The planktonic foraminiferal generaMorozovellaandAcarininarapidly (in ;10 k.y.) diversified during the late Paleocene thermal maximum (LPTM), giving rise to such mor- photypes asM. allisonensis(new species),M. africana, andA. sibaiyaensis. Single-specimen isotopicanalysisconfirmsthatM.allisonensisandA.sibaiyaensisarerestrictedtotheLPTM 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 effectsoftheLPTMonthecalcareousplankton.Single-specimenoxygenisotopedatashow that evolution of M. allisonensis and A. sibaiyaensis was accompanied by migration to deeper water depths. AncestralM. velascoensisandA. soldadoensiswere 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 facili- tatedthemorozovellidandacarininiddiversificationsduringtheoligotrophicconditionsof the LPTM.