Planktonic foraminifera were sampled from the stratigraphic interval between 21 and 10 Ma at ODP Site 744 (southern Indian Ocean). The water-depth hierarchy of 18 taxa was established using δ18O measurements and morphological indicators. The latter were derived from modern foraminifera, and included the presence of spines, test size, wall texture, chamber shape, and type of coiling.
The shallow-water fauna includes predominantly small, microperforate species belonging to Globigerinita, Tenuitellinata, Tenuitella, as well as small Globigerina and Neogloborotalia. The intermediate group comprises heavily ornamented microperforates (Tenuitella and Turborotalita) as well as large Globigerina and possibly Neogloboquadrina. The deep-water taxa include large forms such as Globorotalia, Catapsydrax and possibly Turborotalita.
There is general agreement between isotopic and morphological proxies of water-depth habitats among the analyzed Miocene planktonic foraminifera from southern high latitudes. Large and deep-water dwelling foraminifera have significantly shorter stratigraphic ranges compared to small foraminifera inhabiting shallow-waters. This relationship is probably due to common cryptic speciation among planktonic foraminifera, which may be more pronounced among small taxa of simple test morphology.