Kyrshabaktella davidi Holmer and Ushatinskaya

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 : 500-501

Kyrshabaktella davidi Holmer and Ushatinskaya in Gravestock et al., 2001

Fig. 7D–T .

2001 Kyrshabaktella davidi ; Holmer and Ushatinskaya in Gravestock et al. 2001: 127, pl. 20: 1–10.

2006 Kyrshabaktella davidi ; Jago et al. 2006: 414, fig. 4E, F.

2016 Kyrshabaktella davidii ; Betts et al. 2016: 195, fig. 17Q–T.

Material.— Fifteen dorsal valves and 24 ventral valves from Clast 1, nine dorsal valves and 21 ventral valves from Clast 4 and 219 dorsal valves and 288 ventral valves from Clast 5; 17 figured ( SAM P57345–57261). All from the Dailyatia odyssei Zone, WPC, Kangaroo Island, South Australia.

Description.—Valves oval to circular in outline ( Fig. 7E, G, K, L, O 1 –R ); weakly biconvex, slightly longer than wide with maximum width from mid-length to anterior. Ventral valves 1.10–2.85 mm long (mean 2.04 mm, N = 11), 0.95– 2.56 mm wide (mean 1.75, N = 9), 110–125% long as they are wide; dorsal valves 1.15–2.30 mm long (mean 1.84, N = 12), 1.00– 2.47 mm wide (mean 1.75 mm, N = 12), 95–98% long as they are wide. Ventral valve is gently convex, acuminate posterior and rounded anterior ( Fig. 7D–H 1 , Q). Dorsal valve gently convex and subcircular in outline ( Fig. 7K–M, O 1 ). Metamorphic shell is almost circular and smooth. Adult shells are covered by thin concentric lines and sometimes by fine radial striae on the lateral slopes ( Fig. 7O 2 ).

Ventral pseudointerarea is divided by emarginature into two triangular propareas with well-developed flexure lines ( Fig. 7D–H, S, T ). Ventral interior with thickened visceral area ( Fig. 7D, E, H 2 ), extending to mid-length with umbonal muscle scars barely discernable. Elongate, well-impressed postero-lateral muscle scars located below and distal to the propareas ( Fig. 7D, E, G ).

Dorsal pseudointerarea is moderately high, subtriangular and orthocline ( Fig. 7I–M ). Propareas are narrow, curved and elongate with weak flexure lines. Postero-lateral muscles scars elongate ( Fig. 7K, L ), remaining musculature not preserved. Broad but poorly developed median ridge extending beyond mid-length of valve ( Fig. 7K, L ).

Remarks. —The recent history of this monogeneric linguloid family has been discussed in detail by Skovsted and Holmer (2006) and Streng et al. (2008) and need not be repeated here. The discovery of a pitted larval shell and columnar shell structure in specimens of Kyshabaktella sp. from the lower Cambrian Harkless Formation, Nevada ( Skovsted and Holmer 2006: fig. 3) and the recognition of similar shell structure in valves of K. mudedirri from the middle Cambrian of the Georgina Basin (see Kruse 1991: fig. 6c) indicates that columnar shell structure is relatively widespread within Lingulida . It should be noted that the presence of columnar shell structure and a pitted metamorphic shell has not yet been documented in the type species of the genus, K. certa from the Wuliuan–early Drumian of Kazakhstan. Until these structures are confirmed in the type species, it is possible that the described specimens from Nevada and Australia do not belong to Kyrshabaktella (sensu stricto).

Specimens of Kyrshabaktella davidi from the WPC reach a much larger maximum size (up to 2.85 mm in length, 2.56 mm in width, Fig. 7H 1 ) than those originally described by Holmer and Ushatinskaya in Gravestock et al. (2001), who reported valve lengths of 0.96–1.25 mm and widths of 1.02–1.17 mm for material from the CD-2 drillcore, Parara Limestone on the Yorke Peninsula, South Australia. This is less than half the maximum dimensions of K. davidi from the WPC.

