A PHYLOGENETIC PERSPECTIVE ON THE EVOLUTION OF GIGANTISM IN FUSULINACEA (FORAMINIFERA)

Fusulinacea is a monophyletic clade of fossil Foraminifera that branches within the more inclusive clade Globothalamea, as the sister clade to Endothyracea. The clade Fusulinacea originated in the Late Mississippian, and diversified in the Pennsylvanian and Early Permian. Several lineages went extinct during the end-Guadalupian crisis; none of the remaining lineages survived the end-Permian mass extinction.

A cladistic analysis of representative taxa from each of the major higher taxonomic groups of Fusulinacea (Ozawainellidae, Staffellidae, Schubertellidae, Fusulinidae, Schwagerinidae, Verbeekinidae & Neoschwagerinidae) is used to examine the evolution of gigantism in the subclades Schwagerinidae and Neoschwagerinidae. In the schwagerinids, test gigantism appears to be correlated with increased prolocular size and a greater disparity in the relative sizes of the microspheric (larger) and megalospheric (smaller) tests. In the neoschwagerinids, test gigantism appears to be correlated with increasing complexity of structural elements within the chambers. The evolutionary trends in test morphology that accompany test gigantism in both these clades are interpreted as having been driven by algal endosymbioses.