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.
Source: wiktionary
Mya
Synonyms for "mya" (8 found)
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Noun(2 words)
abbreviationalternative abbreviation
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Noun(2 words)
alternative notationmegaannum
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Noun(4 words)
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6 entriesearly myafossil datinggeologic timescalelate myamillion years agopaleontological dating
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2 total sentences available.
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 4.0 bya and near the end of the era (570 mya) the oldest known fossils of multi-celled organisms appear.
Source: wiktionary