Biology Index
By Richard Bruce BA, MA, and PhC in Economics

A summary of the 3 Periods of the Age of Dinosaurs, the Mesozoic

This simple summary of the periods of the Mesozoic may help you remember them and thus more easily read the literature on the period. I have written this because a similar summary of the Cenozoic has proven popular.

The Cenozoic which follows the Mesozoic is called the Age of Mammals. The Mesozoic is called the Age of Dinosaurs. Before the Mesozoic was the Paleozoic.

The Three Periods of the Mesozoic

  1. Triassic
  2. Jurassic
  3. Cretaceous

Triassic 252 to 201 million years ago

The Paleozoic ended and the Mesozoic began with was a huge extinction event, frequently called the Great Dying. It was even more severe than the extinction event that wiped out all the dinosaurs with the exception of the birds at the end of the Mesozoic. It is believed the extinction event that began the Mesozoic was caused by vast volcanic eruptions in what is now Siberia.

The Triassic was the first period of the Mesozoic. Both dinosaurs and mammals evolved during the Triassic. Mammals evolved from the therapsids, also called proto-mammals or mammal-like reptiles. Dinosaurs evolved from archosaurs also called proto-dinosaurs.

In the scientific literature mammals are therapsids and dinosaurs are archosaurs. Similarly birds are dinosaurs, apes are monkeys, and humans didn't evolve from apes, we are apes. Each species is a considered a member of the group from which it evolved.

Neither dinosaurs nor mammals played a prominent role in the Triassic. The central conflict was the competition between their predecessors: the other groups of therapsids and archosaurs.

There were no famous species from the Triassic. Many of you would not know the name of a single dinosaur, mammal, therapsid, or archosaur from the Triassic. If you bought a bag of plastic animals from the Mesozoic it is likely that none would be from the Triassic.

Jurassic 201 to 145 million years ago

The Triassic ended and the Jurassic began with another extinction event, less severe than the event that divides the Paleozoic from the Mesozoic. In the Jurassic dinosaurs dominated the large terrestrial niches, and one group of dinosaurs, the birds, evolved flight. The real age of dinosaurs began.

The Jurassic is an age of giant dinosaurs, the long-necked sauropods. The other famous dinosaur from this era was the stegosaurus which had a row of bony plates down it back and spikes on its tail. The spiked tail is called a thagomizer after a fictional caveman, named Thag. In a Far Side comic by Garry Larson it is implied that Thag was killed by a thagomizer. The famous large predators belonged to the genus Allosaurus.

Cretaceous 145 to 66 million years ago

The final period of the Mesozoic is the Cretaceous. This is the era of Tyrannosaurus rex, Triceratops, the duck-billed dinosaurs which scientists call Hadrosaurids, the Ankylosaurus with the bony, armored back and a tail with a bony club at the end, and the smaller preditory raptors. Many of the Dinosaurs in the "Jurassic Park" movies were actually from the Cretaceous.

The Cretaceous ended with still another massive extinction. The dinosaurs, except for the birds became extinct. The literature will frequently say the non-avian dinosaurs went extinct. Avian means birds. The birds and mammals lived on into our era, the Cenozoic.

Other pages on Natural History and Biology

If you wish to read more of my summary of natural history, the next period the Cenozoic, the Age of Mammals, is covered here. There are also a number of other essays on Biology you can find through my Biology Index Page. The Biology index provides links to a couple of pages on the Mesozoic. One is my speculative history of the Mesozoic where I suggest explanations for the rise of the archosaurs, the large size of the dinosaurs, and other Mesozoic mysteries. Another looks at why Quetzalcoatlus, the last of the pterosaurs may have been the largest.
Biology Index Page

Written April, 15, 2015

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Biology Index