Sphenosolen draperi Pojeta and Gilbert-Tomlinson, 1977

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 : 918

Sphenosolen draperi Pojeta and Gilbert-Tomlinson, 1977

Fig. 12E, F .

1977 Sphenosolen draperi sp. nov.; Pojeta and Gilbert-Tomlinson 1977: 31, pl. 25: 1, 2.

Type material: Holotype ( CPC 15582 ) is figured by Pojeta and Gilbert-Tomlinson (1977: pl. 25: 1) . Paratype ( CPC 15583 ) figured by Pojeta and Gilbert-Tomlinson (1977: pl. 25: 2) from the type locality. Type locality: Stairway Sandstone , East of Maryvale Homestead, Amadeus Basin, southern Northern Territory, Australia .

Type horizon: Decalcified friable buff/white fine-grained silty sandstone horizon within the Stairway Sandstone.

Material.— 40 specimens, mostly fragmentary internal and external moulds; 4 specimens from the Areyonga Gorge section and 36 specimens from Petermann Creek section, Middle Ordovician of Australia. The two figured specimens are numbered CPC 41485 and CPC 41486 .

Description.—Narrow elongated sub-rectangular orthonotid with flat shell. Dorsal margin straight; anterior margin rounded; posterior margin rounded to straight and oblique toward the dorsum; ventral margin broadly rounded anteriorly, straight posteriorly. Umbo hardly discernible.

Remarks.— Sphenosolen draperi is the only heteroconch recovered from the investigated sections. The shell shape is distinctly different from all other species due to the reduced height and increased length. Cope (2004) placed the genus Sphenosolen within the anomalodesmatans but we follow Pojeta and Gilbert-Tomlinson (1977), who assigned the genus to the heteroconchs; for further discussion see Cope 2002, 2004). Sphenosolen is a middle Darriwilian taxon Cope 2004: fig. 20.3), like the nuculoids Sthenodonta and Johnmartinia .

Stratigraphic and geographic range.—A single specimen from bed PC - 5 in the Petermann Creek section is tentatively assigned to S. draperi , whereas the remaining 35 specimens derive from bed PC +7 to PC + 11 in the upper part of the section ( Fig. 5 ). The species also ranges through the upper part of the Areyonga Gorge section from bed A +1 to A +4 ( Fig. 4 ).

Pojeta and Gilbert-Tomlinson (1977: 31) described and illustrated Sphenosolen draperi based on nine specimens, all from the same locality about 150 km east of the Areyonga Gorge in the Amadeus Basin. All known specimens found are from the Stairway Sandstone. Heteroconchs are uncommon in low-latitude faunas, and seemingly preferred high-latitude environments ( Cope 2002). The range of Sphenosolen draperi 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. 12. Pteriomorphian and heteroconchian bivalves, rostroconchs and gastropods from the Stairway Sandstone, Middle Ordovician, Australia. Internal moulds (A, B, E–M, N1) and latex casts of internal (N2, O) and external (C, D) moulds. A, B. Modiolopsis pojetai sp. nov. from Petermann Creek. A. Holotype (CPC 41481) in left-lateral view showing discrete longitudinal dorsal slit-like mark. B. Paratype (CPC 41482) in right-lateral view with umbo steeply descending anteriorly. C, D. Colpantyx? sp. A from Areyonga Gorge; in right (C, CPC 41483) and left-lateral (D, CPC 41484) views with two umbonal radial ribs anteriorly and one posteriorly. E, F. Sphenosolen draperi Pojeta and Gilbert-Tomlinson, 1977 from Petermann Creek (E, CPC 41485; F, CPC 41486), specimens with flat shell and narrow elongated sub-rectangular shape. G–M. Rostroconchia. G–I. Ribeiria csiro Pojeta, Gilbert- Tomlinson, and Shergold, 1977 from Areyonga Gorge (G) and Petermann Creek (H–I); note the prominent triangular beak and curved ventral margin (G, CPC 41487; H, CPC 41488; I, CPC 41489). J. Ribeiria jonesi? from Petermann Creek, sub-rectangular profile and strong dorsally projecting umbo (CPC 41490). K, L. Pinnocaris sp. A from Petermann Creek with inflated pear-shaped profile (K, CPC 41491; L, CPC 41492). M. Technophorus walteri Pojeta, Gilbert-Tomlinson, and Shergold, 1977 from Areyonga Gorge (CPC 41493) with a single lateral radial rib and prominent nearly erect pegma indicated. N, O. Gastropoda, Scalites? sp. from Petermann Creek. N. CPC 41494 (N1), latex cast with clear growth lines (N2). O. CPC 41495, specimen with 3.5 to 4 whorls. Scale bars 10 mm, except C, D, M, 5 mm.

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

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