The lower few centimeters of the Tertiary (Pine Barren) above the Cretaceous (Prairie Bluff) at Millers Ferry, Alabama, contains an assemblage of very small planktonic Foraminifera with largest dimensions of less than 100 µ. These foraminifers occur in the basal part of the Globigerina edita Zone (=Globorotalia pseudobulloides Zone) which corresponds in part to the lowest nannofossil zone of the Tertiary, the Markalius astroporus Zone. This assemblage occurs at a level similar to that of the central Apennines where Luterbacher and Premoli Silva described a thin zone, named the Globigerina eugubina Zone, of very small planktonic Foraminifera.

A scanning electron-microscope study of the Alabama fossils suggests an evolutionary relation to species of the Globigerina edita Zone and to certain Cretaceous species. The Tertiary genera Chiloguembelina and Globoconusa appear morphologically close to Guembelitria cretacea Cushman of the Cretaceous, whose distribution in Cretaceous strata suggests that it was benthonic or had only a partly planktonic life stage. The Cretaceous planktonic species Hedbergella monmouthensis (Olsson) has morphologic characteristics similar to those of Globigerina edita Subbotina and also seems to be linked phylogenetically to Globorotalia pseudobulloides Plummer).

Perhaps the most significant evolutionary change that occurred in Cenozoic Globigerinacea is the modification of the outer bilamellid layers and the appearance of crustlike deposits of calcite in adult forms. These changes are viewed as adaptations to more efficient use of the water column including, perhaps for the first time, use of mesopelagic zone.

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