Stethacanthus resistens, Ginter, 2002

Ginter, Michał, 2002, Chondrichthyan fauna of the Frasnian-Famennian boundary beds in Poland, Acta Palaeontologica Polonica 47 (2), pp. 329-338 : 332-334

Stethacanthus resistens sp. nov.

Figs. 2 , 3 , 4C–I , 5C .

Stethacanthus sp. ; Ginter 1991: 75, pl. 8: 4, pl. 9: 2, 3.

Cladodus cf. C. thomasi Turner ; Ivanov et al. 1992: 89, pl. 36: 3, 4.

Stethacanthus thomasii Turner [sic]; Derycke 1992: 39–40, fig. 14, pl. 2: 10, 11.

Stethacanthus cf. thomasi (Turner) ; Ginter 1995: fig. 2A.

“symmoriid with button partially divided”; Ginter and Ivanov 1996: fig. 4C.

“stethacanthid?”; Ginter and Ivanov 1996: fig. 5C, D.

Stethacanthus cf. thomasi (Turner) ; Ginter and Ivanov 2000: pl. 1: J.

Holotype: Specimen ZPAL P.IV/216 from the Holy Cross Mts , Płucki near Łagów , sample P−1, latest Frasnian, Palmatolepis linguiformis conodont Zone; Fig. 2C–E .

Etymology: Latin resistens , resistant.

Material.— 27 specimens: eight specimens from Dębnik (samples Dz−13, 14, 15, Early Pa. triangularis Zone ), five specimens from Kowala (samples Ko−35, Late Pa. triangularis Zone ; Kx−12, and Ko−SF, Pa. linguiformis Zone ), seven specimens from Płucki (samples KWK3 and KWK4, Early Pa. triangularis Zone , and P−1, Pa. linguiformis Zone ), three specimens from Kadzielnia (samples K−150, K−151, Pa. rhomboidea Zone ; K−164, Early or Middle Palmatolepis crepida Zone ), two specimens from Psie Górki (sample PG−T, Pa. linguiformis Zone ), and two specimens from Miedzianka (samples M−5, M−EZ, Late Pa. triangularis Zone ).

Diagnosis.—Stethacanth teeth with no more than five cusps in the crown. The angle between the outer lateral cusps is large, almost 90°. The base is lenticular, elongated mesio−distally, with lateral angles usually extending far beyond the foot of the crown. The button and the corresponding labio−basal thickening are mesio−distally elongated and distinct.

Description.—The variation of S. resistens sp. nov. teeth is not very high. It concerns mainly the shape and dimensions of the button, whose mesio−distal to labio−lingual dimension ratio may vary from 2:1 to 5:1, and which may have a form of a straight or slightly curved ridge ( Fig. 2I ) or of a horizontal eight (due to perforations, Fig. 2A ). Corresponding to the button outline, the shape of the labio−basal thickening varies: when the button is more oval, the projection is short ( Fig. 2D, H ), and when the former is elongated, so is the latter ( Fig. 2J , 3 ). Consequently, curved buttons require curved, labially concave, projections ( Fig. 2J ).

The mesio−distal dimension of the base of S. resistens teeth from Polish collections varies from 0.5 to 3.6 mm. Bigger specimens have a larger number of cristae on the labial and lingual faces of the median cusp (up to 11–12 on each side; Figs. 2G , 3B ) than the smaller ones (in some cases less than seven; Fig. 2D, E ). In the latter, the cristae seem to be more prominent compared to the size of a cusp.

There is a single tricuspid specimen in the material from Dębnik, sample Dz−13 ( Fig. 4C, D ). Although typical teeth of S. resistens have five cusps, the lack of intermediate, smaller cusplets in some teeth is rather usual as a variation in stethacanths (see Zidek 1993: fig. 2A and C). Therefore, the specimen is tentatively included here in S. resistens .

