second era of the Phanerozoic eon

The Mesozoic 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 66 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 seem to have originated in the Upper Triassic, their fate was quite different to the dinosaurs. Mammals spent most of the long Jurassic and Cretaceous periods as small nocturnal animals living mainly in forests. They lost their colour vision, and only partly re-evolved it in the Cainozoic era.

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 66 mya also marks the beginning of the Cenozoic era, the one we live in.

The three Mesozoic periods were:


The ecology of this world was quite different to that of the Pleistocene, the era when humans evolved. For much of human history we lived in a cold world with quite a sharp difference between land and sea.

The Mesozoic was a much warmer world, with a higher temperature, and a high water stand. There was little or no ice at the poles and the continents had large areas covered with shallow continental seas.[2]

Moreover, the continents were all quite near each other, or actually touching. Many land animals could walk or swim to other continents. Actually, the Mesozoic starts off with all the continents together in Pangaea. They stayed pretty close for the rest of the Mesozoic.

Textbooks have always described how the Mesozoic was the age of the dinosaurs,[3] which has even more sense now that we know birds descended from land dinosaurs. It was the age, too, of extraordinary marine reptiles. Mammals were mainly small nocturnal animals which scurried around in the night-times of the great forests. We know from their loss of colour vision (which is trichromatic in fish and reptiles) that they lived a nocturnal life.


  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
  2. Van Andel, Tjeerd H. 2nd ed 1994. New views on an old planet. Cambridge University Press. ISBN 0-521-44755-0
  3. Benton, Michael J. 2015. Vertebrate palaeontology, 4th ed. Wiley Blackwell 10.3 Mesozoic world, p204. ISBN 978-1-118-40684-7