Acanthorhachis, a new genus of shark from the Carboniferous (Westphalian) of Yorkshire, England

An association of diverse hollow spines and dermal denticles (ichthyoliths) from the Carboniferous (Westphalian) of Todmorden, Yorkshire, England are attributed to a new genus of enigmatic shark that may lie close to Listracanthus Newberry & Worthen, 1870. Scanning electron microscopy shows that denticle morphology is highly variable, but forms a morphocline including elongate multi-spined elements as well as robust dome-like stellate denticles and recurved spinose elements. Histological analysis suggests an absence of enameloid. Continuous variation of form between elongate multi-cusped spines to boss-like circular denticles shows that all previously described Palaeozoic species of Listracanthus are probably junior synonyms of the type species L. hystrix Newberry & Worthen, 1870. The status of Listracanthus as a surviving 'Lilliputian' taxon after the Permian extinction is questioned. Although the new specimen has affinities with Listracanthus, significant differences in the form of the posterior spines on elongate denticles warrants its placement in the new genus Acanthorhachis gen. nov. The family Listracanthidae is erected to accommodate Listracanthus and Acanthorhachis.