Fish otoliths from the bathyal Eocene Lillebælt Clay Formation of Denmark.

Few deepwater otolith associations from the Eocene have been found so far. The small assemblage of aragonitic-preserved otoliths from the Lillebælt Clay Formation described here therefore adds to the understanding of early Palaeogene deep-sea fish faunas. These otoliths were obtained from a level at about the Ypresian/Lutetian interface and may thus be older than the otoliths previously described from Trelde Næs from mold casts from carbonate concretions. Only 14 otoliths were recovered from about 6,000 kg processed bulk samples. The assemblage also differs in the composition and contains three new species and one new genus: Diaphus? duplex n. sp., Bregmaceros danicus n. sp. and the ophidiid Pronobythites schnetleri n. gen, n. sp. In addition, the new genus Treldeichthys n. gen. in Acanthomorpha incertae sedis is established for T. madseni (Schwarzhans, 2007). The small assemblage also differs in composition from comparable associations described from southwest France and northern Italy on the species level but shows some relationship on a higher systematic level. The mechanism and timing of the colonization of the deep sea by selected groups of fishes is discussed, particularly in respect to the depth migration of demersal fishes.