Ribeiria csiro 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 : 918-919

Ribeiria csiro Pojeta, Gilbert-Tomlinson, and Shergold, 1977

Fig. 12G–I .

1977 Ribeiria csiro sp. nov.; Pojeta et al. 1977: 13, 14, pl. 3: 3, 4.

1977 Ribeiria csiro ; Pojeta and Gilbert-Tomlinson 1977: 31, pl. 29: 17.

Type material: Holotype ( CPC 14715 ), internal mould . Paratype ( CPC 14716 ), largely imbedded in sandstone with partly preserved shell from the type locality. Both figured by Pojeta et al. (1977: pl. 3: 3, 4) .

Type locality: Stairway Sandstone , South of Maryvale Homestead, Amadeus Basin, southern Northern Territory, Australia .

Type horizon: Silicified sandstone horizon within the Stairway Sandstone.

Material. — 21 specimens, all internal moulds, of which 18 are from the Petermann Creek section and 3 from the Areyonga Gorge section, Middle Ordovician of Australia . Three specimens ( CPC 41487–41489 ) are figured ( Fig. 12G–I ) .

Description.—Medium sized Ribeiria with slightly concave and narrow dorsum. Beak projecting above rest of dorsal margin; anterior margin projecting forward of beak; ventral margin arcuate and shell markedly attenuated posteriorly. Internally a prominent oblique pegma and a shell thickening along the posterodorsal margin produce a notch on internal moulds ( Fig. 12H ). The most distinct character for the species is the prominent triangular beak that projects high above the remaining dorsal margin. In front of the steeply forward descending beak the anterodorsal margin flattens out before ascending slightly towards anterior margin, from where the anterior delineation begins to curve ventrally.

Remarks.—The well-defined prominent beak makes species identification easy if preserved (see Fig. 12G–I ). Ribeiria csiro is the most abundant rostroconch in the collection. Ribeiria is a widespread genus reported from Europe, Morocco, China, Australia and North America ( Pojeta and Runnegar 1976). Pojeta et al. (1977) erected five new Cambro-Ordovician species, all endemic to Australia, of which Ribeiria csiro is the only species reported from the Stairway Sandstone.

Stratigraphic and geographic range.― The specimens from the Petermann Creek section were found in the two lowermost fossiliferous beds (PC -6–PC -5), whereas the species is occurring at higher stratigraphic levels in the Areyonga Gorge section (bed A +1–A +4), see Figs. 4 and 5 . Additional to the type material Pojeta and Gilbert-Tomlinson (1977: pl. 29: 17) illustrated an internal mould ( CPC 15601 ) from a different Stairway Sandstone locality very close to the Petermann Creek section. Together there is a total of four occurrences of the species. The range of Ribeiria csiro 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. 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.