Askepasma toddense Laurie, 1986

Topper, Timothy P., Holmer, Lars E., Skovsted, Christian B., Brock, Glenn A., Balthasar, Uwe, Larsson, Cecilia M., Stolk, Sandra Pettersson & Harper, David A. T., 2013, The oldest brachiopods from the lower Cambrian of South Australia, Acta Palaeontologica Polonica 58 (1), pp. 93-109 : 98-102

Askepasma toddense Laurie, 1986

Figs. 2, 3.

1986 Askepasma toddense ; Laurie 1986: 449, figs. 5G, 11A–O.

1998? Aldanotreta sp. ; Williams et al. 1998: 222.

1998 Paterina ? sp.; Williams et al. 1998: 223, pls. 1: 1, 5: 6–8.

2001 Askepasma ? sp.; Holmer and Ushatinskaya in Ushatinskaya and Holmer in Gravestock et al. 2001: 122, pls. 15: 1–10, 16: 1–9.

2006 Askepasma sp. B ; Jago et al. 2006: fig. 4C, D.

2009 Askepasma ; Balthasar et al. 2009: figs. 1C, F, H–J, 2E.

Material.—Consists of 20 illustrated specimens in addition to hundreds of dorsal and ventral valves.

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Emended diagnosis.—Shell strophic, subquadrate in outline and ventribiconvex; ventral valve rarely weakly sulcate, with well−defined, high, apsacline to catacline interarea, lacking a homeodeltidium; pedicle callist present; dorsal valve infrequently with weak to moderate fold; dorsal interarea well defined, low, planar, anacline to catacline; with well developed small, apical homeochilidium; shell ornamentation consisting of irregular concentric growth lines, ornamentation consisting of close−packed polygonal pits that penetrate the entire shell.

Description.—Shell ventribiconvex, subquadrate in outline (maximum width 6.2 mm, maximum length 5.1 mm) with maximum width approximately mid−length. Hinge line straight. Ventral valve moderately to strongly convex, with maximum convexity at umbo ( Fig. 2). Lateral slopes flattened, broad with a shallow sulcus developed in the anterior half of the valve, although not always present. Larval shell (maximum width 642 µm), generally rounded and bulbous with four lobes ( Fig. 3A 3). Ventral interarea well−defined, high and varies from apsacline to catacline. Delthyrium open, width of delthyrium is 1.5 to 1.92 times the height of the delthyrium (e.g., Fig. 2D–F) with the apex occupied by a large depressed pedicle callist ( Fig. 2F 2). Dorsal valve weakly convex to flattened, infrequently with weak to moderate fold ( Fig. 3). Dorsal interarea well−defined, low and varies from anacline to hypercline ( Fig. 3A 2, B 2, C 2). Notothyrium broad, closed entirely by a convex (weak to strong), triangular homeochilidium ( Fig. 3A 3, B 3, C 3). Post larval shell external ornament of well developed, irregularly spaced, concentric growth lamellae bearing a ornamentation of hexagonally close−packed, deep hemispherical pits (width of pits range from 4 to 10 µm, maximum depth of pits 5 µm), separated by high walls ( Figs. 2C 2, 6A ). Larval shell covered by irregular shallow, polygonal pits, separated by low, rounded walls ( Fig. 3A 3, B 3). Larval shell ornament frequently not well developed, giving a wrinkled appearance. Ventral and dorsal interior with variably developed ridges, typically two, primary submedian ridges diverging anteriorly at approximately 10–15 ° ( Fig. 3F 2). Some ventral valves display a series of ridges (up to 9) diverging anteriorly from the pedicle callist ( Fig. 2G). Dorsal interior may also display a slightly more prominent, submedian ridge, towards the anterior half of the valve ( Fig. 3E 2, G). Juveniles are ventribiconvex, rarely with any fold or sulcus present ( Figs. 2I, 3H). Juvenile ventral valves are weakly to moderately convex with interareas moderately high and nearly always procline ( Fig. 2I). Dorsal valves of juveniles, flattened with poorly defined interareas and almost always orthocline ( Fig. 3H). Ornament is the same as in adult specimens. Shell microstructure displays a rhythmic succession of alternating thin heavily mineralised compact laminae (around 5 µm thick; Fig. 7) and thicker less strongly mineralised laminae ( Fig. 7) with prominent elliptical cavities ( Figs. 6C , 7D). Lateral margins thickened with thin lamellae extensions giving a frayed or hook−like appearance ( Figs. 6B , 7D).

