Around 340 million years ago, the fragments of what had been a supercontinent, Rodinia, reassembled into a new and greater landmass—Pangaea. At its greatest, Pangaea stretched almost from pole to pole. The union of the continents into a single landmass had drastic consequences for life both on land and in the oceans. On land, forms of life that were once endemic to particular continents mixed and mingled with others. Competition between natives and newcomers was fierce, and many kinds of animals died out.
Sealife was most abundant on the continental shelf, the part of the sea closest to land. When the continents merged, there was less continental shelf to go around, and competition for living space in the sea was intense.
The climate itself became more challenging. The interior of Pangaea was mainly dry, even if punctuated by annual monsoon inundations, and often very hot. Although the cool southern regions of Pangaea were clothed in a seemingly endless scrub of a tree-fern called Glossopteris, plant life was not as luxuriant as it had been, in the age of the great Coal Forests. Less plant life meant that there was less oxygen than there once was, so much so that by the end of the Permian, breathing at sea level would have been like trying to catch a breath in the Himalayas today. Terrestrial life was left gasping.
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