Eohadrotreta sp.

Betts, Marissa J., Claybourn, Thomas M., Brock, Glenn A., Jago, James B., Skovsted, Christian B. & Paterson, John R., 2019, Shelly fossils from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia, Acta Palaeontologica Polonica 64 (3), pp. 489-522 : 504

Eohadrotreta sp. cf. E. zhenbaensis Li and Holmer, 2004

Fig. 10 .

2017 Eohadrotreta sp. cf. E. zhenbaensis ; Betts et al. 2017b: 269, fig. 15A–O.

Material.— Twelve dorsal and four ventral valves from Clast 4; approximately 1675 dorsal and 1060 ventral valves from Clast 5, 14 figured ( SAM P57276–57289). From the Dailyatia odyssei Zone, WPC, Kangaroo Island, South Australia.

Description. — See Holmer and Popov (2007: 2560–2562).

Remarks. — Specimens illustrated here are similar to those figured by Betts et al. (2017b: fig. 15A–O) from the upper Mernmerna Formation (10MS section) in the Bunkers Graben, southern-central Flinders Ranges. Brock in Betts et al. (2017b) outlined taxonomic difficulties associated with Eohadrotreta zhenbaensis from the Shuijingtuo Formation in South Shaanxi, China. Key issues are the variable development of characters such as the dorsal medium septum and the apical process and apical pits on the interior of the ventral valves ( Li and Holmer 2004: 207). Like the specimens from the Flinders Ranges, the material from the WPC clasts do not have the well developed apical pits manifest in the Chinese specimens, hence are referred to Eohadrotreta sp. cf. E. zhenbaensis .

Zhang et al. (2018) defined three distinct ontogenetic stages for Eohadrotreta zhenbaensis in specimens from the Shuijingtuo Formation (Cambrian Series 2, Stage 3, Ajiahe and Wangjiaping sections, Three Gorges area, western Hubei Province, China), some of which are relevant to the identification of Eohadrotreta . Firstly, in valves <450 μm in length, the foramen is developed from a pedicle notch “pedicle foramen forming stage”, T 1). Secondly, when the valve is 450–750 μm in length, a shallow intertrough and apical process develops in the ventral valve (“pedicle foramen enclosing stage”, T 2) ( Zhang et al. 2018: figs. 4L, 5A). Thirdly, when the ventral valve interior has a valve length of>750 μm, the vascula lateralia is developed ( Zhang et al. 2018: fig. 5G), and on dorsal valves <900 μm in length, the bifurcating median septum is developed (“intertrough increasing stage”, T 3) ( Zhang et al. 2018: fig. 5K, L). These growth patterns are observed in Eohadrotreta sp. cf. E. zhenbaensis from the WPC, as well as Eohadrotreta sp.

cf. E. zhenbaensis from the upper Mernmerna Formation (10MS section of the Bunkers Graben, South Australia, Cambrian Series 2, D. odyssei Zone ; Brock in Betts et al. 2017b). In the Australian material, an impressed vascula lateralia is not developed in smaller ventral valves ( Fig. 10C, D ; Betts et al. 2017b: fig. 15B), but is present in larger specimens (cf. Fig. 10K ). In contrast, the median septum of Eohadrotreta sp. cf. E. zhenbaensis from the WPC is not as well developed as in specimens from western Hubei, even in individuals>900 μm in length ( Fig. 10H, N ).

Eohadrotreta sp. cf. E. zhenbaensis from the WPC can be distinguished from Eohadrotreta zhujiahensis Li and Holmer, 2004 from Cambrian Series 2 Shuijingtuo Formation, South China by its pedicle foramen becoming enclosed early in ontogeny ( Li and Holmer 2004: fig. 14C, J; Zhang et al. 2018: fig. 7). The ventral valve intertrough of E. zhujiahensis remains vestigial throughout ontogeny, but becomes better developed in Eohadrotreta sp. cf. E. zhenbaensis ( Fig. 10G 2 ; Zhang et al. 2018: fig. 7). Eohadrotreta sp. cf. E. zhenbaensis can be distinguished from E. haydeni Popov et al. 2015 from the Kaotaia parvatya Trilobite Zone of the Parahio Formation (late Cambrian Stage 4 to Wuliuan Stage of the Parahio Valley, Indian Himalaya), by its better developed ventral valve intertrough and dorsal cardinal muscle scars ( Fig. 10H, I, J, G 2 ; Popov et al. 2015: fig. 22F, H–J, M).

Stratigraphic and geographic range. — Lower Cambrian, South Australia, Arrowie Basin ( D. odyssei Zone ): Upper Mernmerna Formation and limestone beds near the base of the Oraparinna Shale (section 10MS), Bunkers Graben, southern-central Flinders Ranges. Stansbury Basin ( D. odyssei Zone ): WPC clasts, Kangaroo Island.

Figures

Fig. 10. The lingulid brachiopod Eohadrotreta sp. cf. E. zhenbaensis Li and Holmer, 2004 from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia. Dorsal (A, B, H–J, M, N) and ventral (C–G, K, L) valves. A. SAM P57276 in external view (A1); A2, detail of the larval shell. B. SAM P57277 in external view. C. SAM P57278 in internal view. D. SAM P57279 in internal view. E. SAM P57280 in external view. F. SAM P57281 in external view, tilted to show intertrough (F1); F2, detail of the foramen. G. SAM P57282 in external view, tilted to show intertrough (G1); detail of the foramen (G2) showing the intertrough. H. SAM P57283 in internal view. I. SAM P57284 in internal view; I2, close up of interarea; I3, detail of shell microstructre. J. SAM P57285 in internal view. K. SAM P57286 in internal view. L. SAM P57287 in internal view. M. SAM P57288 in internal view. N. SAM P57289 in internal view.

Fig. 2. Correlation chart showing shelly fossil biozones against the stratigraphic succession in the Flinders Ranges, Fleurieu Peninsula, and Kangaroo Island (adapted from Betts et al. 2018: fig. 27). Note: The succession in the NE Kangaroo Island is based on the Investigator-1 drill core in which the Mt. McDonnell Formation is not apparent (Gehling et al. 2011). Lower boundary of White Point Conglomerate (WPC) is likely faulted.Abundant and diverse shelly fauna (SSF) from the bioclastic limestone clasts in the WPC have an upper Dailyatia odyssei Zone age. Dashed lines indicate uncertain stratigraphic relationship. Abbreviations: K., Kulparina; P., Parabadiella; S., Sunnaginia.