Cladogenetic, Extinction and Survivorship Patterns from a Lineage Phylogeny: The Paleogene Planktonic Foraminifera

Paleontological analyses of patterns of evolution at the 'species level' too often make uncritical use of taxonomic range data gathered primarily for biostratigraphic purposes. In this contributon, taxonomic evolution is studied from a stratophenetic lineage phylogeny of Paleogne planktonic foraminifera. This phylogeny, previously published in Micropaleontology, was constructed to eliminate 'pseudospeciation' and 'pseudoextinction' from the record, leaving only genuine cladogenesis (lineage branching) and terminal extinction. The phylogeny thus obtained obeys a valid cladistic geometry. Following the Cretaceous/Paleogene boundary extinctions, the planktonic foraminifera made a rapid initial recovery in less than one million years to a more stable level of diversity. Lineage diversity and morphologic disparity ultimately reached a maximum in the middle Eocene. Diversity fluctuations in the group during most of the Paleogene result primarily from the expansion and decline of one sub-clade, the muricates. Rates of cladogenesis and extinction in the planktonic foraminifera as a whole did not covary. Cladogenesis peaked in the earliest Paleocene and the late early Eocene. Enhanced extinction occurred in the mid-Paleocene and through the middle Eocene. Neither the Paleocene/Eocene nor Eocene/Oligocene boundaries were exceptional events in the evolutionary history of planktonic foraminifera. Survivorship curves, both uncorrected and normalised to variations in the general extinction probability (the 'corrected survivorship score' technique) are approximately straight, in contrast with the case of morphospecies longevities, which show a strongly age-dependent pattern (Pearson 1992). This suggests that the latter pattern is largely an artifact of taxonomic data gathering. A further interesting feature is evident from the lineage phylogeny: those lineages which terminate in branching have a mean duration which is less than half that of those which terminate in extinction, pointing to a strongly radiative pattern of evolution in the group.