Abstract.— Marine planktonic microfossils have provided some of the best examples of evolutionary
rates and patterns on multi-million-year time scales, including many instances of gradual evolution.
Lineage splitting as a result of speciation has also been claimed, but all such studies have used
subjective visual species discrimination, and interpretation has often been complicated by relatively
small sample sizes and oceanographic complexity at the study sites. Here we analyze measurements on
a collection of 10,200 individual tests of the Eocene planktonic foraminifer Turborotalia in 51
stratigraphically ordered samples from a site within the oceanographically stable tropical North Pacific
gyre. We use novel multivariate statistical clustering methods to test the hypothesis that a single
evolutionary species was present from 45 Ma to its extinction ca. 34 Ma. After identification of a set of
biologically relevant traits, the protocol we apply does not require a prior assignment of individuals to
species. We find that for most of the record, contemporaneous specimens form one morphological
cluster, which we interpret as an evolving species that shows quasi-continuous but non-directional
gradual evolutionary change (anagenesis). However, in the upper Eocene from ca. 36 to ca. 34 Ma there
are two clusters that persistently occupy distinct areas of morphospace, from which we infer that
speciation (cladogenesis) must have occurred.