A reappraisal of the lampridiform Palaeocentrotus boggildi Kühne, 1941 from the Eocene Fur Formation, Denmark.

The systematic affinities of Palaeocentrotus boggildi within acanthomorph fishes have never been properly discussed. A comparative analysis of the skeletal anatomy of the genera Cyttus and Zeus was used by Kühne to support the placement of Palaeocentrotus within the zeiforms, despite certain features are completely different from those characteristic of zeiforms. Bonde (1966) was the first that recognised the lampridiform affinities of Palaeocentrotus and allocated the genus to its own family, Palaeocentrotidae. However, this hypothesis was not supported by a discussion of the relevant features to justify a possible relationship with the Lampridiformes. The anatomical survey of the known material of Palaeocentrotus boggildi presented here resulted in the identification of a suite of features that is regarded as diagnostic of the Lampridiformes, including: anterior palatine process absent, mesethmoid posterior to lateral ethmoids, elongate ascending premaxillary process longer than the alveolar process and inserting into the frontal vault, premaxilla and dentary edentolous, endopterygoid teeth absent, urohyal bearing a large ventral laminar expansion, second ural centrum free from fused first ural and preural centra and fused posteriorly to the upper hypural plate, presence of two supraneurals and of a single postcleithrum. The most striking feature of the Palaeocentrotidae, comprising only Palaeocentrotus and Natgeosocus, is the unique configuration of the dorsal fin wherein true spines occur exclusively in the central portion. This peculiar condition represents a further example of the remarkable plasticity of the acanthomorph median fins.