On ventral valves of K. davidi from the WPC, the pedicle groove is typically more adpressed to the valve floor ( Fig. 7D–H, S, T ), whereas specimens from the Parara Limestone have a pedicle groove raised above the valve floor (Holmer and Ushatinskaya in Gravestock et al. 2001: pl. 20: 1b, 6). Specimens from the WPC lack the vascula lateralia and vascula media described figured by Holmer and Ushatinskaya in Gravestock et al. (2001: pl. 20: 1b, 5b), though this may be preservational.

The specimens described here can be readily distinguished from the type species Kyrshabaktella certa Koneva, 1986 from the Amgan of Kazahkstan (Wuliuan Stage–earliest Drumian Stage), which has well-developed flexure lines on the much narrower propareas of the ventral valve ( Holmer et al. 2001: 34, pl. 5: 7, 10). Kyrshabaktella mudedirri from the Tindall Limestone, Northern Territory, Australia ( Kruse 1990: pl. 10: D; Terreneuvian, Stage 2–Wuliuan) shares some similarities with K. davidi from the WPC. Both have similarly adpressed pedicle grooves and relatively narrow propareas on the ventral valve. However, they can be distinguished, as K. mudedirri has a beak at the posterior apex of the dorsal valves ( Kruse 1990: pl. 10: H, J), a shorter pedicle groove, and a more rounded delthyrial opening on the ventral valves ( Kruse 1990: pl. 10: C, D). The recently described Kyrshabaktella diabola Skovsted, Knight, Balthasar, and Boyce, 2017 from the Forteau Formation of Laurentian Newfoundland and southern Labrador (mid-Dyeran; Cambrian Series 2, Stage 4) can be distinguished from K. davidi by its narrow pseudointerarea with elongated propareas on the ventral valve ( Skovsted et al. 2017: fig. 14.3), and well-developed diamond-shaped dorsal pedicle groove ( Skovsted et al. 2017: figs. 14.9, 14.10). Kyrshabaktella diabola also lacks flexure lines and has a broader pedicle groove than K. davidi ( Skovsted et al. 2017: figs. 14.2, 14.3).

Stratigraphic and geographic range.— South Australia, lower Cambrian: Stuart Shelf, Andamooka Limestone. Arrowie Basin ( Micrina etheridgei – Dailyatia odyssei zones): Ajax Limestone, Mt. Scott Range ; Nepabunna Siltstone, north-east Flinders Ranges; Winnitinny Creek Member of the Wilkawillina Limestone; Bunkers Range, Second Plain Creek Member of the Wilkawillina Limestone, Bunkers Graben; Wirrapowie Limestone, south-west Arrowie Syncline; Six Mile Bore, Linns Springs and Third Plain Creek members of the Mernmerna Formation, Bunkers Range and Bunkers Graben; Upper Mernmerna Formation, Donkey Bore Syncline; Mernmerna Formation, Southern Arrowie Syncline, Chambers Gorge region, eastern Flinders Ranges. Stansbury Basin: Curramulka Quarry and CD-2 drillcore on the Yorke Peninsula, WPC clasts, Kangaroo Island.

Figures

Fig. 7. The lingulid brachiopods Eoobolus sp. (A–C) and Kyrshabaktella davidi Holmer and Ushatinskaya in Gravestock et al., 2001 (D–T) from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia. Ventral (A–H, N, Q–T) and dorsal (I–M, O, P) valves. A. SAM P57242 in internal view. B. SAM P57243 in internal view. C. SAM P57244, in external view (C1); C2, detail of post larval shell. D. SAM P57245 in internal view. E. SAM P57246 in oblique view. F. SAM P57247 in internal view. G. SAM P57248 in internal view. H. SAM P57249 in internal view (H1); H2, detail of the interarea. I. SAM P57250 in internal view. J. SAM P57251 in internal view. K. SAM P57252 in internal view. L. SAM P57253 in internal view. M. SAM P57254 in internal view. N. SAM P57255 in external view. O. SAM P57256 in external view (O1); O2, detail of external radial and concentric ornament. P. SAM P57257 in external view. Q. SAM P57258 in external view. R. SAM P57259 in external view. S. SAM P57260 in internal view. T. SAM P57261 in internal view.

Fig. 1. Map showing study area on northern Kangaroo Island, Australia. Dashed line indicates Stansbury Basin extent.

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.