TREE-THINKING AND MOLECULAR PHYLOGENIES OF FORAMINIFERA: WHERE DO FOSSILS FIT in?

Molecular phylogenetics and systematics have revolutionized our current understanding of the evolutionary relationships of Foraminifera; however, molecular studies are constrained by the necessary requirement that only living organisms can be sampled. Molecular phylogenetic trees, therefore, depict only crown groups (clades delimited by extant taxa). Where do fossil foraminiferans fit into these trees? Most microfossils recognized as foraminiferans fall within the crown clade Foraminifera. For example, Platysolenites, the oldest fossil unequivocally recognized as a foraminiferan, shares a suite of morphological characters with modern Ammobaculites and Bathysiphon species, indicating that this taxon is more closely related to multi-chambered foraminiferans than to single-chambered, allogromiid-grade foraminiferans. Reconciling the named subclades depicted in molecular phylogenies with the traditional rank-based classification of Foraminifera (e.g., Miliolida, Textulariida, Rotaliida) is complicated by the fact that traditionally defined taxonomic groups include both extant and extinct organisms. Thus, without explicit phylogenies based on morphological characters, it may be impossible to determine whether specific fossils fall within a crown group or represent stem lineages. This discrepancy may be problematic when researchers try to use the fossil ranges of traditional rank-based groups to calibrate the divergence times of crown groups in molecular studies. Unambiguous morphological characters do not currently exist that distinguish these crown groups. Therefore, It is essential that phylogenetic studies based on morphological data are conducted on each of the major subclades of Foraminifera, in order to distinguish crown vs. stem fossils, and to establish the sequences of character evolution.