Sthenodonta sp. B

Jakobsen, Kristian G., Brock, Glenn A., Nielsen, Arne T. & Harper, David A. T., 2016, A Darriwilian (Middle Ordovician) bivalve-dominated molluscan fauna from the Stairway Sandstone, Amadeus Basin, central Australia, Acta Palaeontologica Polonica 61 (4), pp. 897-924 : 910-911

Sthenodonta sp. B

Fig. 11A .

Material.― 46 specimens, of which we only figure CPC 41468 as most of the specimens are poorly preserved. The bulk of the material was recovered from beds PC -6–PC 2 in the Petermann Creek section and only two specimens from bed A + 3 in the Areyonga Gorge section ( Figs. 4 , 5 ). Most specimens (39 out of 46) were found in bed PC -5. All Middle Ordovician of Australia .

Description.―Inequilateral sub-ovate Sthenodonta species carrying a rounded and anteriorly pointing umbo; shell profile relatively flat; dorsal margin straight, ventral margin straight to sub-rounded, anterior margin straight to rounded ventrally and posterior margin rounded. Ornament of growth lines distinct. Outline of teeth uncertain due to poor preservation.

Remarks.― Sthenodonta sp. B is quite similar to Sthenodonta sp. A and S. eastii . As most of the specimens are preserved as external moulds it complicates comparison of internal characters with the other species. Sthenodonta sp. B is separated from Sthenodonta sp. A due to a more extended posterior part of the valve, making the overall shell shape sub-ovate rather than sub-circular (compare Fig. 11A vs. Fig. 10N ).

The species is separated from S. eastii due to the much better defined growth lines (compare Fig. 11A vs. Fig. 10J 2 ) as well as the overall lower shell profile and the less demarcated umbo. The discussed Sthenodonta species have an umbo pointing in an anterior direction and generally the umbo on Sthenodonta sp. B is slightly more dorsally directed as compared to the two other Sthenodonta species. Sthenodonta sp. B likely represents a new, as yet unnamed species. However, due to the generally poor state of preservation among the specimens and lack of preserved characteristic taxonomic features, Sthenodonta sp. B has not been described formally as a new species.

Figures

Fig. 11. Nucularcid and pteriomorphian bivalves from the Stairway Sandstone, Middle Ordovician, Australia. Internal moulds (B–K) and latex casts of external moulds (A, L, M). A. Sthenodonta sp. B (CPC 41468) from Petermann Creek, left-lateral view showing well marked growth lines. B. Johnmartinia cordata Pojeta and Gilbert-Tomlinson, 1977 (CPC 41469) from Areyonga Gorge; left-lateral view with dorsal border (B1), anterior view showing the pointy beak (B2). C–F. Nuculites wattii Tate, 1896 from Areyonga Gorge. C–E. Specimens in left (C, E) and right-lateral (D) dorsal views, showing well-defined anterior myophoric buttress, D additionally shows dorsal teeth (C, CPC 41470; D, CPC 41472; E, CPC 41472). F. Right-lateral view of fragmentary specimen with chevron shaped posterior teeth pointing anteriorly (CPC 41473). G. Cyrtodonta carberryi Pojeta and Gilbert-Tomlinson, 1977 (CPC 41474) from Areyonga Gorge; in right-lateral view with growth lines and at least one dorsal lateral tooth (G1), dorsal view illustrating the height of shell profile (G2). H–K. Cyrtodonta staffordae Pojeta and Gilbert-Tomlinson, 1977 from Petermann Creek (H–I, K) and Areyonga Gorge (J). H. Specimen in right-lateral view showing distinct growth lines (CPC 41475). I. Specimen in left-lateral view with three posterior lateral teeth (CPC 41476). J. Fragmentary posterior margin with duplivincular ligament and indications of three posterior lateral teeth (CPC 41478). K. Specimen in right-lateral view with a posterior lateral tooth (CPC 41477). L, M. Cyrtodonta sp. B. L. Anterior (L1) and left-lateral (L2) views of sub-ovate shell-profile (CPC 41479) from Petermann Creek. M. Sub-ovate shell-profile with growth lines (CPC 41480) from Areyonga Gorge. Scale bars 10 mm.

Fig. 4. Composite log of the Stairway Sandstone and abundance-charts showing distribution of bivalves and rostroconchs in the Areyonga Gorge section.

Fig. 5. Composite log of the Stairway Sandstone and abundance-charts showing distribution of bivalves and rostroconchs in the Petermann Creek section.

Fig. 10. Nucularcid bivalves from the Stairway Sandstone, Middle Ordovician, Australia. Internal (A–H, I1, J1, K–N) and external (I2, J2) moulds. A–J. Sthenodonta eastii Pojeta and Gilbert-Tomlinson, 1977 from the Petermann Creek. A–D. Specimens in left-lateral views with chevron-shaped palaeotaxodont teeth (A, CPC 41455; B, CPC 41456; C, CPC 41456; D, CPC 41457). E. Specimen in dorsal view with teeth (CPC 41458). F, G. Specimens in right-lateral view with teeth and ventral border (F, CPC 41459; G, CPC 41460). H. Specimen in left-lateral view with anterior myophoric buttress preserved (CPC 41461). I. Specimen in right-lateral view showing indistinct growth lines (CPC 41462). J. Specimen in left-lateral view (CPC 41463). K, L. Sthenodonta paenesymmetrica sp. nov. from the Areyonga Gorge (K) and Petermann Creek (L), right-valves in dorsomedial views. K. Holotype (CPC 41464). L. Juvenile specimen (CPC 41465). M, N. Sthenodonta sp. A. M. Specimen (CPC 41467) from the Petermann Creek in right-lateral view with anterior muscle scars indicated and part of original shell material preserved. N. Specimen (CPC 41466) from Areyonga Gorge in right-lateral view with curved anterodorsal margin. Scale bars 10 mm.

Fig. 2. Distribution of the Larapintine Sea in the Middle Ordovician. The extent of the Canning, Amadeus, and Georgina basins, all containing Middle Ordovician deposits, have been inferred together with the extent of continental landmass (marked with horizontal lines). Base map modified from Webby 1978. A. Webby’s interpretation (1978). B. New interpretation based on faunal correlation. Note the broken connection between the Amadeus–Georgina basins and the Canning Basin.