Cenozoic Eras have periods. Earth was dominated by bug-like creatures called trilobites 2. Earth was dominated by coral 4. Earth was dominated by fish with jaws 5. Earth was dominated by small reptiles 2.
Earth is believed to be almost five billion years old.
Early Earth was a hot mass which cooled to form a crust. Inside the Earth it is still very hot today. The dense parts of the Earth do not float and these form the beds of oceans. However, what we call land is less dense and floats on the liquid core of the Earth. Land formation gave rise to early continents.