Sthendonta paenesymmetrica, Jakobsen & Brock & Nielsen & Harper, 2016

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

Sthendonta paenesymmetrica sp. nov.

Fig. 10K, L .

Etymology: The new name alludes to the almost symmetrical shell shape, which is unusual among sthenodontids in the material.

Holotype: CPC 41464 , an internal mould ( Fig. 10K ).

Type locality: Areyonga Gorge, Amadeus Basin, central Australia.

Type horizon: Bed A - 5 in the Areyonga Gorge section ( Fig. 4 ). Darriwilian (Middle Ordovician), Stairway Sandstone ( Figs. 1, 3).

Material.― 68 specimens, of which two are figured ( CPC 41464 and CPC 41465 ); all specimens are internal moulds recovered from the Petermann Creek section, Middle Ordovician of Australia .

Diagnosis.―Small, nearly equidimensional, sub-circular sthenodontid. Indistinct palaeotaxodont teeth; umbo wide, well-defined, rounded and posteriorly projecting.

Description.―Dorsal margin rounded, sloping rapidly into anterior margin; posterior margin rounded towards dorsum, not projecting. Sub-circular shell shape carrying a wide, rounded and dorsally pointing umbo; shell profile relatively flat. Weakly developed external ornament of growth lines on some specimens ( Fig. 10K ), otherwise a smooth surface. The holotype is 12 mm high and 13.5 mm long, making the species nearly equidimensional (about 90% as high as long).

Remarks.―The outline of the species is similar to Sthenodonta etheridgei ( Tate, 1896) figured by Pojeta and Gilbert-Tomlinson (1977: pl. 7: 8, 9). These specimens are latex replicas of external moulds showing strongly developed commarginal striae on the surface. It is difficult to compare the external characters of S. etheridgei with S. paenesymmetrica sp. nov., as the latter almost entirely consists of internal moulds. The distinct commarginal striae presented on the former is not even indicated on S. paenesymmetrica sp. nov., and for this reason the two species are separated.

The two species have the most circular shell outline of the bivalves recovered from the Stairway Sandstone and are as such atypical nucularcids, which usually possess an inequilateral shell shape. Pojeta and Gilbert-Tomlinson (1977: 18) assigned the species S. etheridgei to Sthenodonta due to the arrangement of teeth (long posterior tooth row and short anterior tooth row). The teeth of Sthendonta paenesymmetrica sp. nov. are poorly preserved but the presence of few indistinct teeth indicates a very similar arrangement.

Stratigraphic and geographic range.― Sthendonta paenesymmetrica sp. nov. was recovered from bed PC -5 to PC + 10 in the Petermann Creek section and bed A -5 to A + 3 in the Areyonga Gorge section ( Figs. 4 , 5 ). Almost 85% of the species were found in from the Petermann Creek section. The same distributional pattern between the sections is observed for the other sthenodontids. The range of the species is within the Stairway Sandstone Formation, Darriwilian (Middle Ordovician). The region is in the southern Northern Territory, corresponding to the central part of the Amadeus Basin ( Fig. 1).

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.