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During which geologic eras were chondrosteans most numerous and diverse?

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Russian sturgeon (Acipenser gueldenstaedtii) — chondrostean Russian sturgeon (*Acipenser gueldenstaedtii*) swimming along the seafloor of the Black Sea.

Chondrosteans were most numerous and displayed their greatest diversity during the latter part of the Paleozoic Era and the beginning of the Mesozoic Era.1