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Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry (Lochkovian), as well as serial sections of an articulated specimen, reveal the three-dimensional shape and structure of fin spines, scapulocoracoid and dermal plates, and the histological structure of dermal and endoskeletal hard tissues of the climatiid acanthodian Climatius reticulatus. Globular calcified cartilage is the only form of mineralization of the head endoskeleton, with the jaws preserved as double-layered globular calcified cartilage. Tooth whorls are borne on both the upper and lower jaws and comprise a vascularized bone base and tooth cusps composed of a vascular network and mesodentine, without a central pulp cavity. The short admedian spine is the only strongly laterally compressed spine; the anterior dorsal fin spine has a wide, splayed base. No evidence was found of large dermal plates between the pectoral fin spine and the median lorical plates, with the prepectoral pinnal plates with spines being the only large paired dermal components of the shoulder girdle. The anterior lorical plate, pinnal plates, and tesserae on the scapulocoracoid bear ornament comparable to the postbranchial apronic ornament in acanthothoracid placoderms. Branchiostegal plates have a thin, dense inner bone layer and an outer dentinous ornament layer but lack a middle osteodentine layer; other postcranial plates and fin spines have a smoothsurfaced inner layer of bone and a thick middle osteodentine layer. Body scales have a crown with areal growth zones formed of Stranggewebe and syncitial mesodentine on a cellular bone base.

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Circumorbital dermal bones are found in most groups of early vertebrates that have dermal bony plates on the head. Taxonomic distribution of dermal sclerotic plates on the eye itself is less clear, partly because the eyeball is rarely preserved and sometimes because sclerotic bones have been misinterpreted as circumorbital bones. Based on the examination of climatiid Climatius plus mesacanthid, cheiracanthid and acanthodid acanthodiform acanthodians, we conclude that most, if not all, acanthodiforms and climatiids had sclerotic rings. Presence and number of these elements should be included as a character in phylogenetic analyses of early jawed vertebrates.

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