Askepasma saproconcha, Topper & Holmer & Skovsted & Brock & Balthasar & Larsson & Stolk & Harper, 2013

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 : 102-106

Askepasma saproconcha sp. nov.

Figs. 4, 5.

2006 Askepasma sp. A ; Jago et al. 2006: 414, fig. 4A, B.

2006 Askepasma sp. ; Holmer et al. 2006: 3, fig. 2a–h.

Etymology: From the Greek sapro, rotten and concha, shell. In reference to the weakly phosphatised shell that easily disintegrates when submerged in acid and the subsequent disappointment of a worker having spent hours in the acid room only to retrieve nothing but fragments from the resulting residues.

Type material: Holotype: SAMP47091 ( Fig. 5A) from Wirrapowie Limestone, originally collected by Brian Daily from a spot locality north of Quorn , supposedly near the Kanyaka ruins, Southern Arrowie Basin . Paratypes: SAMP47085 ( Fig. 4C) , SAMP47087 ( Fig. 4E) , SAMP47088 ( Fig. 4F) , SAMP47089 ( Fig. 4G) , SAMP47092 ( Fig. 5B) , SAMP47093 ( Fig. 5C) and SAMP47095 ( Fig. 5E) from Wirrapowie Limestone , spot locality near the Kanyaka ruins, southern Arrowie Basin . SAMP47084 ( Fig. 4A) and SAMP41646 ( Fig. 4B) from Wirrapowie Limestone , Chace Range, sample CR1−136 . SAMP47086 ( Fig. 4D) from the Wilkawillina Limestone , 4.4 km Northwest of Point Well, Arrowie Basin.

Type locality: Wirrapowie Limestone , spot locality north of Quorn , near Kanyaka ruins, Arrowie Basin .

Type horizon: Spot Locality in the Wirrapowie Limestone.

Diagnosis.—Shell strophic, uniplicate, subquadrate in outline and ventribiconvex; ventral valve strongly convex with maximum convexity at umbo and with well developed high apsacline interarea; delthyrium is wide and triangular lacking homeodeltidium and restricted by pedicle callist; dorsal valve convex with well−developed median fold and well−defined anacline to catacline interarea; ornamentation consisting of concentric lines only; ornament consisting of close−packed polygonal pits that penetrate the entire shell. Partially mineralised shell with inner surface commonly lacking mineralisation.

Description.—Shell ventribiconvex, subquadrate in outline (maximum width 2.39 cm, maximum length 2.04 cm) with maximum width at approximately mid−length. Hingeline straight. Ventral valve strongly convex, with maximum convexity at umbo. Lateral slopes flattened, broad with a well−defined, sharp, sulcus developed in the anterior half of the valve ( Fig. 4C–F, G). Dorsal valve weakly to moderately convex with well−defined fold developed in the anterior half of the valve ( Fig. 5A, B, D 3). Ventral interarea well−defined, moderately high and varies from apsacline to catacline. Delthyrium open and wide, with an obtuse delthyrial angle (114–115 °), width of delthyrium is 3 to 3.2 times the height of the delthyrium (e.g., Fig. 4B 2) with an obvious pedicle callist ( Fig. 4G 3, H). Dorsal valve moderately to strongly convex ( Fig. 5). Dorsal interarea well−defined and low to

http://dx.doi.org/10.4202/app.2011.0146

moderately high. Notothyrium broad, closed entirely by a convex, triangular homeochilidium ( Fig. 5E 3). Post larval shell external ornament of well−developed, irregularly spaced, concentric growth lamellae bearing a micro−ornamentation of hexagonally close−packed, deep hemispherical pits, separated by high walls ( Fig. 4J). Growth lamellae variably coarse and angular ( Fig. 5E). 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) rarely with prominent elliptical cavities ( Figs. 6C , 7D). Shell only partially mineralised with the degree of mineralisation decreasing towards the interior of the shell with the inner surface commonly lacking mineralisation ( Fig. 7). Lateral margins thickened with thin lamellae extensions giving a frayed or hook−like appearance ( Figs. 6B , 7D).

http://dx.doi.org/10.4202/app.2011.0146

Remarks.— Askepasma saproconcha sp. nov. is based on approximately 30 specimens derived from the Wirrapowie and Wilkawillina limestones mainly recovered from the southern Arrowie Basin. The collection of A. saproconcha sp. nov. predominantly consists of crack−out macro−specimens with very few complete valves retrieved using acid−etching techniques. The large majority of acid−etched material is fragmentary as the shell of A. saproconcha sp. nov. is only weakly mineralised, a feature discussed below in the description of the shell microstructure. This species is considered the oldest known brachiopod from the lower Cambrian of South Australia and occurs as the sole brachiopod species in the lowermost Wilkawillina and Wirrapowie limestones. Many specimens of A. saproconcha sp. nov. have been collected from spot localities in the Arrowie Basin making correlation difficult, however, the lack of distinct Abadiella huoi Zone faunal constituents lends support to an early, pre−trilobitic occurrence of this species. This early occurrence of the species in South Australian Cambrian strata may prove useful for regional correlation in future studies. Askepasma saproconcha sp. nov. was first reported by Jago et al. (2006) as occurring in the basal part of the Wilkawillina Limestone type section in the Bunkers Graben and the Wirrapowie Limestone in the Chace Range. Jago et al. (2006: fig. 4A, B) illustrated a single specimen from the Chace Range and referred to it as simply Askepasma sp. A , specimen re−illustrated here ( Fig. 4B).

