Holocephali Bonaparte, 1841

Ginter, Michał & Sun, Yuanlin, 2007, Chondrichthyan remains from the Lower Carboniferous of Muhua, southern China, Acta Palaeontologica Polonica 52 (4), pp. 705-727 : 716

Superorder Holocephali Bonaparte, 1841 Order Helodontiformes Patterson, 1965 Family Helodontidae Patterson, 1965 Genus Diclitodus Davis, 1883

Diclitodus denshumani ( Newberry and Worthen, 1866) Fig. 7A .

1866 Helodus denshumani sp. nov.; Newberry and Worthen 1866: 76, pl. 4: 21.

Material.—One tooth.

Description.—The crown of Diclitodus denshumani is convex labially and concave lingually. Its lingual/lateral ends slope down at 45 ° and are gently crenulated. The biting edge is almost horizontal, with lateral corners only slightly higher than the median part. The base is strongly expanded lingually and excavated at the lingual end. The orolingual side of the base is perforated with numerous nutritive canals and grooves. Width of the tooth is 1.5 mm.

Remarks.—An almost identical tooth was described by Newberry and Worthen (1866: pl. 4: 21a–c) from the Viséan Keokuk Limestone of Illinois as Helodus denshumani ; although the illustrations are somewhat confusing, the description and diagrammatic sketch of the lateral view leave no doubt. However, as the tooth is much different from the specimens typically referred to as Helodus , and, on the other hand, resembles the type specimen of Diclitodus scitulus Davis, 1883 ( BMNH P.49629; Fig. 10A ) from the Carboniferous Limestone of Yorkshire, it should be included in Diclitodus . D. scitulus is much larger (7 mm wide) than D. denshumani and shows more prominent lateral corners of the crown, but the exposed labial part of the holotype (the rest is embedded in limestone) reveals a similar shape of the base.

Based on indirect reasoning, it was suggested (Woodward, unpublished notes, fide Stahl 1999: 54) that Diclitodus is a symphyseal tooth of a helodontiform. Such a position could explain the extreme rarity of such forms in comparison to other bradyodont teeth and tooth−plates. This being the case, it is quite possible that Diclitodus sp. is in fact a symphyseal tooth of the next described species, Helodus coniculus , and, consequently, represents the genus Helodus , as originally proposed.

Figures

Fig. 7. Holocephalian dental elements from sample MH−1, middle Tournaisian of Muhua, China. A. Diclitodus denshumani (Newberry and Worthen, 1866), PKUM02−0153, in lingual (A1), lingual/aboral (A2), labial (A3), and lateral (A4) views. B–E. Helodus coniculus Newberry and Worthen, 1866. B. PKUM02−0154 in oral (B1) lingual (B2), and lateral (B3) views. C. PKUM02−0155 in lingual (C1), oral (C2), and two lateral (C3, C4) views. D. PKUM02−0156 in lateral (D1), labial (D2), and oral (D3) views. E. PKUM02−0157 in aboral/lateral (E1) and aboral (E2) views. F. Deltodus cf. sublaevis (Agassiz, 1838), tooth plate (PKUM02−0158) in labial view.

Fig. 10. Reference specimens from Laurussia. A. Diclitodus scitulus Davis, 1883, holotype (BMNH P.49629), from the Carboniferous Limestone, Richmond, Yorkshire, England, in labial view. B. “?Psephodus laevissimus”, specimen BMNH P.9914, from the Lower Limestone shales bone bed, Avon Gorge near Bristol, in oral view. C. Chomatodus davisi (Woodward, 1889), a specimen of the type series (BMNH P.2656), from the Viséan Mountain Limestone of Armagh, Northern Ireland, in lingual view. D. Tamiobatis wachsmuthi (St. John and Worthen, 1875), a specimen of the type series (MCZ 6371), from the Kinderhookian of Burlington, Iowa, in labial view. E. Deltodus sublaevis (Agassiz, 1838), specimen BGS 5964, from the Viséan Mountain Limestone of Armagh, Northern Ireland, in oral view (left side broken). F.?Goodrichthys eskdalensis (Moy−Thomas, 1936), a tooth from the specimen NMS 2000.14.2, from the lower Viséan of Glencartholm, Dumfriesshire, Scotland, in labial view. G. Chomatodus linearis (Agassiz, 1838), specimen BMNH P.2653, from the Lower Carboniferous Yoredale Beds, Richmond, Yorkshire, England, in oral view. H. Sibyrhynchus denisoni Zangerl and Case, 1973 (FMNH PF6408), palatal tooth plates.