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The Cretaceous Period: (144 million years ago)

 

The Cretaceous is usually noted for being the last portion of the "Age of Dinosaurs", but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared. Also during this time, we find the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants.

 

The breakup of the world-continent Pangaea, which began to disperse during the Jurassic, continued. This led to increased regional differences in floras and faunas between the northern and southern continents.

 

On land, mammals were a small and still relatively minor component of the fauna. Early marsupial mammals evolved in the Early Cretaceous, with true placentals emerging in the Late Cretaceous period. The fauna was dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse stage.

 

 

 

Tyrannosaurus rex	Pterosaur

Climate

 

There was no polar ice during the mild warm, subtropical Cretaceous. The land was covered with forests surrounded by shallow seas. Seasonality was increasing. Most of the land mass was at or around sea level until the mid-Cretaceous, a time of high tectonic activity (continental plate movement) and accompanying volcanic activity . This is when many mountain ranges were formed, including California's Sierra Nevadas, the Rocky Mountains in the western USA and the European Alps. The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. Also, the continents were taking on their modern-day forms.

The End of the Cretaceous Period

At the end of the Cretaceous period, 65 million years ago, an asteroid hit Earth in the Yucatán Peninsula, Mexico, forming what is today called the Chicxulub impact crater. It has been estimated that half of the world's species went extinct at about this time, but no accurate species count exists for all groups of organisms. Some have argued that many of the species to go extinct did so before the impact, perhaps because of environmental changes occuring at this time. Whatever its cause, this extinction event marks the end of the Cretaceous and of the Mesozoic Era.

 

Map of the Earth during the Cretaceous Period

 

 

 

 

 

 

Paleozoic Era

Mesozoic Era