Anatomy and relationships of †<i>Aipichthys pretiosus</i> and †‘<i>Aipichthys</i>’ <i>nuchalis</i> (Acanthomorpha: Lampridomorpha), with a review of Late Cretaceous relatives of oarfishes and their allies

The Cenomanian acanthomorph teleost †<i>Aipichthys</i> is best known from two species: †<i>Aipichthys minor</i> and †<i>Aipichthys velifer</i>. Two poorly studied species of †<i>Aipichthys</i>, †<i>Aipichthys pretiosus</i> from Slovenia and †<i>Aipichthys nuchalis</i> from the English Chalk, have been excluded from all recent studies of the genus. Previous accounts of †<i>A. pretiosus</i> – the type species of †<i>Aipichthys</i> – are inadequate, while †<i>A. nuchalis</i> has been described as too incomplete to interpret. Here we completely redescribe †<i>A. pretiosus</i> and utilize computed tomography to reveal previously hidden skeletal anatomy of †<i>A. nuchalis</i>. †<i>Aipichthys pretiosus</i> has a long sagittal crest, with the supraoccipital, frontal and mesethmoid bones contributing to this structure. Based on this and other features, we find that †<i>A. pretiosus</i> belongs to a clade that is more closely related to extant Lampridiformes than all other nominal species of †<i>Aipichthys</i>. As †<i>A. pretiosus</i> is the type species for the genus, we recommend all other species of the genus should be temporarily referred to as †‘<i>Aipichthys</i>’. †‘<i>Aipichthys</i>’ <i>nuchalis</i> possesses a concave posterior margin and a thickened anterior margin to the supraoccipital crest, and is placed in a polytomy alongside †‘<i>A.</i>’ <i>minor</i>, †‘<i>A.</i>’ <i>velifer</i>, †‘<i>Aipichthys</i>’ <i>oblongus</i> and †<i>Freigichthys</i>. Further investigation is needed to determine whether †‘<i>Aipichthys</i>’ is congeneric with and can be reassigned to †<i>Freigichthys</i>, or whether a new generic name is needed for these taxa. We also include two additional taxa in our analysis: †<i>Nardovelifer altipinnis</i>, the oldest fossil previously attributed to the lampridiform crown, and †<i>Gigapteryx tethyestris</i>, an <i>incertae sedis</i> acanthopterygian. We find that †<i>Nardovelifer</i> has a number of primitive characters not found in crown Lampridiformes, and resolve it as a stem lampridiform. We find that †<i>Gigapteryx</i> is closely related to †<i>Pycnosteroides</i>, together forming the sister clade to all other lampridomorphs. Finally, we examine the diversity of Cretaceous lampridomorphs and discuss their palaeobiogeography and palaeoecology.