Hamiltonichthys mapesi (Maisey 1989, Early Carboniferous, 27cm) was considered the earliest hybodontid shark. Here it nests as a small shark between the Palaeobates clade and the Xenacanthus clade, basal to the seven-gill shark, Chlamydoselachus. Maisey considered Tristychiusa basal member of the Hybontiformes, distinct from the Neoselachii (= all extant sharks and rays).

Xenacanthus decheni (Beyrich 1848; originally Pleuracanthus Agassiz 1837; Devonian to Triassic, 1m up to 4m in length) is traditionally considered a genus of prehistoric shark. At least 21 species are known. This freshwater elasmobranch has distinctive two knife-like cusps on each teeth. A serrated postparietal spine arises from the back of the skull. The second anal fin is actually the lower lobe of the caudal fin. The short broad pectoral and pelvic are the same size. As in Tristichius the root of each fin is a slender jointed cone of cartilage. Distinct from most reconstructions, here the skull bones are not fused to one another.

Stethacanthus altonensis (St. John and Worthen 1875; Late Devonian to Early Carboniferous; 70cm; below) has an anvil-like post-cranial structure with elaborate brush hooks on top. This is the former placoderm thoracic shield (carapace). Traditionally considered a holocephalian with a large orbit, here it nests with Xenacanthus, a basal shark. Note the great distance between the skull and pectoral girdle. The pectoral fin (rather than the pelvic fin) has clasper-like projections.

Akmonistion zangerli (HMV8246; Coates and Sequeira 2001; Early Carboniferous; above) had a larger spine-brush complex, a former placoderm carapace.