<i>Cynopodius</i> , a long-toothed chondrichthyan from the Carboniferous of Scotland and the USA

ABSTRACT The distinctive isolated elements of Cynopodius , characterised by their deep narrow base and spatulate crown, were first described (as spine-like elements) by Ramsay Heatley Traquair in the late 19th Century, based on specimens from the Burghlee Ironstone (Serpukhovian, Carboniferous) at Loanhead near Edinburgh, Scotland. The type species Cynopodius crenulatus is also known from older Calciferous Sandstone Measures (Viséan) of Fife. Here we provide evidence that the elements are teeth rather than spines or dermal claspers, as hypothesised elsewhere. Thin sections of the teeth, illustrated for the first time, show osteodentine and tubate dentine forming the crown, covered by a thin outer hypermineralised layer on the cusps, and trabecular osteodentine or acellular bone forming the base. In recent decades, teeth of Cynopodius have also been collected from the lower Carboniferous Sainte Genevieve Formation (Mississippian: Viséan) of Iowa, Kentucky, and West Virginia in the USA. We assign these teeth from Iowa to Cynopodius robustus n. sp. They are distinguished from the type species by their relative robustness, with a ratio of maximum crown length to root length of c. 2:3 for C. traquairi and c. 1:2 for C. crenulatus , and lack of longitudinal curvature. The Scottish and American occurrences, though widely separated in the early Carboniferous, are the only known localities for the genus. The similarity between these over 300-million-year-old teeth and those of Recent long-toothed teleost reef fishes like Ctenochaetus , for instance, suggests that the Cynopodius animals might also have been specialised detritivores.