Johnmartinia cordata 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 : 912

Johnmartinia cordata Pojeta and Gilbert-Tomlinson, 1977

Fig. 11B .

1977 Johnmartinia cordata sp. nov. Pojeta and Gilbert-Tomlinson

1977: 14, pl. 8: 1–14.

Holotype: Incomplete specimen CPC 15514 , figured by Pojeta and Gilbert-Tomlinson (1977: pl. 8, 4–7).

Type locality: James Range, Amadeus Basin, southern Northern Territory, Australia.

Type horizon: Sandstone moulds from the Stairway Sandstone, Middle Ordovician (no specific horizon mentioned).

Material.― Only one specimen ( CPC 41469 ) has been identified with certainty from the Areyonga Gorge section, Middle Ordovician of Australia. Fragmentary internal moulds from the same bed may also belong to this species but they are too poorly preserved to be confidently identified .

Description.―Shell markedly inequilateral. Dorsal margin arcuate, anterior and posterior margins rounded and projecting, ventral margin rounded and projecting. Umbones highly prominent and strongly divergent ( Fig. 11B 2 ), not projecting above dorsal commissure. Teeth anterior to umbo very short or absent (Pojeta and Gilbert-Tomlinson 1977).

Remarks.― Johnmartinia cordata shares a number of characters with Sthenodonta eastii , including the course of the posterior tooth row, development of umbo and inequilateral shell form. However, the row of teeth anterior to umbo is virtually absent in Johnmartinia and there is significant interumbonal growth, which place the umbo far from the midline, precluding assignment to the Nucularcidae .

Johnmartinia cordata has the most acutely tapered and prominent umbo among the bivalves investigated. The figured specimen ( Fig. 11B 1) also exhibits an elongate border area following the dorsal delineation. This distinct character is also evident on the internal mould illustrated by Pojeta and Gilbert-Tomlinson (1977: pl. 8: 1–3).

Stratigraphic and geographic range.―In addition to the occurrence in bed A- 9 in the Areyonga Gorge section Pojeta and Gilbert-Tomlinson (1977: 15) reported the species from the Stairway Sandstone at four localities within the Amadeus Basin. Johnmartinia is endemic to Australia. The range of the species 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. 11. Nucularcid and pteriomorphian bivalves from the Stairway Sandstone, Middle Ordovician, Australia. Internal moulds (B–K) and latex casts of external moulds (A, L, M). A. Sthenodonta sp. B (CPC 41468) from Petermann Creek, left-lateral view showing well marked growth lines. B. Johnmartinia cordata Pojeta and Gilbert-Tomlinson, 1977 (CPC 41469) from Areyonga Gorge; left-lateral view with dorsal border (B1), anterior view showing the pointy beak (B2). C–F. Nuculites wattii Tate, 1896 from Areyonga Gorge. C–E. Specimens in left (C, E) and right-lateral (D) dorsal views, showing well-defined anterior myophoric buttress, D additionally shows dorsal teeth (C, CPC 41470; D, CPC 41472; E, CPC 41472). F. Right-lateral view of fragmentary specimen with chevron shaped posterior teeth pointing anteriorly (CPC 41473). G. Cyrtodonta carberryi Pojeta and Gilbert-Tomlinson, 1977 (CPC 41474) from Areyonga Gorge; in right-lateral view with growth lines and at least one dorsal lateral tooth (G1), dorsal view illustrating the height of shell profile (G2). H–K. Cyrtodonta staffordae Pojeta and Gilbert-Tomlinson, 1977 from Petermann Creek (H–I, K) and Areyonga Gorge (J). H. Specimen in right-lateral view showing distinct growth lines (CPC 41475). I. Specimen in left-lateral view with three posterior lateral teeth (CPC 41476). J. Fragmentary posterior margin with duplivincular ligament and indications of three posterior lateral teeth (CPC 41478). K. Specimen in right-lateral view with a posterior lateral tooth (CPC 41477). L, M. Cyrtodonta sp. B. L. Anterior (L1) and left-lateral (L2) views of sub-ovate shell-profile (CPC 41479) from Petermann Creek. M. Sub-ovate shell-profile with growth lines (CPC 41480) from Areyonga Gorge. Scale bars 10 mm.

Fig. 2. Distribution of the Larapintine Sea in the Middle Ordovician. The extent of the Canning, Amadeus, and Georgina basins, all containing Middle Ordovician deposits, have been inferred together with the extent of continental landmass (marked with horizontal lines). Base map modified from Webby 1978. A. Webby’s interpretation (1978). B. New interpretation based on faunal correlation. Note the broken connection between the Amadeus–Georgina basins and the Canning Basin.