Evolutionary trends and stratigraphic significance of the late Cretaceous Helicorbitoides-Lepidorbitoides lineage

Abstract

Several thousands of specimens of Lepidorbitoides and its ancestor Helicorbitoides have been studied. All populations are considered to belong to one phylogenetic lineage. A subdivision is proposed of this lineage into seven species. The most important evolutionary trend is nepionic acceleration. Other trends have been observed as well, but these are less consistent and refer to features which may also be influenced by environmental factors. These trends are: increase in size of test, increase in flatness of test, increase in size of the initial chambers, increase in relative size of deuteroconch (i.e. ratio diameter deuteroconch/protoconch), crowding of the equatorial chambers and increasing complexity of equatorial chamber connections. All trends may be brought under the general aspect of increased rate of growth. The lineage originates from Pseudosiderolites vidali in the lower Upper Campanian and continues up to the uppermost Maastrichtian. Contrarily to the opinion of earlier investigators, the Campanian-Maastrichtian boundary should be drawn between the Lepidorbitoides campaniensis and L. bisambergensis Zones or in the upper part of the L. campaniensis Zone. Several types of wall structure of equatorial chambers are discussed and it is concluded that that of Lepidorbitoides agrees best with the rotaliid type. The Lepidorbitoididae are completely unrelated to Orbitoides and are to be classified in the superfamily Rotaliacea.