompanied by oceanic crust subduction and the formation of several deep sea trenches and seamounts. Deep sea sedimentation occurred in this space, with the oceanic crust overthrusting the adjacent tectonic blocks in some locations, and a metamorphosis of the deep-sea sediments taking place at the same time. The entire Dinaric Tethys was thus affected by complex tectonic movements, volcanic eruptions, metamorphism and complex sedimentation processes on both sides of the Adriatic carbonate platform. The platform further converged on Eurasia in the Upper Cretaceous, with more pronounced landscape differentiation. Parts of the platform became mainland, some retained the same features, and deep-sea trenches were formed in some locations, characterised by sedimentation of deep-sea limestone deposits. Most of the platform became mainland at the end of the Cretaceous, interspersed with occasional seas.

 

Extinction

A global catastrophe caused the extinction of around 60% species at the end of the Cretaceous. This was not the biggest extinction event in Earth history, but it is the most famous one, because it ended the age of the dinosaurs. The extinction event was caused by a large asteroid impact in the Yucatan Peninsula, but also by volcanic eruptions in present-day India, North America, Greenland, Great Britain and Western Pacific. The asteroid impact caused an enormous tsunami that flooded all continental lowlands. The substantial amount of dust, steam, carbon dioxide and ash released by the asteroid impact in several stages of volcanic eruption resulted in global cooling and warming episodes that caused many species to become extinct.

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