Gradualistic evolution in Belemnitella from the middle Campanian of Lower Saxony, NW Germany

Abstract

Nine samples of the Belemnitella lineage, including the uppermost Lower and lower Upper Campanian B. mucronata and the uppermost lower Upper Campanian B. misburgensis sp. nov., from the expanded uppermost Lower and lower Upper Campanian succession of the Lehrte West Synclinee east of Hannover, Lower Saxony, northern Germany have been subjected to univariate and bivariate statistical analyses. The succession consists of monotonous marly limestones and calcareous marls, which were deposited during 4–5 m.y. in a fairly stable environment. Based on superficial resemblance alone the nine samples are nearly identical, but trends have been recognized in the morphological characters when treated quantitatively. Three characters, the length from the apex to the protoconch, the slenderness of the guard and the alveolar angle, show evolutionary reversals (socalled zig-zag evolution) with net decreases or increases. The Schatzky distance shows stasis in B. mucronata and decreases in B. misburgensis sp. nov. The fissure angle displays unidirectional evolution and increases gradually. These morphological changes are interpreted as long term phyletic gradualism, although this model of evolution has almost universally been depicted as one of unidirectional change. B. misburgensis sp. nov. is closely allied to B. mucronata (its ancestor), but differs in its slightly smaller and more slender guard, in addition to its smaller Schatzky distance and larger fissure angle. B. praecursor is recorded from the uppermost five metres of the Lower Campanian, and this is the first record of this species from the highest Lower Campanian of northern Germany. The uppermost Lower and Lower Upper Campanian belemnite assemblages of the Misburg/Höver area in northern Germany are reviewed. The Upper Campanian Belemnitella successions of Norfolk in England, the Maastricht-Liège district in The Netherlands and Belgium, the Mons Basin in southern Belgium, Scania in southern Sweden and the Misburg-Höver area in northwest Germany are compared and contrasted.