Otoliths from the late Oligocene Brejning Formation of Denmark – a testimony of a deep, cool-water fish fauna

This is the first comprehensive description of late Oligocene otoliths from the northern North Sea Basin, i.e., the Norwegian–Danish Basin. More than 2100 otolith specimens had been collected from 17 localities in Denmark and from the erratic Sternberger Gestein of Mecklenburg, Germany. A total of 60 species have been identified from the Danish localities, of which 9 are new species, and four from the Sternberger Gestein and three further species are described from localities in the southern North Sea Basin that have previously remained as nomina nuda. The new species in the sequence of description are Sardinella mecklenburgensis, Palaeoesox scandicus, Paraulopus superstitius, Lampanyctus morsensis, Lampanyctus vilsundensis, Enchelyopus dybkjaerae, Palimphemus pinguis, Trisopterus brevicollum, Trisopterus cylindratus, Trisopterus weileri, Coelorinchus ignotus, Nomeus sternbergensis, Artediellus iutlandicus, Myoxocephalus aculeatus, Dapalis bradicae and Lophiodes sliwinskae. The faunal composition exhibits some substantial differences to well-known otolith assemblages of the southern North Sea Basin in western Germany, The Netherlands and Belgium. These differences are thought to be related to cooler water temperatures in the northern North Sea Basin during the late Oligocene compared to the postulated warm pool in the southern North Sea Basin. Certain localities in the northwestern region of Jutland (Denmark) were deposited in greater water depth on the lower shelf or upper bathyal zone and contained for the first time in the upper Oligocene otoliths of the mesopelagic Myctophidae and the bathybenthic Macrouridae in the North Sea Basin. In combining the knowledge of otoliths from the southern and northern parts of the basin, we arrive at an improved understanding of the evolution of bony fishes in the region during the late Oligocene, a time when the North Sea Basin had transformed into a cul-de-sac basin open only to the north. Climatic changes such as cold pulses (Oi-2 and Mi-1 events) and warm influx (Asterigerina Horizon) are reflected in the composition of the fish fauna. The enhanced otolith database enabled us to establish a consistent, formal otolith zonation for the Oligocene of the North Sea Basin based on gadid otoliths. Moreover, we performed 87Sr/86Sr isotope analyses for certain localities in addition to those published, and as a result, we propose to expand the range of the Brejning Formation from about 22.4 Ma to 26.0 Ma, therewith extending into the basal Miocene.