The mass-extinction event at the Cretaceous/Tertiary boundary (C/T in text; K-T on figures) exterminated all but one species of the planktonic Foraminifera (Guembelitria cretacea Cushman). Although not all details have been clarified yet, all Paleocene planktonic Foraminifera may have evolved from this sole survivor. Globigerina minutula Luterbacher and Premoli Silva is the first true Paleocene species to appear; it develops into Globigerina fringa Subbotina, and later on probably into Globigerina eugubina Luterbacher and Premoli Silva. Essentially, the stratigraphy of several complete sections shows a similar pattern: abrupt extinction of plankton followed by deposition of a thin lamina with high concentrations of siderophile elements, considered as the direct fallout level of the impact event. This lamina is followed by a 1- to 30-cm-thick clay or marl layer (which represents the normal background supply of hemipelagic clay) and a gradual return to calcareous sedimentation associated with the appearance of the first new Paleocene species.

The biostratigraphy of the Gredero section in southeast Spain is analyzed in detail, complemented with data from the Kef section in Northern Tunisia. A new zone, the Guembelitria cretacea Zone, which contains only G. cretacea, and possibly Globotruncanella monmouthensis (Olsson) and Globigerinelloides messinae Brönnimann in situ, is established at the base of the Tertiary. It is essentially the same as the C/T boundary clay. New taxonomic descriptions are given of G. cretacea, Globigerina minutula, G. fringa, G. eugubina, and Globotruncanella caravacaensis n. sp. Preliminary results of a paleomagnetic survey are used to calculate sediment accumulation rates, from which the mass extinction is inferred to have occurred within 50 years and a new stable planktonic fauna re-established within 35,000 years. The earliest Paleocene faunas are highly unstable. Different species successively become dominant and show a rapid evolutionary development. The initial development thus is both explosive and unstable and seems to be consistent with the punctuated mode of evolution.