Sthenodonta eastii ( Tate, 1896 )

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 : 909

Sthenodonta eastii ( Tate, 1896)

Fig. 10A–J .

1896 Isoarca eastii sp. nov.; Tate 1896: 106, pl. 2: 12a.

1896 Isoarca eastii var. modiolaeformis ; Tate 1896: 106, pl. 2: 12b.

1977 Sthenodonta eastii ; Pojeta and Gilbert-Tomlinson 1977 [partim]: 17, pl. 12: 1–10; pl. 13: 1–6, 8; non pl. 13: 7 [= Sthenodonta sp. A ]. Type material: Syntypes (SAM T 1274a, b) investigated by Pojeta and Gilbert-Tomlinson (1977: 17), of which they designated lectotype (SAM T 1274a; original of Tate 1896: pl. 2: 12a). It was refigured by Pojeta and Gilbert-Tomlinson (1977: pl. 12: 3, 4).

Type locality: Middle Valley (Tempe Downs), which is in the Amadeus Basin, southern Northern Territory, Australia .

Type horizon: In sandstone and quartzite underlying the fossiliferous limestone, Ordovician.

Material.― 550 specimens recovered from the Stairway Sandstone at Areyonga Gorge (n = 130) and Petermann Creek (n = 420), Middle Ordovician of Australia. The nine illustrated specimens, including latex casts, are numbered CPC 41455–41463 .

Description.―Highly inequilateral and tumid nucularcid with prominent umbo; shell elongated ovate and extends only slightly anteriorly beyond umbo; dorsal and ventral margins arcuate with anterior and posterior margin rounded; strongly developed anterior myophoric buttress; uninterrupted anterior and posterior taxodont tooth rows that meet at an obtuse angle (>100°), with anterior tooth row much shorter than posterior tooth row; peg-like teeth below umbo with chevron or half-chevron shaped teeth in posterior direction; apex of teeth points toward umbo (see Fig. 10A–G ). A distinct border following the ventral margin is particularly pronounced on adult specimens ( Fig. 10F ). Growth lines are faintly discernible on external moulds ( Fig. 10I 2, J 2 ).

Remarks.―The Nucularcidae include two genera: Nucularca Pojeta and Stott, 2007 and Sthenodonta Pojeta and Gilbert-Tomlinson, 1977 . The former is found in North America and the latter is endemic to central Australia (Pojeta and Gilbert-Tomlinson 1977; Pojeta and Stott 2007). Nucularcids differ from other Ordovician palaeotaxodonts by having a hinge plate that is broad and arcuate.

Sthenodonta eastii comprises 43% of the determinable bivalves in the two investigated sections. It is readily identifiable if the strongly developed anterior myophoric buttress is preserved ( Fig. 10H ). Within the investigated sections four times as many specimens were recovered from Petermann Creek as the Areyonga Gorge section (see Figs. 4 and 5 ). The Petermann Creek section contains more very fine sand to silt-dominated beds, which appears to have been the preferred substrate of S. eastii although the species is also found in most facies types in the sections (albeit in lower numbers).

Pojeta and Gilbert-Tomlinson (1977: 17–19) described five species assigned to Sthenodonta , of which two were reported from the Nora Formation of the Georgina Basin, one or possibly two from the Stairway Sandstone and two to possibly four from the Stokes Siltstone in the Amadeus Basin (see Fig. 6). Sthenodonta eastii is the only species recorded with certainty from the Stairway Sandstone. Tate (1896: 106) reported Isoarca etheridgei from “limestone, Middle Valley at Tempe Downs, Petermann Creek by Camp at Laurie´s Creek, north of Tempe Vale, and Chandler Range”. Thus, the type suite of I. etheridgei was probably obtained from several localities, but the lithology of most syntypes suggests that they originated from the Stairway Sandstone (Pojeta and Gilbert-Tomlinson 1977: 17).

In the material at hand, Sthenodonta is represented by four species, of which three are new. Sthenodonta eastii can be separated from Sthenodonta sp. A in having a more asymmetric shape and a higher shell profile. Sthenodonta eastii is separated from Sthenodonta sp. B by its higher shell profile and from S. paenesymmetrica sp. nov. by a much more asymmetric shape; whereas the height in shell profiles of these species are very similar.

An example of a well-preserved juvenile internal mould with associated external mould is illustrated on Fig. 10I, J . The external latex cast displays faint growth lines, which separate the species from Sthenodonta sp. B having well marked growth lines.

Stratigraphic and geographic range.― Sthenodonta eastii has a stratigraphic range from bed PC -6 to PC + 11 in the Petermann Creek section encompassing 65 m true thickness and from bed A -9 to A + 5 in the Areyonga Gorge section (85 m true thickness). The species is present in almost every fossiliferous bed in these sections ( Figs. 4 , 5 ).

S. eastii is a widespread species in the Amadeus Basin, where it is documented from 27 localities by Pojeta and Gilbert-Tomlinson (1977). In 25 of these localities the species was recorded from unknown levels within the Stairway Sandstone. The two additional localities represent occurrences in the overlying Stokes Siltstone. The species is also reported from an uncertain stratigraphic level (probably the Nora Formation) in the Georgina Basin (Pojeta and Gilbert-Tomlinson 1977: 17; see Fig. 6). The range of the Nora Formation is Early to Middle Ordovician (the major part of the formation is Middle Ordovician). The region is the south-eastern Northern Territory, and the locality is located east of Mt Ultim. This location corresponds to the south-western limit of the Georgina Basin.

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. 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.

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.