Mesozoic Era is the stage that goes from the end of the Paleozoic with the great extinction of the Permian (252 million years ago) and ends with the entry into the Cenozoic 65 million years ago (great extinction of the Cretaceous Period).
The Mesozoic Era is comprised of three stages:
Triassic Period: It begins 252 million years ago and ends 200 million years ago.
Jurassic Period: It begins 200 million years ago and ends 145 million years ago.
Cretaceous Period: It begins 145 million years ago and ends 65 million years ago. It’s the longest period of the Mesozoic.
The Mesozoic is characterized by a warm and humid Earth. Oxygen levels were very similar to today’s levels, thanks to the large coniferous forests that proliferated in the early Triassic period but were replaced by Angiospermae forests in the early Cretaceous.
We pass from a dry and arid climate at the beginning of the Triassic as we leave an extinction that left the continents dry, to still warm climates but with greater humidity at the beginning of the Jurassic and remain so until the end of the Cretaceous. It is thought that in the Cretaceous, the North Pole was able to hold ice in winter.
We began the Mesozoic Era with a supercontinent “massif” Pangea, with a large desert and no water in the center of the continent, surrounded by the great Panthalassa Ocean. It begins to open to the right, giving a C shape, giving rise to two supercontinents, Laurasia (North) and Gondwana (South), separated in the center by the Tethys Sea.
Life in the Mesozoic
The entrance to the Mesozoic is after a great extinction, one of the three largest that has occurred on Earth. Life re-emerged again with only a small percentage of species before the great Permian extinction, something incredible.
Recent studies show that, after a catastrophe that leaves few survivors (but different enough from each other to have several species, not only animals but also plants), evolution is accelerating, that is, there are a large number of niches and empty spaces that can house any kind of life, so species compete to be the first to arrive and stay in that territory.
It is this process that has allowed the Earth to continue to live after each extinction, and that after extinction life has changed and varied. In addition, in the Mesozoic, there are two important facts, one in plants and the other in animals.
Angiosperms appear on the plants. In the Carboniferous (approximately 359 million years ago), gymnosperms appeared and displaced ferns from terrestrial ecosystems. During the Paleozoic period conifer forests were very abundant, covering practically the entire Earth, but in the Cretaceous period angiosperms (plants with flowers) appeared and began to displace gymnosperms.
It is known with certainty that this fact was in the Cretaceous, obtaining a complete fossil plant, but some authors think that the plants with flowers could appear before (in the Jurassic) and that later in the Cretaceous they would begin to displace in great measure the conifers.
Birds appear in animals. Although the Mesozoic is known as the “Age of the Dinosaurs”, and is a very striking fact since they would be the largest land animals that dominated the Earth, at an evolutionary level and looking at a more current point in the Mesozoic we find the origin of birds, the current descendants of the dinosaurs (and that, therefore, we can consider them without fear of doubt as dinosaurs).
The birds come from a family of Saurian dinosaurs, theropod dinosaurs, that is, dinosaurs with hips similar to those of a reptile and carnivores.
The appearance of the feather was essential for the dinosaurs to become the current birds. The pen did not originate as an instrument for the airborne environment but originated for thermoregulation.
The feather is a great insulator of the heat and allows to regulate the corporal temperature, besides being covered in a wax that waterproofs the animal. The primitive feather (proto-plume) is more similar to a hair than to a current macroscopic feather, but if we look a little closer we can see the rachis (central rod) from which the threads that make up the feathers fork.
End of the Mesozoic
The Mesozoic ended about 65 million years ago in the great extinction of the Cretaceous, ending the “Age of the Dinosaurs” and giving way to the Cenozoic and the “Age of Mammals”.
This extinction is the best known, and not only the non-avian dinosaurs but also other groups of Archosaurs such as pteranodons (aerial reptiles) or plesiosaurs and ichthyosaurs (aquatic reptiles) become extinct. Therefore, the great extinction of the Cretaceous affects mainly the Archosauromorpha clade, although there was also extinction at the plant level and of other animal taxa such as amphibians, fish or mammals (although to a lesser extent compared to reptiles).