Ribeiria jonesi? Pojeta, Gilbert-Tomlinson, and Shergold, 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 : 919

Ribeiria jonesi? Pojeta, Gilbert-Tomlinson, and Shergold, 1977

Fig. 12J .

?1977 Ribeiria jonesi sp. nov.; Pojeta et al. 1977: 15, pl. 4: 1, 2, 6–9;

pl. 11: 8–15.

Type material: Holotype ( CPC 14787 ) figured by Pojeta et al. (1977: pl. 11: 14). Eight paratypes ( CPC 14709 , 14711 , 14782 , 14783 , 14784 , 14785 , 14786 , 14788 ) figured by Pojeta et al. (1977: pl. 4: 1, 2, 6–9; pl. 11: 8–13, 15) from type locality.

Type locality: Pacoota Sandstone in the Amadeus Basin, Australia .

Type horizon: Saukiid trilobites found in the same beds indicate a late Cambrian (Furongian) age.

Material.―A single internal mould (CPC 41490) recovered from bed PC +8. It is the only rostroconch from the upper part of the Petermann Creek section ( Fig. 5 ), Middle Ordovician of Australia. The facies in this upper part is more silty than the quartzite in which Ribeiria csiro occurs in the lower part of the section.

Description.―Small Ribeiria with sub-rectangular profile. Strong dorsally projecting umbo, the dorsal margin is convex but flattens out in posterior direction. Anterior margin projecting forward of beak; ventral margin straight or nearly straight; posterior margin nearly straight to slightly rounded. Pegma strong and oblique.

Remarks.―Due to the fragmentary umbo on the specimen it cannot be assigned to Ribeiria jonesi with certainty. However, part of the umbo may be partly preserved above the specimen ( Fig. 12J ) and indicates a strong and prominent umbo. The strong pegma also resembles that of Ribeiria jonesi figured by Pojeta et al. (1977: pl. 11: 8–15) and the straight ventral margin is also characteristic. In comparison with Ribeiria csiro the beak of specimen CPC 41490 appears more posteriorly directed versus straight dorsally directed in R. csiro (compare Fig. 12H vs. Fig. 12J ). The overall shape also differs with R.? jonesi being more rectangular and sharply demarcated versus more rounded ventral, anterior and posterior margins in R. csiro .

Comparison of R.? jonesi with one of the paratypes figured by Pojeta et al. (1977, pl. 11: 11) shows no obvious differences. The specimen certainly belongs to Ribeiria and if not representing Ribeiria jonesi , it probably belongs to a new closely related species.

Stratigraphic and geographic range.― All of the type material was found in the late Cambrian (Furongian) to Early Ordovician Pacoota Sandstone, Australia. However, additional material from the coeval Tomahawk sandstone beds in the Georgina Basin was mentioned but not illustrated by Pojeta et al. (1977: 15). Ribeiria jonesi ranges from the late Cambrian (Furongian) to the Middle Ordovician (Darriwilian), if the specimen found in the Stairway Sandstone is correctly assigned .

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.