The planktonic foraminiferal genera Morozovella and Acarinina rapidly (in ∼10 k.y.) diversified during the late Paleocene thermal maximum (LPTM), giving rise to such morphotypes as M. allisonensis (new species), M. africana, and A. sibaiyaensis. Single-specimen isotopic analysis confirms that M. allisonensis and A. sibaiyaensis are restricted to the LPTM carbon isotope excursion recorded at Ocean Drilling Program Site 865 (equatorial Pacific Ocean). The short-lived (50 to several 100 k.y.) “excursion” taxa attest to the ephemeral effects of the LPTM on the calcareous plankton. Single-specimen oxygen isotope data show that evolution of M. allisonensis and A. sibaiyaensis was accompanied by migration to deeper water depths. Ancestral M. velascoensis and A. soldadoensis were extremely rare or absent during the early stages of the LPTM, but immigrated back into the study area to coexist with their descendants in later LPTM horizons. Photosymbiosis may have facilitated the morozovellid and acarininid diversifications during the oligotrophic conditions of the LPTM.