Remarks.—The genus Askepasma was originally erected on the basis of eighteen specimens recovered from the lower Cambrian Todd River Dolostone in the Northern Territory of Australia ( Laurie 1986). The collection of Askepasma toddense from the lower Cambrian of the Arrowie Basin currently consists of hundreds of dorsal and ventral valves and within the assemblage there is a wide range of ontogenetic and morphological variation, primarily in inclination of the interarea and prominence of the dorsal fold. Dorsal valves for instance, display a variation in the inclination of the dorsal cardinal area from hypercline to anacline (compare Fig. 3B and Fig. 3C). Ventral valves predominantly display a slightly concave, apsacline interarea ( Fig. 2D 2), rarely showing variation to a catacline and procline inclination of the ventral cardinal area. Juveniles though, almost always exhibit a procline ventral interarea (see Fig. 2I) with the interarea in dorsal valves poorly developed ( Fig. 3H). Ventral valves rarely display sulcation, however dorsal valves invariably display a weak to moderate fold that is developed in the anterior half of the valve ( Fig. 3B, F). Such morphological variation is present in individual samples as well as throughout stratigraphic sections and is herein treated as intra−specific variation.

Ushatinskaya and Holmer in Gravestock et al. (2001: 122) described a largely fragmentary assemblage from various Arrowie and Stansbury basin localities, noting the particular wide range of morphological variation within the collection, specifically shell shape and sulcation. Despite documenting isolated valves and fragments from five different localities, Ushatinskaya and Holmer in Gravestock et al. (2001: 122, pls. 15: 1–10, 16: 1–8) only illustrated specimens from the Ajax Limestone, Mount Scott Range and a single specimen from the Parara Limestone retrieved from drill core SYC−101 (Ushatinskaya and Holmer in Gravestock et al. 2001: pl. 16: 9). Despite illustrated specimens (Ushatinskaya and Holmer in Gravestock et al. 2001: 122, pls. 15: 1–10, 16:

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1–9) displaying morphological variation all individuals fit within the scope of Askepasma toddense and whilst it is possible that their collection contained specimens of Askepasma saproconcha sp. nov. without further information its presence is difficult to confirm.

In a biostratigraphic review of the Cambrian of South Australia, Jago et al. (2006: fig. 4C, D) illustrated two specimens of Askepasma toddense , there referred to as Askepasma sp. B from Abadiella huoi Zone equivalent strata in the shallow−water setting of the Wirrealpa Hinge Zone, specimens that are re−illustrated here ( Fig. 3A, F). Unlike Askepasma saproconcha sp. nov. which is restricted to pre−trilobitic strata, Askepasma toddense , ranges from immediately below the base of the Abadiella huoi Zone into the upper Atdabanian where it co−occurs with other linguliform brachiopods, such as Eoobolus , Eodicellomus , and Kyrshabaktella ( Jago et al. 2006) . The stratigraphic separation of the two species suggests use in further biostratigraphic studies and may be used to facilitate correlation across Cambrian basins in Australia.

Peng et al. (2010: 372, pl. 3) recently documented a new species of Askepasma from the lower Cambrian Balang Formation in Eastern Guizhou. Askepasma transversalis Peng, Zhao, Qin, Yan, and Ma, 2010 represents the first occurrence of the genus outside of Australia extending its geographic range to South China. Askepasma transversalis shares many similarities with the type species and is characterised by a straight hinge line, well−defined dorsal and ventral interareas with a wide delthyrium capped by a large pedicle callist. The main morphological difference between the two species is the prominent pedicle beak displayed by A. transversalis ( Peng et al. 2010: pls. 3, 6, 9). Differences in preservation between the Chinese and Australian material makes comparison difficult; the Chinese material is represented by crack out specimens deposited in deep−water, shelf to slope environment of the Balang Formation ( Peng et al. 2010). Askepasma transversalis is also slightly younger than A. toddense with the Balang Formation of mid to late Botomian age ( Palaeolenus –Megapalaeolenus Assemblage zones) (see Steiner et al. 2007).

Geographic and stratigraphic range.—Lower Cambrian (middle–upper Atdabanian, Cambrian Series 2, Stage 3–4?), Ajax Limestone (Mount Scott Range), Wilkawillina Limestone (Bunyeroo Gorge, Ten Mile Creek, and Bunkers Range), Wirrapowie Limestone (Druid Range and Elder Range), Mernmerna Formation (Ten Mile Creek) and Parara Limestone (Horse Gully and SYC−101).

Figures

Fig. 6. Backscatter SEM images of polished cross−sections of Cambrian paterinate brachiopods, Askepasma saproconcha sp. nov. (A) and Askepasma toddense Laurie, 1986 (B–E). A. SAMP47096, Wilkawillina Limestone, Narina Road, showing polygonal ornament exposed along a crack (top left corner of image) and the traces of polygonal structures through the shell. B. SAMP43361, Wilkawillina Limestone, sample MMF0.0, in limestone matrix (B1), transversal cross section showing thin fringes along the external shell surface; boxes show position of D and E; sketches of the lateral margins (B2). Details of B showing the polygonal structures of the fringes along the external shell surface (D1, E1); labelled sketches (D2, E2) showing different stages of closure of polygonal compartments; dotted line indicates a hypothetic lamina that sealed the compartments and acted as surface for apatite along the top of the compartments (ceiling deposits). C. SAMP 43332, Wilkawillina Limestone, sample MMF0.0, longitudinal cross−section of a specimen that was isolated by acid maceration and embedded in resin.