Remarks.—The Late Devonian Stethacanthus resistens sp. nov. is very close to the Early Carboniferous S. thomasi ( Turner, 1982) . The only difference is the number of lateral cusps; in S. resistens sp. nov. there are no more than two cusps on each side, and the number of cusps in S. thomasi is usually greater, sometimes different on each side ( Turner 1982: figs. 6C, 8J; Ginter 1995: fig. 5E). Stethacanthus cf. altonensis from the lower Chesterian (= late Viséan) of Caney Shale, Oklahoma ( Zidek 1993: fig. 2A–D) is another very similar form. However, two differences may be pointed out: first, the base of S. cf. altonensis is more lingually extended than that of S. resistens and therefore, its outline is rather triangular than lenticular; second, the angle between the lateral cusps of the younger form is much smaller. The problem with diagnosing S. resistens teeth is that their characters, although probably unique for the species in the late Frasnian and early Famennian, are general for the genus and they tend to repeat in younger stethacanthids in different configurations.

After having submitted the final version of the manuscript I had an opportunity to examine original specimens of “ Cladodus ” wildungensis Jaekel, 1921, in the Museum für Naturkunde der Humbodt−Universität, Berlin. These specimens come from a latest Frasnian Kellwasserkalk layer of Bad Wildungen, Germany. Teeth of the latter species, preserved as natural moulds together with well preserved jaws ( Gross 1938), seem to be very similar to those of S. resistens sp. nov. This suggests that “ C.” wildungensis and S. resistens are closely related or perhaps even conspecific, but further detailed morphological study is necessary before any taxonomic decision is taken. Thus far “ C. ” wildungensis used to be considered as a relative of either xenacathiforms (e.g., Maisey 2001) or ctenacanthoids ( Coates and Sequeira 2001). However, in his recent study, Alexander Ivanov (personal communication 2002) suggested its stethacanthid affinity.

Stratigraphic range in Poland.—Frasnian–Famennian, Pa. linguiformis – Pa. rhomboidea zones.

Distribution in the world.— Russia (South Urals, Palmatolepis rhenana –? Pa. rhomboidea zones; Kuznetsk Basin, Pa. triangularis Zone ), Morocco (Famennian), Moravia ( Pa. triangularis through Pa. crepida zones).

Figures

Fig. 2. Stethacanthus resistens sp. nov. A, B. IGPUW/Ps/1/215 from Miedzianka, sample M−EZ, Late Pa. triangularis Zone, lingual and occlusal views, × 40. C–F. Holotype, ZPALP.IV/216 from Płucki, sample P−1, Pa. linguiformis Zone, occlusal, labial, lateral and lingual views, × 60. G. IGPUW/Ps/4/1 from Płucki, sample KWK3, Early Pa. triangularis Zone, lingual view, × 40. H. IGPUW/Ps/4/2 from Kowala, sample Kx−12, labial−basal view, × 27. I, J. IGPUW/Ps/1/216 and IGPUW/Ps/1/217 from Psie Górki, sample PG−T, Pa. linguiformis Zone, lingual and basal views, × 32 and × 40 respectively. All SEM micrographs.

Fig. 3. Stethacanthus resistens sp. nov., large tooth, IGPUW/Ps/4/3 from Kowala, sample Ko−SF, Pa. linguiformis Zone, basal (A) and labial (B) views, × 20. All SEM micrographs.

Fig. 4. Chondrichthyan microremains from Dębnik, sample Dz−13, Early Pa. triangularis Zone. A, B. “Ctenacanth−type” scales, IGPUW/Ps/4/4 and IGPUW/Ps/4/5. C–I. Stethacanthus resistens sp. nov. teeth. C, D. Problematic tricuspid tooth, IGPUW/Ps/4/6, labial and occlusal views. E. IGPUW/Ps/4/7, basal view. F. IGPUW/Ps/4/8, occlusal view. G–I. IGPUW/Ps/4/9, occlusal, lingual and labial views respectively. All × 80. All SEM micrographs.

Fig. 5. Chondrichthyan microremains from Płucki. A, B. Branchial denticles?, IGPUW/Ps/4/10 and IGPUW/Ps/4/11, sample KWK3. C. Stethacanthus resistens sp. nov. tooth, IGPUW/Ps/4/12, sample KWK4, basal−labial view. All × 60. All SEM micrographs.