Blackwell Publishing
Precambrian Paleozoic Mesozoic Cainozoic
Cambrian 590 MYRBP Ordovician 438 MYRBP Silurian 438 MYRBP Devonian 408 MYRBP Carboniferous 360 MYRBP Permian 286 MYRBP

The earliest tetrapods - four footed vertebrates adapted for life on land, such as this icthyostega - are thought to have appeared during the Devonian. All life on land faced the following problems:

• without the buoyancy of water, the body weight of the animal can crush internal organs.

• the extreme temperature changes out of water require homeostatic mechanisms.

Strong bones and sophisticated metabolisms were among many of the new evolutionary innovations which enabled vertebrates to colonize the land.

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