Four consecutive planktic foraminiferal faunas characterize the planktic transformation recorded from continuous Cretaceous-Tertiary boundary successions of Mexico; from older to younger, these are mayaroensis, aegyptiaca, relict, and cancellate. The mayaroensis fauna, containing the nominal taxon and lacking gansseri, is followed by the aegyptiaca fauna, characterized by the absence of mayaroensis, indicating that this taxon disappeared before the major extinction of Cretaceous Hedbergellacea. The relict (“small globigerinas”) fauna contains taxa whose wall structure and umbilical apparatuses are directly linked to Cretaceous Hedbergellacea rather than Tertiary Globigerinacea. The cancellate fauna represents the first appearance of true Tertiary forms. The mayaroensis and aegyptiaca faunas are found immediately below the spherulite and clastic units attributed to the asteroid impact in Yucatan. The relict fauna is found immediately above the clastic unit, whereas the cancellate fauna is usually found ∼30 cm above this unit. The stratigraphic positions of the four consecutive faunas indicate that (1) the faunal changes observed started before the physical features attributed to the impact, and (2) the major evolutionary planktic event, i.e., the appearance of the cancellate wall, is the only unequivocal feature that defines the entry of the Tertiary planktic foraminiferal fauna.

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