Era
Cretaceous
18
Permian
10
Mississippian
9
Proterozoic
8
Precambrian
6
Cenozoic
4
Paleozoic
4
Quaternary
4
Tertiary
4
Devonian
3
1952-
2
Carboniferous
2
Eocene
2
Pennsylvanian
2
Pleistocene
2
1935-
1
1935-1998
1
1941-
1
1942-
1
1943-2017
1
1951-
1
1954-
1
1955-
1
1961-
1
1965-
1
20th century
1
Jurassic
1
Mesozoic
1
Paleocene
1