Pre-Slavic Finno-Ugric peoples of the Volga-Oka forest zone — the Merya (attested by the 6th century, per Jordanes, and detailed in the Primary Chronicle), the Muroma (archaeologically dated to the 7th century), and the Meshchera — occupying the lands later forming the heartland of Vladimir-Suzdal and Ryazan, well before Slavic settlement.
Peacefully assimilated by incoming East Slavs as their territories were absorbed into Kievan Rus': the Merya by the 10th century, the Muroma and Meshchera by the 11th-12th centuries. Some inhabitants of the Kostroma/Yaroslavl region and the Oka basin identify with Meryan or Meshcheran heritage today, but the peoples ceased to exist as distinct groups.
Standard survey covering the Finno-Ugric substrate peoples of the Volga-Oka basin absorbed during the formation of Rus'.