Blackwell Publishing
Precambrian Paleozoic Mesozoic Cainozoic
Tertiary 65 MYRBP Quaternary 1.8 MYRBP

Horses first appear in the Tertiary, and provide a good example of Cope's Law - the trend for evolution to lead to larger animals. The first horse-like mammals, hyracotheria, were considerably smaller than a member of the modern genus Equus.

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