Ecophenotypic variation of the larger foraminifer Orbitoides apiculata from the Maastrichtian stratotype

Abstract

Morphometric analysis of the larger foraminifer Orbitoides apiculata Schlumberger from the type Maastrichtian shows that the general evolutionary trend within the genus continued in Maastrichtian times, but that the local evolutionary pattern is not gradual and unidirectional. Specimens from the lower part of the type section on the average have smaller embryons and fewer epi-auxiliary chambers than those from the upper part of the section. Besides, a new phenomenon can be observed: in the latest Maastrichtian the species had developed an asymmetric test, with one thin, superficial lateral layer of the ordinary Orbitoides type but with the other layer much thicker and lacking the usual subdivision into lateral chamberlets. The pattern, however, is not a strictly evolutionary one, but seems to be related to the prevailing environmental conditions. The more advanced populations are only found in debris layers overlying hardgrounds. In the intervals between the hardgrounds, O. apiculata is much less abundant and, moreover, has the same morphology as the older, more primitive populations. A comparison with O. apiculata from deposits in southern France, sometimes considered to be slightly younger than the type Maastrichtian, suggests that the development within the Maastrichtian stratotype may be the result of geographic separation.