The extinction of the Morozovella velascoensis lineage, a group of planktonic foraminifera that dominated tropical assemblages for ∼6.3 m.y., is investigated using an upper Paleocene/lower Eocene section from the central Pacific Ocean (ODP Site 865). Uppermost stratigraphic occurrences of the lineage consist solely of diminutive M. edgari, a species roughly half the size of its ancestor, M. velascoensis. Eigenshape analysis indicates that adult specimens of M. edgari are morphologically similar to small, presumably juvenile, forms of M. velascoensis. The overall stratigraphic succession, where the uppermost occurrences of a lineage are represented by relict populations of minute, juvenilized forms, is consistent with a process called “terminal progenesis.” Various lines of evidence indicate that stress-induced size reduction foreshadowed the extinction of other planktonic foraminiferal lineages as well.

The ecological cause(s) for the extinction of ancestral M. velascoensis is unclear. Stable isotopic evidence for a progressive deterioration of algal symbiosis was not detected. Specifically, none of the individually-analyzed shells of M. velascoensis were depleted significantly in ¹³C, nor was there a decrease in the covariance of the δ¹³C/size relationship.

Planktonic foraminiferal δ¹⁸O values exhibit a subtle increase over the stratigraphic interval in which the M. edgari extinction is recorded. This inferred tropical cooling appears to be correlative with terrestrial records of cooling from the North American continental interior and northwestern Europe. Thus, it is postulated that climatic variation during the earliest Eocene (∼54.02Ma) played a significant role in shaping biodiversity in the pelagic realm, sealing the doom of the declining M. velascoensis lineage.