First record of the genus Cylindracanthus Leidy, 1856 (Osteichthyes, Actinopterygii) from the Eocene of the Pamplona Basin (Navarre, South Pyrenean area): systematics and taphonomy

Cylindracanthus (Osteichthyes, Actinopterygii) fossils from the middle–upper Eocene (Ilundain Marl Formation, Bartonian–Priabonian) of the Pamplona Basin (Navarre, Southern Pyrenean area) are described for the first time. The fossils correspond to rostral fragments from the anterior and posterior regions. The absence of alveoli and teeth allows them to be assigned to the species Cylindracanthus rectus, a species with a wide distribution in the Eocene. The rostrum is composed of francolite, in consequence of the fossil-diagenetic replacement of biogenic hydroxyapatite. Sometimes, the external surface of the rostrum, formed by ridges and grooves, is covered by a thin band of early celestite, which partially protects the original microstructure. The central channels are filled with different mineral phases, with voids corresponding to previous euhedral pyrite crystals surrounded by natrojarosite, with colloidal shapes. The whole complex was affected by late fractures filled by fibrous gypsum crystals. The possible physical-chemical processes that gave rise to the formation of pyrite, natrojarosite, celestite and gypsum in different environments during the fossil-diagenetic stage are discussed.