Lapworthella fasciculata Conway Morris and Bengtson

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 : 509-511

Lapworthella fasciculata Conway Morris and Bengtson in Bengtson et al., 1990

Fig. 13G–N.

1990 Lapworthella fasciculata ; Conway Morris and Bengtson in Bengtson et al. 1990: 122, figs. 74–76.

1990 Lapworthella cf. fasciculata ; Conway Morris and Bengtson in Bengtson et al. 1990: 124, figs. 77, 78.

2001 Lapworthella fasciculata ; Demidenko in Gravestock et al. 2001: 116, pl. 8: 1–3.

2004 Lapworthella fasciculata ; Demidenko 2004: pl. 1: 1–9, pl. 2: 1–7.

2009 Lapworthella fasciculata ; Topper et al. 2009: 214, fig. 5A–H.

2016 Lapworthella fasciculata ; Betts et al. 2016: fig. 21A–G, K.

Material.— One specimen from Clast 1, and 17 from Clast 5; eight figured ( SAM P57317–57324 ). From the Dailyatia odyssei Zone, WPC, Kangaroo Island , South Australia .

Description.—See Topper et al. (2009: 214).

Remarks. — Lapworthella fasciculata is a common component in early Cambrian shelly fossil assemblages from South Australia. It can be extremely abundant in some horizons, e.g., probable lag deposit accumulations in the Second Plain Creek Member of the Wilkawillina Limestone, Bunkers Graben, Flinders Ranges ( Smith 2006; Betts et al. 2016: app. 8). Lapworthella fasciculata in the WPC is represented by isolated sclerites that are either curved, conical shells with a sub-quadrate cross-section (Fig. 13H, I, M, N), spine-like forms (Fig. 13J, K) or are broad and flattened (Fig. 13L). Growth sets are separated by slightly raised ridges (Fig. 13H, J, M), usually ornamented by small (5–10 μm across) pustules (Fig. 13G). Lapworthella fasciculata is often septate (though septae are not visible in the WPC material).

Lapworthella sclerites occur as either sinistrally and dextrally twisted forms, and conjoined elements are rare ( Demidenko 2004: pl. 1: 1–9; Gravestock et al. 2001: pl. 8: 3). Fused sclerites demonstrate that the skeletal elements were closely spaced on the body, merging during growth to form an external skeleton ( Demidenko 2004). The broad, flattened sclerite from the WPC appears to have had multiple apices (Fig. 13L), though none of the sclerites from the WPC exhibit the elaborate spinose morphology of the conjoined sclerites described by Demidenko (2004).

Lapworthella fasciculata from the WPC are similar to L. fasciculata described and figured by Topper et al. (2009: fig. 5A–H) from the Mernmerna Formation in the Flinders Ranges ( D. odyssei Zone ). Both bear the pustulose ridges and distinctive fasciculate microornament between co-marginal ribs, and both assemblages include spine-like forms and curved, pyramidal forms with quadrate apertures. Betts et al. (2017: fig. 21A–G, K) also figured L. fasciculata from the Wilkawillina Limestone and Mernmerna Formation in the Flinders Ranges that exhibit the same sub-quadrate pyramidial morphology and microornament as in the WPC specimens.

Species of Lapworthella were globally distributed during the early Cambrian,and have been recovered from east and west Avalonia, Baltica, Laurentia, Siberia, South China and West Gondwana (Missarzhevsky in Rozanov and Missarzhevsky 1966; Qian and Bengtson 1989; Bengtson 1980; Devaere et al. 2014a; Devaere and Skovsted 2017). Lapworthella fasciculata is only known from the lower Cambrian of South Australia, where it ranges from the Micrina etheridgei Zone to the Dailyatia odyssei Zone (Terreneuvian, Stage 2–Series 2, Stage 3) in the new shelly fossil biostratigraphic scheme of Betts et al. (2016, 2017b). Material assigned to L. fasciculata by Wrona (1989) from Antarctic glacial erratics may be a species of Kelanella . Both have septate sclerites with similar concentric and radial ornament ( Devaere et al. 2014a; Betts et al. 2017b). The scleritome of Kelanella is still poorly resolved (see below), though sclerites assigned to Kelanella are often large (fragments up to ~ 5 mm in width; Betts et al. 2017b: fig. 14N–U). Lapworthella fasciculata sclerites are usually up to ~ 1 mm long (Fig. 13I–N).

A single, ~ 1 mm long, elongate, flattened tube-like sclerite from the WPC clasts is assigned to Kelanella sp. (Fig. 13O). This sclerite and L. fasciculata both have similar external ornament of concentric co-marginal growth sets. Growth sets consist of fine, raised ribs bordering thicker zones with longitudinal, fasciculate striae (Fig. 13G, N, O 4). The taxa can be distinguished by the presence of pustulose ridges separating growth sets in L. fasciculata and the presence of fine pustules on the internal surface of Kelanella sp. (Fig. 13O 7, O 8), which is not present in L. fasciculata .

A tommotiid-like “mitrosagophoran sclerite” was reported from Cambrian erratics from King George Island, West Antarctica ( Wrona 1989: pl. 10: 3; Wrona and Zhuravlev 1996), which was referred to Lapworthella fasciculata by Topper et al. (2009). Later, Wrona (2004) reported Lapworthella fasciculata from the same glacial erratics. These specimens however, while septate, are more likely to be Kelanella based on external ornamentation, relatively large size and their overall compressed shape.

Stratigraphic and geographic range. — Lower Cambrian of South Australia: Arrowie Basin ( Micrina etheridgei – Dailyatia odyssei zones): Andamooka Limestone, Stuart Shelf; Ajax Limestone, Mt Scott Range , northern Flinders Ranges; Winnitinny Creek Member of the Wilkawillina Limestone, Mernmerna Formation and Nepabunna Siltstone, Arrowie Syncline, northeastern Flinders Ranges; Winnitinny Creek Member of the Wilkawillina Limestone, Linns Springs and Third Plain Creek members of the Mernmerna Formation, Bunkers Range, central Flinders Ranges; Winnitinny Creek and Second Plain Creek members of the Wilkawillina Limestone, Six Mile Bore, Linns Springs and Third Plain Creek members of the Mernmerna Formation, Bunkers Graben, southern-central Flinders Ranges; Wirrapowie Limestone, Elder Range, southern Flinders Ranges and Arrowie Syncline northeastern Flinders Ranges; Mernmerna Formation, Mt. Chambers area, eastern Flinders Ranges. Stansbury Basin ( M. etheridgei – D. odyssei zones): Kulpara Formation and Parara Limestone, Yorke Peninsula; Sellick Hill Formation, Fleurieu Peninsula; WPC clasts, Kangaroo Island.