An enigmatic gnathostome vertebrate skull from the Middle Devonian of Bolivia

A new taxon, Ramirosuarezia boliviana n. gen., n. sp. is erected for a single, articulated jawed fish (gnathostome) skull from the Middle Devonian (Eifelian) Icla Formation of Bolivia. The specimen displays an elasmobranch-like braincase, but lacks unambiguous elasmobranch and even chondrichthyan characters, although its peculiar tooth-bearing 'labial' elements evoke certain stem-holocephalans. Its endoskeletal elements seem lined with either perichondral bone or non-prismatic calcified cartilage, but show no evidence of endochondral bone. Although devoid of large dermal bones and scales, R. boliviana shares with certain 'ostracoderms', placoderms and holocephalans the lack of an otico-occipital fissure, but lacks a hypophysial fenestra. Certain features (elongated braincase, 'labial elements', sharp denticles and teeth) are also suggestive of the equally enigmatic coeval stensioellids, once regarded as either primitive placoderms or stem holocephalans. The jaws are armed with platelets that bear blunt to pointed and sharp teeth, in which synchrotron radiation microtomography yields evidence of a large pulp cavity, a possibly osteichthyan-like character. No character clearly supports affinities of R. boliviana to any of the currently known major gnathostome groups. Tenuous hints suggest a relationship to the enigmatic fossil Zamponiopteron, from the Eifelian of Bolivia, known by peculiar calcified 'fin plates' and isolated shoulder girdles.