Exoskeletal micro-remains of an Ordovician fish from the Harding Sandstone of Colorado

Three dimensional scales and fragments of the dermal skeleton from a hitherto little known fish have been isolated from the Ordovician Harding Sandstone of the type area around Cañon City, Colorado, USA, allowing exoskeletal morphology to be correlated with tissue structure and arrangement. Scales with a small crown sculpted into ridges over an extended base exhibit the same histology as material in bone-bed thin sections previously described as 'Vertebrate indet. A' by Denison (1967) and here formalized as Skiichthys halsteadi gen. et sp. nov. Possible taxonomic relationships with the osteostracans and primitive gnathostomes, elasmobranchs, acanthodians and placoderms are discussed. The microsquamous exoskeleton exhibits a variety of histological characters: enameloid, mesodentine with odontocytes, basal bone with osteocytes and numerous extrinsic fibre bundles in groups crossing at right angles and inserted at the visceral surface. The suprageneric assignment of Skiichthys is uncertain, but a taxonomic relationship to either the acanthodians or the placoderms is likely. The contemporaneous and co-occurring pteraspidomorph agnathans, Astraspis and Eriptychius, have an exoskeleton of plates and tesserae of acellular bone (aspidin) and tubular dentine and are not closely related to Skiichthys.