Mesozoic

The Mesozoic Era was the geological era in which dinosaurs lived, as well as the first mammals. It lasted about 186 million years, starting 252.2 mya (million years ago) with the P/Tr extinction and ending 65 mya with the K/T extinction (the one that killed all dinosaurs except birds).^[1]

Dinosaurs appeared around 231 mya (21 million years after the beginning of the Mesozoic). They evolved from the reptiles called archosaurs. Modern birds and crocodiles are part of that group.

Although mammals originated in the Upper Triassic, they did not live like dinosaurs. Mammals spent most of the long Jurassic and Cretaceous periods as small nocturnal animals living mainly in forests. Most lost their color vision. Mammals today have some color vision. Most have red-green color blindness, with only two types of cone cells. Some primates evolved a third type of cone cell, which gives them trichromacy. Color is needed in forests to spot ripe fruit.

The Mesozoic is the middle of the three eras that make up the Phanerozoic eon. Before the Mesozoic was the Palaeozoic era. The K/T extinction in 65 mya also marks the beginning of the Cenozoic era, the one we live in.

The three Mesozoic periods were:

• Triassic (252–201 mya)
  • Lower Triassic (252–247 mya)
  • Middle Triassic (247–237 mya)
  • Upper Triassic (237–201 mya)
• Jurassic (201–145 mya)
  • Lower Jurassic (201–176 mya)
  • Middle Jurassic (176–161 mya)
  • Upper Jurassic (161–145 mya)
• Cretaceous (145–66 mya)
  • Lower Cretaceous (145–100 mya)
  • Upper Cretaceous (100–65 mya)

1. ↑ Gradstein, Felix M. James G. Ogg, Alan G. Smith (eds) 2005. A geologic time scale 2004. Cambridge University Press. ISBN 0-521-78673-8