mya

Both tabulates and rugosans evolved independently as part of the Ordovician Radiation; the tabulates appeared first in the Early Ordovician (~488 Mya), followed by rugosans about 20 My later.

The first (oldest) unit of time is the Precambrian Supereon, which covers much of Earth's history from its origins roughly 4.55 bya to 545 mya. Very primitive, one-celled life-forms arose as early as