Sthenodonta sp. A

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

Sthenodonta sp. A

Fig. 10M, N .

1977 Sthenodonta eastii ; Pojeta and Gilbert-Tomlinson 1977 [partim]: pl. 13: 7.

Material.― 12 specimens (3 from the Areyonga Gorge section and 9 from the Petermann Creek section), Middle Ordovician of Australia. Illustrated specimens: CPC 41466 and CPC 41467 . The material was recovered from bed interval PC -6–PC + 11 in the Petermann Creek and beds A +1–A + 4 in the Areyonga Gorge section ( Figs. 4 , 5 ). The species is endemic to the Amadeus Basin and is currently known only from the two studied sections (Fig. 3) as well as from a single specimen from a locality close to Lake Amadeus (Pojeta and Gilbert-Tomlinson 1977: 51, fig. 1). The latter specimen was collected by Peter. J. Cook (Australian Bureau of Mineral Resources) in 1962 and assigned to Sthenodonta eastii by Pojeta and Gilbert-Tomlinson 1977 .

Description.―Inequilateral Sthenodonta species carrying an anteriorly pointing umbo; shell relatively flat and elongate ovate to sub-circular; dorsal margin arcuate, ventral margin straight to rounded anteriorly and posteriorly, anterior margin slightly rounded, posterior margin strongly rounded; taxodont tooth rows meet at an obtuse angle (>100°), with anterior tooth row much shorter than the posterior tooth row; peg-like teeth below beak anteriorly. Admittedly the dentition is difficult to see but CPC 41467 shows dentition and it is more obvious on the actual specimen. Ornament of growth lines weakly developed; circular anterior adductor muscle scar placed about mid-way on valve (ex. sagittal) close to anterior margin ( Fig. 10M, N ).

Remarks.― Sthenodonta sp. A has a less ovate and more sub-circular shell shape than S. eastii . The anterior myophoric buttress is not observed in Sthenodonta sp. A , and this key difference also separates it from S. eastii . Sthenodonta sp. A 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, Sthenodonta sp. A has not been described formally as a new species.

Figures

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. 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.