Modiolopsis pojetai, Jakobsen & Brock & Nielsen & Harper, 2016

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 : 915-916

Modiolopsis pojetai sp. nov.

Fig. 12A, B .

Etymology: In honour of John Pojeta Jr. (Smithsonian Institution, National Museum of Natural History, Washington D.C., USA), who did much pioneering work on Ordovician molluscs from central Australia. Type material: Holotype CPC 41481 ( Fig. 12A ). Paratype CPC 41482 ( Fig. 12B ), derived from the type locality.

Type locality: Petermann Creek section, close to Tempe Downs, Amadeus Basin , southern Northern Territory .

Type horizon: Bed PC +9 of the Stairway Sandstone in the Petermann Creek section (Figs. 3, 5) .

Material.— 17 specimens from the Petermann Creek section and 16 specimens from the Areyonga Gorge section, Middle Ordovician of Australia. The material consists exclusively of internal moulds .

Diagnosis.—Inequilateral Modiolopsis species with sub-rounded and well-defined umbo. Shell expands in posterior direction and is delineated by rounded posterior margin; umbo prominent and steeply descending anteriorly. A discrete longitudinal slit-like mark reaching far posterior on internal moulds. No teeth present in the material.

Description.—Shell ovate-elongate with rounded posterior margin and straight to oblique dorsal and ventral margins. Shell profile high at umbonal area but fades out in posterior direction and becomes relatively flat. Internal moulds show a longitudinal slit-like mark, probably due to a longitudinal rib that has been present ( Fig. 12A ).

Remarks.— Modiolopsis Hall, 1847 is regarded as being edentulous by Ulrich (1924), and the absence of this important character supports the generic assignment of the Stairway Sandstone specimens. Ulrich (1924) described the ligament of Modiolopsis (Modiodesma) as having both inner and outer parts, the inner part being supported by a longitudinal rib which left a slit-like mark on moulds (see e.g., Pojeta 1971: pl. 12: 4). This slit-like mark appears present in Modiolopsis pojetai sp. nov. (see Fig. 12A ). Usually, however, silicified specimens of Ordovician modiolopsids show no sign of an internal longitudinal rib ( Pojeta 1971: 21) and it is possible that the “longitudinal slit” was made by the dorsal margin of the shell. Admittedly the slit has been difficult to photograph and may not be perfectly clear on the figures. However, it is clearly present when looking at the actual specimens. The slit may also be referred to as a preduplivincular ligament (see Carter et al. 2012).

Pojeta and Gilbert-Tomlinson (1977: 26) discussed Modiolopsis ? sp. A based on three specimens found in the Stairway Sandstone in the James Range about 150 km east of Areyonga Gorge. One poorly preserved internal mould was illustrated (Pojeta and Gilbert-Tomlinson 1977: pl. 23: 10) and the ovate and elongate shape somewhat resembles that of M. pojetai sp. nov., but the dorsal and ventral margins are straight without being oblique. More importantly, the shell of Modiolopsis ? sp. A is not nearly as expanding posteriorly as in M. pojetai sp. nov.

Modiolopsis gordonensis was reported from the Ordovician Gordon Limestone (Gordon Group) of Tasmania by Johnston (1888). This species is truncate so that the umbo is terminal and this feature differentiates it from M. pojetai sp. nov. In this species the umbo descends steeply towards the anterior margin, but is not terminal positioned. For further discussion on M. gordonensis , see Pojeta and Gilbert-Tomlinson (1977: 26).

The North American species Modiolopsis modiolaris Conrad, 1838 ) is somewhat similar to Modiolopsis pojetai sp. nov., except that the projection of the anterior margin descends more steeply, resulting in a somewhat shorter transverse) anterior margin in M. pojetai sp. nov. (compare Fig. 12A vs. Pojeta 1971: pl. 15: 1 and pl. 16: 2). However, M. pojetai sp. nov. somewhat resembles Modiolopsis cuyana Sánchez, 1990 , as reported from the Upper Ordovician in the Precordillera, Argentina by Sánchez (compare Fig. 12A vs. Sánchez 1990: pl. 1: 8). No obvious differences can be distinguished and Modiolopsis cuyana Sánchez, 1990 is probably more closely related to M. pojetai sp. nov. compared to the North American species of Modiolopsis modiolaris .

Stratigraphic and geographic range.— Modiolopsis pojetai sp. nov. ranges from bed PC -6 to PC + 10 in the Petermann Creek section and from bed A -4 to A + 4 in the Areyonga Gorge section (see Figs. 4 , 5 ). The new species is endemic to the Amadeus Basin and it is one of very few modiolopsids recovered from Middle Ordovician successions in the Southern Hemisphere. The genus ranges through most of the Ordovician from the Floian to the end Hirnantian (e.g., Cope 2004) and may eventually prove useful for palaeobiogeographic correlation. The range of Modiolopsis pojetai sp. nov. is in the middle part of 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.