Pinnocaris sp. A

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

Pinnocaris sp. A sensu Pojeta et al. 1977

Fig. 12K, L .

1977 Pinnocaris sp. A ; Pojeta et al. 1977: 18, pl. 8: 11.

Material.― Four specimens from the Petermann Creek section, Middle Ordovician of Australia. Additional specimens probably belonging to this species are too badly preserved to be safely identified. The two illustrated specimens are numbered CPC 41491 and CPC 41492 .

Description.― Pinnocaris sp. A is pear-shaped in outline being widest anteriorly and tapering rearwards. Shell profile relatively inflated. Ventral margin is arcuate throughout, with dorsal margin tapering from the distinct beak towards posterior margin. Anterior and posterior margins rounded.

Remarks.―According to Pojeta et al. (1977: 18) the species has a highly placed nearly erect pegma. This character cannot be observed in the available specimens due to the state of preservation, but may be indicated by a near vertical trace anterodorsal in one of the illustrated specimens (compare Pojeta et al. 1977: pl. 8: 11 vs. Fig. 12K ).

The specimens at hand somewhat resemble one of the paratypes of Pinnocaris wellsi Pojeta, Gilbert-Tomlinson, and Shergold, 1977 : pl. 9: 9 from the Pacoota Sandstone. However, P. wellsi , as well as other Australian species assigned to Pinnocaris possesses a more narrow-elongated pear-shape.

Pojeta and Gilbert-Tomlinson (1977: pl. 29: 18) illustrated a specimen of Conocardioid sp. A from Tempe Downs (near Petermann Creek). The specimen was originally figured by Tate (1896: pl. 2: 13) and probably derives from the Stokes Siltstone. It resembles specimen CPC 41492 ( Fig. 12L ) regarding overall shape, but the beak is much more prominent in Conocardioid sp. A, which distinguishes it from Pinnocaris sp. A (compare Pojeta and Gilbert-Tomlinson 1977: pl. 29: 18 vs. Fig. 12L ).

Due to the generally poor state of preservation among the specimens and lack of preserved characteristic taxonomic features, Pinnocaris sp. A has not been described formally as a new species.

The species was originally described by Pojeta et al. 1977) based on a single specimen from the Stairway Sandstone. In the present study four specimens were recovered from bed PC - 6 in the Petermann Creek section. Pojeta et al. (1977: 17, 18) described two other species and two other species of Pinnocaris from central Australia of late Cambrian (Furongian) age, making Pinnocaris sp. A the youngest Australian species. Pinnocaris is also known from North America and Scotland ( Pojeta and Runnegar 1976) and these occurrences are not older than Middle Ordovician.

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.