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Subject (Era)
- Quaternary54
- Pleistocene22
- Cretaceous20
- Paleozoic18
- From 2 million years ago15
- Devonian13
- Precambrian13
- From 65 to 140 million years ago11
- Cenozoic10
- Ordovician10
- Pre-Quaternary10
- Tertiary10
- Silurian9
- From 230 to 570 million years ago8
- Jurassic8
- Pennsylvanian8
- Mesozoic7
- Mississippian7
- Cambrian6
- From 2 to 65 million years ago6