Large-sized Late Turonian–Early Coniacian (Late Cretaceous) inoceramid bivalves from Germany: taxonomic issues, temporal framework and palaeoecological implications

Abstract

A systematic-stratigraphic revision of selected large Late Turonian–Early Coniacian inoceramids of the lamarcki group from Germany showed that both historical specimens of Goldfuss (1836), introduced as Inoceramus annulatus and later becoming the types of Late Turonian I. lamarcki stuemckei Heinz, 1928a and Early Coniacian I. annulatus Goldfuss, 1836, respectively, are of Late Turonian age. I. stuemckei Heinz, 1928a, as emended herein, is well represented by its lectotype. Bulky isolated hinges from Upper Turonian strata, used by Heinz (1932a) to erect Heroceramus hercules, are just fragments of contemporaneous large I. stuemckei; H. hercules is thus invalid. The common Late Turonian I. stuemckei occurs facies-independent from near- to offshore settings and its first common appearance is an excellent marker for the substage base. I. annulatus Goldfuss, 1836 was later applied to very large forms first appearing in the mid-Early Coniacian. Thus, I. annulatus sensu Walaszczyk and Wood (1998) is poorly represented by its medium-sized latest Turonian lectotype, causing some nomenclatorial issues. I. stuemckei (heightmax 500 mm) and I. annulatus (heightmax one metre) are characterized by a stratigraphically abrupt shell enlargement and hinge buttressing across the Middle/Upper Turonian boundary and within the Early Coniacian, respectively. This shift to larger sizes, a common passive defense strategy against predation, is accompanied by widespread evidence for increased Late Turonian predation pressure by marine durophages. We thus speculate that the size increase in Late Turonian–Early Coniacian inoceramid bivalves is part of an escalating arms race between prey and predators, ultimately triggering Late Cretaceous inoceramid gigantism.