New quantitative data on Omphalocyclus from the Maastrichtian in Northern Iraq

Abstract

Omphalocyclus is a well-known genus of large benthic foraminifera; species in the genus mainly occur in the argillaceous limestones and carbonates of the Aqra Formation of the early to late Maastrichtian age from Kurdistan foreland basin in Northern Iraq. Its presence is a significant indicator of its paleogeographic distribution in the Middle East. The Aqra formation was divided by lithology into lower, middle, upper, and transitional parts using Google Earth satellite images. Evolutionary changes were documented in successive Omphalocyclus assemblages for the first time in Iraq, and a clear increase in embryon size was found towards the end of the Maastrichtian. This evolutionary development was followed by an increase in the number of epi-embryonic chamberlets. The earliest known Omphalocyclus specimens in the studied strata predominantly have three to four primary epi-embryonic chamberlets and one to two accessory epi-embryonic chamberlets, and these occur in horizons attributable to the Pseudotextularia intermedius zone. The gain of radial stolons in the tritoconch wall may have led to accessory epi-embryonic in consecutive Omphalocyclus populations in horizons attributable to the Racemiguembelina fructicosa zone; accordingly, only a few additional epi-embryonic chamberlets arise from the tritoconch. Epi-embryonic chamberlets tend to be relatively large and comprise a large part of the embryon around its dense external enclosure. Notable differences in embryon size, the number of epi-embryonic chamberlets, as well as other morphological features in successive populations, two prominent species were distinguished: O. cideensis in the early Maastrichtian strata and O. macroporus in the late Maastrichtian. Omphalocyclus specimens exhibited the same evolutionary age and a similar embryonic developmental stage, according to a morphometric analysis of O. macroporus embryon size. However, tests of some of these specimens from the uppermost Aqra Formation were ellipsoidal, biconcave, and centrally depressed, and other tests showed disk forms and central depressions with a well-rounded equatorial margin. These were interpreted as two new subspecies: O. macroporus ellipsoides subsp. nov. and O. macroporus maukabensis subsp. nov.