Cyrtodonta 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 : 915

Cyrtodonta sp. B

Fig. 11L, M .

Material.— Three specimens from the Petermann Creek section and four specimens from the Areyonga Gorge section, Middle Ordovician of Australia. The material is found in bed PC+ 10 in the Petermann Creek section and beds A +1 to A + 4 in the Areyonga Gorge section ( Figs. 4 , 5 ). The two illustrated specimens ( CPC 41479 and CPC 41480 ) are latex casts made from external moulds .

Description.―Near equilateral sub-ovate cyrtodontid with slightly curved dorsal and ventral margins; slightly pointy anterior margin and broadly rounded posterior margin. Umbo subdued, pointy and anterodorsally projecting. No teeth preserved in the specimens, possibly due to a preservational artefact. Shell profile low and ornament of commarginal growth lines.

Remarks.― Cyrtodonta sp. B is the most flat-profiled and symmetrically shaped cyrtodontid in the collections ( Fig. 11L 1). It is differentiated from Cyrtodonta carberryi by the lower shell profile, more rounded dorsal and ventral margins as well as a less pronounced umbo. Furthermore, Cyrtodonta sp. B exhibits a rounded and gradually declining shell from the midline towards anterior margin, whereas the course of the shell in C. carberryi declines abruptly about halfway from the midline towards the anterior margin.

The overall equilateral shape of Cyrtodonta sp. B separates it from the inequilateral C. staffordae , whereas the latter exhibits a more symmetrical rounded overall shape. Cyrtodonta sp. B likely represents a new, as yet unnamed species. Due to the generally poor state of preservation among the specimens and lack of preserved characteristic taxonomic features, Cyrtodonta 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.