Xenacanthidae Fritsch, 1889

Johnson, Gary D. & Thayer, David W., 2009, Early Pennsylvanian xenacanth chondrichthyans from the Swisshelm Mountains, Arizona, USA, Acta Palaeontologica Polonica 54 (4), pp. 649-668 : 658

Family Xenacanthidae Fritsch, 1889 Genus Triodus Jordan, 1849

Type species: Triodus sessilis Jordan, 1849. Early Permian, “Lebacher Toneisenstein−Layer”, upper Lauterecken−Odenheim member, Lebach, Saar−Nahe basin, Germany ( Hampe 2003: 221).

Diagnosis.—Limited to dentition. Slightly heterodont; teeth small. Three cusps nearly always present; lateral cusps and usually the intermediate cusp bear straight vertical cristae, sometimes bifurcated, largely limited to distal halves.

Remarks.— Schneider (1996: 330) described Bohemiacanthus in a manuscript that remained in press for at least two years, as Schneider and Zajíc (1994: 123) had already recognized this taxon. They and Schneider (1996: 325–326, fig. 2) assigned to Bohemiacanthus those species with teeth showing cristae on the principal cusps that are simple and straight (as in Hampe 1989: fig. 3), although they may be bifurcated ( Schneider and Zajíc 1994: fig. 21); and they restricted Triodus to those species that possess cristae restricted to the labial side of the principal cusps, or at most, one lingual crista as well. Furthermore, the labial cristae in Triodus possess an inverted Y−shaped bifurcation below the apex of the principal cusps ( Schneider and Zajíc 1994: 125, 133). Thus, Triodus would include only T. sessilis and T. kraetschmeri . Triodus species assigned to Bohemiacanthus by Schneider and Zajíc (1994) include T. carinatus , T. lauterensis , T. palatinus , and T. obscurus , with the latter three species, in this order, showing a stratigraphically older to younger decrease in the number of labial and lingual cristae (Schnei− der 1996: fig. 8). Other morphological features in Triodus and Bohemiacanthus teeth are not significantly different (compare characteristics in Schneider 1996: 326) and their histology is the same ( Schneider 1996: table 1). Soler−Gijón and Hampe (1998: 343 and table 2) and Hampe (2003: 221) argued that Bohemiacanthus is a junior synonym of Triodus for these reasons, and also because both Y−shaped bifurcations of the cristae and straight cristae appear together in T.? frossardi teeth ( Soler−Gijón and Hampe 1998: fig. 4). This combination is approached in T. obscurus ( Hampe 1989: fig. 5d) and T. serratus ( Hampe 2003: fig. 20); and Schneider and Zajíc (1994: figs. 21.1, 5a, 9, 12) show cristae with straight and Y−shaped bifurcations in “ Bohemiacanthus ” carinatus . Schneider (1996: 326) mentioned that Bohemiacanthus teeth possess “simple to occasionally forked carinae”. Given the variability in the pattern and number of cristae in Triodus teeth, Soler−Gijón and Hampe’s (1998) argument is valid. However, Bohemiacanthus has continued to be used ( Werneburg et al. 2007).