WHAT EXACTLY ARE LARGER BENTHIC FORAMINIFERA? WHY DEFINITIONS MATTER

Early in the 20^th century, the petroleum industry recognized the importance of forams as stratigraphic indicators. With discoveries in Late Paleozoic strata in Texas and Russia, fusulinid fossils became a focus of study. As exploration expanded to Cretaceous and Cenozoic limestones of the Neotethys, the discocyclinids, nummulitids, and other large-shelled taxa were recognized for their biostratigraphic value and for the limestones they produced. Moreover, because their external features were non-descript, while their internal structures were characteristically complex, the term larger benthic foraminifera (LBF) was defined as taxa that grew to large sizes and that required identification using petrographic thin sections.

Although Joseph Cushman noted in 1940 that large, living forams likely hosted commensal algae, Ronald Hedley from the British Museum challenged that assumption in the 1964. John Lee subsequently published extensively on the algae associated with live forams. Amphistegina, while technically not “larger” by the original definition, host diatom symbionts, grow to sufficient sizes to produce substantial carbonate sands, and have occurred with the LBF, as historically defined, throughout the Cenozoic. Thus, since the 1980s they have been commonly included as LBF.

In the 21^st century, with growing interest in bioindicators and biodiversity, the definition of LBF has drifted further from the original definition. Because they depend upon photosynthesis of their symbionts, Amphistegina and modern taxa historically considered to be LBF can be used as bioindicators of water transparency. Some researchers have extended the term to include any taxa that host algal symbionts, regardless of size; even tiny peneroplids have been counted as LBF in biodiversity studies. Comparing LBF species-richness or assemblage studies from the past decade with 20^th century studies can be misleading if changes in both taxonomy and terminology are not recognized.