During the Paleozoic Era (542 to 248 million years ago), which is itself
divided into various periods (Cambrian, Ordovician, Silurian, Devonian,
Carboniferous, Permian), the first fish, and later amphibians, reptiles
and mammal-like reptiles, all appeared. At the the end of the
Era, there was some terrible disaster (the "Permian mass extinction")
when many creatures became extinct.
During the early part of Mesozoic Era,
the mammal-like reptiles
dramatically declined, apparently out competed by the
they did give rise to the first mammals, which were mostly tiny mouse-size
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