The new species displays many similarities with the other early Cambrian South Australian paterinate, Askepasma toddense , such as a subquadrate shell shape, apsacline ventral interarea, low, hypercline dorsal interarea and the distinctive polygonal pitting micro−ornament. Askepasma saproconcha sp. nov. can be discriminated by the presence of a well−developed, pronounced chevron−shaped fold and deep sulcus ( Fig. 4F) and a wider delthyrium (compare Fig. 2D–F with Fig. 4B 2). The width of delthyrium in the type species is approximately 1.5 to 1.92 times that of the delthyrium height with a delthyrial angle between 75 and 88 ° (e.g., Fig. 2D 1, F 1 , H). Askepasma saproconcha sp. nov. displays a delthyrium with width that is 3 to 3.2 times that of its height and with a delthyrial angle of 114–115 ° (e.g., Fig. 4B 2). Askepasma toddense is also invariably plicate, however, the degree and development of the fold and sulcus in the two species is strikingly different. The fold in the dorsal valve of A. toddense , if present, is relatively shallow ( Fig. 3B 2, F 1 ) compared to the well−defined, pronounced, very sharp fold displayed in the dorsal valves of Askepasma saproconcha sp. nov. ( Fig. 5A, B, D 3). The shell microstructure also differs between the species with A. saproconcha being much less strongly mineralised. The combination of morphological characteristics possessed by Askepasma saproconcha sp. nov. is unique and readily distinguishes the taxon from other species of Askepasma .

The shell of Askepasma saproconcha sp. nov. is additionally perforated by a type of hollow, phosphatic tubular canals (see Holmer et al. 2006). These canals are invariably oriented subparallel to the shell lamination and were originally documented by Holmer et al. (2006) in specimens recovered from the Wirrapowie Limestone, south of Hawker, near the ruins of Kanyaka Station. However, these canals are only sporadically preserved and it seems that their presence is reliant on the degree of phosphatisation at that particular locality. The specimens documented by Holmer et al. (2006) from the Wirrapowie Limestone for example, are heavily phosphatised in comparison with etched shells from other localities that are invariably fragmentary and even partly dissolved during the acid preparation stage. The reoccurring pattern of shell−penetrative setal canals in brachiopod−like problematic fossils, such as Micrina , Heliomedusa , and Mickwitzia ( Holmer et al. 2002; Williams and Holmer 2002; Balthasar 2004) suggests that shell−penetrative setae are a character trait that may be deeply rooted within the brachiopod stem−group ( Holmer et al. 2002, 2008; Holmer and Popov 2007). Canal−like structures, however, have subsequently been found in other Cambro−Ordovician taxa including the probable stem group linguliformean, Setatella ( Skovsted et al. 2010) and Ordovician orthoid brachiopods ( Jin et al. 2007; Jin and Zhan 2008) as well as in Ordovician siphonotretids ( Popov et al. 2009). The canaliform shell structure observed in Askepasma saproconcha sp. nov. is not a characteristic morphological feature of the genus having not been reported from specimens of A. toddense . The presence of setal canals in unrelated forms and their absence in related forms (e.g., present in A. saproconcha and absent in A. toddense ) may indicate that this is a plesiomorphic feature having been developed and subsequently lost multiple times, proving unhelpful for phylogenetic studies.

Stratigraphic and geographic range.— Lower Cambrian (lower–middle Atdabanian, Terreneuvian, Stage 2, Cambrian Series 2, Stage 3?), basal part of the Wilkawillina Limestone, Wilkawillina Limestone type section, Bunkers Graben. Wirrapowie Limestone, Chace Range and Kanyaka area , Southern Arrowie Basin , South Australia .

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.

Fig. 1. Locality and geological maps showing position of sampled stratigraphic sections and spot localities. A. Mount Scott area with positions of sections AJX−M and AJX−N. B. Bunkers Range showing location of section MMF. C. Geographical position of localities in the Flinders Ranges. D. Location of spot sample taken near the Kanyaka ruins. E. Bunkers Graben, showing location of section 10MS and the Wilkawillina Type Section. F. General position of field area in South Australia. G. Chace, Druid, and Elder ranges, showing position of sections ER9 and CR1 and spot locality in the Druid Range.