Morphological adaptations of the brachiopods from a Late Cretaceous rocky shore, Ivö Klack, southern Sweden

Abstract

In 1965 D.V. Ager highlighted the concept of morphological adaptations of Mesozoic brachiopods to different environments. Rocky shores were only indirectly included in his seven habitats, undoubtedly due their rarity in the fossil record. A diverse brachiopod fauna, comprising 19 species, lived on the late early Campanian gneissic rocky shore at the palaeo-island of Ivö, southern Sweden. The brachiopods show many different morphological adaptations to the contrasting local habitats in the turbulent and highly variable environment. They are grouped into guilds and related to preferred habitats according to their morphology, substrate strategies, mode of life and preservation. Four main local habitats are recognized, representing different energy and illumination conditions, ranging from protected areas below and between gneiss boulders and hummocks to the most exposed areas on the vertical sides of these rocks. Medium-sized delicate species preferred the protected habitats, whereas large, robust species occupied the most exposed habitats. No small-sized brachiopod species are known from Ivö Klack in marked contrast to contemporaneous faunas of the more basinal chalks. The only observed direct indication of predation on the brachiopods is boreholes after carnivorous gastropods. The brachiopod fauna is by far the richest in terms of species diversity compared not only to other Late Cretaceous rocky shore faunas worldwide, but rocky shore faunas in the entire Phanerozoic. This difference is so remarkable that it cannot be explained by taphonomic factors alone and the density and diversity of the well-preserved brachiopods at Ivö Klack and the great variety in shell morphologies gives a unique opportunity to examine the variety in attachment strategies used in such a highly variable environment.