Maastrichtian orbitoidal foraminifera in Turkey show a variety of unusual features. They include remarkable specimens of their reproductive life cycle, lateral chambers within the embryo, teratological individuals (polyvalence), microboring activity within tests, equatorial chamber growth, and conical forms. These features provide evidence for Maastrichtian orbitoidal foraminiferal systematics, biological life cycle, palaeoecological conditions and fossilization. The growth of lateral and equatorial chambers is thought to have been controlled largely by genetic characteristics. Microboring activity is the result of their fossilization and life relationships with other micro-organisms such as Talpinella conicularia. Conical forms and polyvalence are thought to be related to sedimentological characteristics of the palaeoenvironment, such as currents. Abnormal growth of the equatorial chambers is accepted as one of the main distinguishing features of Orbitoides gruenbachensis. (C) 2000 Academic Press.