8.I: Introduction And Geological Time

Chapter 8 mostly belongs to what geologists call the Mesozoic or “Middle Life” era.  (See linear timeline).  Chapter 9 left off with the Paleozoic or “Old Life” era.  After the Mesozoic came the present-day Cenozoic or “New Life” era.  These eras are all part of the gigayear Phanerozoic Eon, which is not labeled on the timeline below but was put into broader context in Chapter 9.

Recall that eras are divided into periods, based on changing layers of sedimentation and fossil records.  Eras run hundreds of millions of years, and periods are subunits spanning tens of millions of years.  The last period of the Paleozoic Era was the Permian.  The three periods of the Mesozoic Era were the Triassic, then the Jurassic, and finally the Cretaceous.  The first period of the Cenozoic Era was the Paleogene.  It is almost impossible to keep these crazy names straight, so I find the timeline indispensable to summarize these divisions of time. 

Section II is a global overview of the Mesozoic Era.  This era was dramatically bookended by two major mass extinctions.  The mass extinction that ended the Paleozoic era, called the Permian-Triassic or simply the P-T event, was the worst known mass extinction in world history.  It set the stage for dinosaurs, the dominant and most famous lifeform of the Mesozoic.  The dinosaurs themselves, along with other orders of life, went extinct in the Cretaceous-Paleogene or K-Pg event.  Both of these mass extinctions had significant influence on the evolution of our ancestors.  The P-T event was an evolutionary bottleneck, narrowing down the gene pool and leaving only a lucky minority of species to repopulate the Earth. 

When dinosaurs perished in the K-Pg event, it cleared the way for mammals to flourish in a variety of niches.  Section III takes a closer look at mammals, our biological class.  By the end of this timescale, our ancestors were primates who were already beginning to show signs of social and neurological sophistication. 


Continue to Section 8.II: The Mesozoic World

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