Colpantyx 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 : 916-917

Genus Colpantyx Pojeta and Gilbert-Tomlinson, 1977

Type species: Colpantyx woolleyi Pojeta and Gilbert-Tomlinson, 1977 from the late Cambrian to Early Ordovician Pacoota Sandstone, Amadeus Basin, Australia; original designation .

Colpantyx ? sp. A

Fig. 12C, D .

Material.—10 fragmentary internal and external moulds found in beds A +1 to A + 3 in the Areyonga Gorge section only ( Fig. 4 ), Middle Ordovician of Australia. The two figured specimens ( CPC 41483 and CPC 41484) are latex casts made from external moulds.

Description.—Small slightly rounded triangular colpomyid with well-defined umbonal radial ribs, of which two are present anteriorly and one posteriorly ( Fig. 12D ). Except for radial ribs the surface appears smooth. Anterior ribs short and almost straight, radiating until they reach anterior-most part of ventral margin. Posterior rib long and curving towards posterior margin. Anterior and ventral margins straight to broadly rounded; posterior margin straight, oblique towards dorsum. Umbo subdued ( Fig. 12C ) and shell profile low.

Remarks.— Colpantyx is endemic to Australia. The only other species assigned is Colpantyx woolleyi Pojeta and Gilbert-Tomlinson, 1977 from the upper Tremadocian part of the Pacoota Sandstone of the Amadeus Basin (e.g., Shergold 1991; Cope 2004).

The slightly rounded triangular overall shape, the radial well-defined umbonal ribs and the delineation of the margins are quite similar among the two Colpantyx species. However, C. woolleyi has pronounced commarginal ornament and only one or two posterior umbonal radial ribs. Furthermore the posteroventral margin is sinuous between the ribs. In comparison Colpantyx ? sp. A possesses a smooth surface completely lacking commarginal ornament. It is unknown whether these differences are due to preservational artefacts, as the material from the Stairway Sandstone is dolomitized. The posteroventral margin is somewhat sinuous between the ribs but not nearly as pronounced as observed on C. woolleyi . The three umbonal ribs present on Colpantyx ? sp. A are very consistent and differ from the condition in C. woolleyi where only one or two ribs are developed (compare Fig. 12D vs. Pojeta and Gilbert-Tomlinson 1977: pl. 24).

The related species Colpantyx woolleyi occurs in the Pacoota Sandstone at 11 different localities in the Amadeus Basin (Pojeta and Gilbert-Tomlinson 1977: 28). When more material from the Stairway Sandstone becomes available the generic assignment should be investigated, and if identified with confidence as Colpantyx , the range of this genus should be extended from the Tremadocian into the Darriwilian.

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.