The Geologic Time Scale

Eons

Phanerozoic (visible life) Eon (0.544 billion years [b.y. or Ga {giga annum}]->the present)

Proterozoic (earlier life) Eon (2.5->0.544 b.y.)

Archean (ancient life) Eon (4->2.5 b.y.)

Hadean Eon (4.55->4 b.y.)

Proterozoic + Archean + Hadean represents about 90% of Earth history

Phanerozoic Eras

Cenozoic (recent life)(65 m.y. ->the present)

Mesozoic (middle life)(250->65 m.y.)

Paleozoic (old life)(544->250 million years [m.y. or Ma {mega annum}])

Phanerozoic Periods

Cenozoic - Quaternary (2 m.y. ->the present)

Tertiary (65->2 m.y.)

Mesozoic - Cretaceous (144->65 m.y.)

Jurassic (206->144 m.y.)

Triassic (250->206 m.y.)

Paleozoic - Permian (290->250 m.y.)

Carboniferous (355->290 m.y.)

Devonian (410->355 m.y.)

Silurian (438->410 m.y.)

Ordovician (510->438 m.y.)

Cambrian (544->510 m.y.)

^ note: the Triassic/Mesozoic starts at 245 m.y. in your text

Mesozoic Period Subdivisions

Cretaceous

Late 65 - 97 m.y.

Early 97 - 144 m.y.

Jurassic

Late 144 - 157 m.y.

Middle 157 - 178 m.y.

Early 178 - 206 m.y.

Triassic

Late 206 - 231 m.y.