SIZE, EXTINCTION, SURVIVORSHIP AND PHYLOGENY IN FORAMINIFERA

Definitions of the Lilliput Effect leave the role of phylogeny ambiguous. This is unfortunate because size is heritable and failure to take phylogenetic relations into consideration can mask important aspects of the evolutionary history of size variation. The foraminiferal fossil record provides many examples of the problems associated with treating size data in a non-phylogenetic context. Both planktonic and benthic foraminiferal species exhibit intraspecific decreases in gross test size across the K-T, P-E, and E-O boundaries. In all instances these size changes begin prior to the stage boundaries suggesting species adjust test size in response to long-term (10⁵-10⁶ years) secular changes in marine environments. In the case of the K-T event there is some evidence that the planktonic foraminiferal Lilliput event was caused by the differential survival of smaller species that subsequently served as ancestral stock for the Danian fauna, than by dwarfing per se, though there are also suggestions genuine dwarfing may have played a role. Detailed analysis of a Late Eocene dwarfing event suggests life-history traits, rather than test size, may be the true target of selection during planktonic foraminiferal Lilliput events. In a larger context, when evolutionary size change is viewed as the difference between sister species couplets, the Late Cretaceous interval is identified as a time of a remarkable net test size decrease in planktonic foraminifera. This contrasts with almost 100 years of accepted wisdom for this group and simply points up the need for, and value of, a more phylogenetic approach to data analysis. Inspection of Mesozoic-Cenozoic trends in phylogenetic size differentials identifies the Early Cretaceous and Eocene as intervals of particularly rapid and coordinated planktonic foraminiferal size increase. In addition to the Late Cretaceous, the Oligocene and Middle Miocene are seen as intervals of net phylogenetic size decrease. These trends are broadly consistent with the record of marine environmental variation, but contrast strongly with historical patterns of physical test size. Likely both phylogenetic and physical trends will be important in understanding the natural history of foraminiferal test size.