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" (1 found)
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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
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Data sourced from Wiktionary, WordNet, CMU, and other open linguistic databases. Updated March 2026.