A new Priabonian Heterostegina from the eastern Tethys (Sulaiman fold belt, west Pakistan); implications for the development of eastern Tethyan heterostegines and their paleobiogeography

The stratigraphic utility of Eocene Heterostegina in the Western Tethys prompts an interest in the Eastern Tethyan domain, where virtually no information exists on this group. The classic Eocene sections, Zinda Pir and Rakhi Nala, in West Pakistan offer a unique opportunity to fill this information gap. The genus here is confined only to the upper part of the Drazinda Formation ('Pellatispira beds' of the obsolete Kirthar series) associated with Pellatispira, Silvestriella, reticulate Nummulites, rare orthophragminids and other less significant larger benthic foraminifera. Heterostegina specimens are characterised by notably small, nearly flat to flat tests with fine granulation over the test surface, a small proloculus and tight early spirals. The early operculinid chambers, few in number, are followed by the heterosteginid stage consisting of rectangular chamberlets, developed only in the median part of the test, but not in alar prolongations. The latter are remarkably wide and extend to the umbilical part of the test. A combination of these features permits their differentiation from the Western Tethyan and Pacific Eocene Heterostegina, also demonstrated morphometrically, and a new species, H. indusensis n. sp., is erected. Our data favor the polyphyletic origin for the Eocene heterostegines in the Tethys and do not support a previous hypothesis that the genus was confined to the high-latitudes in Priabonian times.