Adaptive brachiopod morphologies in four key environments of the Late Cretaceous–Danian Chalk Sea of northern Europe: A comparative study

Abstract

Rich brachiopod faunas are known from the four main, but highly different, palaeoenvironments in the Late Cretaceous–Danian Chalk Sea of northern Europe: Maastrichtian soft-bottom chalk (Møns Klint, Stevns Klint), lower Danian bryozoan mounds (Stevns Klint), middle Danian coral mounds (Faxe), all in the Danish Basin, and a late Campanian rocky shore (Ivö Klack) in the Kristianstad Basin, southern Sweden. There is almost no genus or species overlap between the rocky shore fauna and the three other faunas; the former is interpreted as endemic, having evolved in the small basin which is situated adjacent to the eastern end of the Danish Basin. The Maastrichtian and Danian faunas differ significantly at the species level, reflecting the mass extinction at the Cretaceous–Paleogene (K/Pg) boundary. In spite of having inhabited very different environments, the brachiopod faunas are remarkably similar in terms of modes of life and population structure, and by a marked predominance of micromorphic species. The main difference is the total absence in the Danian faunas of highly specialised, secondarily free-living hemispherical species, which are a characteristic element of the Maastrichtian chalk fauna. This adaptation was developed in several unrelated taxonomic groups during the c. 40-Myr-long existence of the Late Cretaceous interval of the Chalk Sea. No species of this group survived the K/Pg mass extinction. Identification of brachiopod morphogroups reported as characterising different environments is thus successful for the rocky shore environment, but tends to break down between the soft chalk sea floor and the bryozoan or coral mounds. This is interpreted to reflect the availability of a multitude of small, hard substrates, notably slender fragments of bryozoans, in all the three of the last-named environments and by the fact that the survivors of the mass extinction were represented mainly by micromorphic species in addition to a few generalists from which the new, Danian fauna rapidly evolved. Three new major environments, each characterised by distinct brachiopod morphotypes, are recognised in addition to the seven described previously in the literature, namely the sublittoral chalk sea floor, the bryozoan mounds, and the cool and deep-water coral mounds. The rocky shore may be considered as representing a fourth new distinct environment.