Periods of the Mesozoic Era

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Periods of the Mesozoic Era

A group of Plesiosaurs relaxing on a Jurassic Day. Getty/Mark Stevenson/Stocktrek Images

Just like all of the major Eras on the Geologic Time Scale, the Paleozoic Era ended with a mass extinction.  The Permian Mass Extinction is thought to be the largest loss of species in the history of the Earth.  Nearly 96% of all living species were wiped out due to large amounts of volcanic eruptions which led to a massive and relatively quick climate change in the Mesozoic Era.

The Mesozoic Era is often called the “age of the dinosaurs” because this is the time period in which dinosaurs evolved and eventually went extinct.  The Mesozoic Era is divided into three periods: the Triassic, Jurassic, and Cretaceous.

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Triassic Period (251 million years ago - 200 million years ago)

Pseudopalatus fossil from the Triassic Period. National Parks Service

The beginning of the Triassic Period was a pretty desolate time in terms of life forms on Earth.  Since there were so few species leftover from the Permian Mass Extinction, it took a very long time to repopulate and increase biodiversity once again.  The layout of the Earth was also changing during this time period.  At the beginning of the Mesozoic Era, all of the continents were together in one big landmass.  This supercontinent was named Pangaea.  The Triassic Period saw the beginnings of the continents breaking apart due to Plate Tectonics and Continental Drift.


As animals began to once again come out of the oceans and start to colonize the near empty land masses, they also began to learn to burrow to protect themselves from environmental changes.  Also appearing for the first time in history were amphibians like frogs and then reptiles like turtles, crocodiles, and eventually dinosaurs.  Towards the end of the Triassic Period, birds also first came into existence, branching off of the dinosaur line of the phylogenetic tree.

Plants also were few and far between, at first.  Throughout the period, they started to flourish once again.  Most land plants were conifers or ferns at the time.  Towards the end of the Triassic Period, some of the ferns had evolved seeds for reproduction.  Unfortunately, yet another mass extinction put an end to the Triassic Period.  This time, about 65% of the species on Earth did not survive.

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Jurassic Period (200 million years ago - 145 million years ago)

Plesiosaur cast from the Jurassic Period. Tim Evanson

After the Triassic Mass Extinction event, life and species diversification happened quickly to fill the niches that were left open.  Pangaea had now broken into two large pieces - Laurasia was the land mass in the north and Gondwana drifted southward.  Between these two new continents was the Tethys Sea.  The varied climates on each land allowed many new species to make their appearance for the first time, including lizards and small mammals.  However, the dinosaurs reigned supreme and even flying reptiles could be found in the skies during the Jurassic Period.


In the oceans, ray finned fish were plentiful.  Plants on land had flowers for the first time, too.  There were very lush and numerous areas for herbivores to feed and predators to stalk their prey in.  The Jurassic Period was somewhat like a renaissance time for life on Earth.

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Cretaceous Period (145 millions years ago - 65 million years ago)

Pacycephalasaurus fossil from the Cretaceous Period. Tim Evanson

The final period in the Mesozoic Era is known as the Cretaceous Period.  The favorable conditions for life on Earth continued from the Jurassic Period into the early Cretaceous Period.  Laurasia and Gondwana began to break into even more pieces and eventually would form the seven continents we see today.  As the land forms drifted further apart, the climate all over the Earth was warm and humid.  These were very favorable conditions for plant life to thrive.  Flowering plants began to proliferate and dominate on land.  Since the plant life was plentiful, herbivores also grew larger and, in turn, their predators also increased in size. Mammals were also starting to diversify, as did the dinosaurs.

Life in the oceans very similarly did well.  The warm and humid climates kept the sea levels high.  This helped increase biodiversity in marine biomes.  The entire tropical regions of Earth were encircled by this water, so the climatic conditions were pretty much ideal for diversity of life.

Once again, these nearly perfect conditions would have to come to an end sooner or later. This time, it is believed the mass extinction that ended the Cretaceous Period, and subsequently the entire Mesozoic Era, was caused by one or more large meteors crashing into the Earth.  The ash and dust that was thrown into the atmosphere mostly blocked out the sun, slowly killing off all of the lush plant life that had accumulated on land.  Likewise, much of the aquatic species also died off at this time.  Since the plants were fewer in numbers, anything that relied on them as a food source also died off.  This includes everything from insects to large birds and mammals, and, of course, all of the dinosaurs.  Only smaller animals that could burrow or survive on very little food made it through to see the Cenozoic Era.