Stoibostrombus crenulatus 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 : 508-509

Stoibostrombus crenulatus Conway Morris and Bengtson in Bengtson et al., 1990

Fig. 13A–F.

1990 Stoibostrombus crenulatus ; Conway Morris and Bengtson in Bengtson et al. 1990: 145, figs. 93–97.

1990 Stoibostrombus cf. crenulatus ; Conway Morris and Bengtson in Bengtson et al. 1990: 147, fig. 98.

1993 Stoibostrombus crenulatus ; Brock and Cooper 1993: 768, 770, figs. 8.4–8.6.

2001 Stoibostrombus crenulatus ; Ushatinskaya and Holmer in Gravestock et al. 2001: 90, pl. 12: 1a–c, 2.

2001 Stoibostrombus mirus ; Demidenko in Gravestock et al. 2001: 91, pl. 12: 3.

2007b Stoibostrombus crenulatus ; Skovsted and Brock in Paterson et al. 2007b: 141, fig. 5J–L.

2009 Stoibostrombus crenulatus ; Topper et al. 2009: 215, fig. 5I–O.

2011 Stoibostrombus crenulatus ; Skovsted et al. 2011: 653–658, figs. 2, 3.

2016 Stoibostrombus crenulatus ; Betts et al. 2016: fig. 21H–J, L.

Material.— One specimen from Clast 1 and 14 specimens from Clast 5; 6 figured ( SAM P57311–57316 ). From the Dailyatia odyssei Zone, WPC, Kangaroo Island , South Australia .

Description.—See Topper et al. (2009: 215).

Remarks. — Stoibostrombus crenulatus is represented by curved, conical sclerites known only from the lower Cambrian of South Australia (Arrowie and Stansbury basins), and are commonly found as disarticulated components in shelly fossil residues. They have a distinctive external ornament that is often abraded, especially toward the apex. Skovsted et al. (2011) demonstrated that the external ornament is variable and can range from nodose plates to pustules, synonymising Stoibostrombus mirus (Demidenko in Gravestock et al., 2001), S. cf. crenulatus (Conway Morris and Bengtson in Bengtson et al., 1990) and S. crenulatus . The WPC specimens appear highly abraded and are often smooth or with very weak external texture (Fig. 13A, B, C 1, E, F). Ornament is often best developed around the base or abapical edge (Fig. 13D).

Original composition is likely to have been phosphatic, and S. crenulatus does not show evidence of growth by marginal accretion (Skovsted et al. 2011). The abapical edge of sclerites is irregular, and composite (fused) specimens are rare (Skovsted et al. 2011). Despite recovery of articulated sclerites, the gross scleritome morphology remains elusive, and phylogenetic affinity continues to be problematic. Skovsted et al. (2011) suggest that the sclerites of S. crenulatus may have been the dermal armour of an ecdysozoan, possibly a palaeoscolecid worm (Skovsted et al. 2011: 656).

Stoibostrombus crenulatus is a key taxon in the new shelly fossil biostratigraphic scheme of Betts et al. (2016, 2017b), as its stratigraphic range is used to define the D. odyssei Zone (Cambrian Series 2, Stage 3) in the absence of the eponym ( Betts et al. 2016, 2017b). However, occurrence in Toyonian-equivalent strata (late Stage 4) in the Stansbury Basin ( Brock and Cooper 1993) suggest that S. crenulatus may have a relatively long stratigraphic range.

Stratigraphic and geographic range. — Lower Cambrian of South Australia: Arrowie Basin ( D. odyssei Zone ): Andamooka Limestone, Stuart Shelf; Ajax Limestone, Mt. Scott Range , northern Flinders Ranges; Mernmerna Formation, Arrowie Syncline, northeastern Flinders Ranges, Bunkers Range, central Flinders Ranges; Second Plain Creek Member of the Wilkawillina Limestone, Bunkers Graben, southern-central Flinders Ranges; Six Mile Bore, Linns Springs and Third Plain Creek members of the Mernmerna

Formation, central Flinders Ranges. Stansbury Basin ( D.

odyssei Zone): Parara Limestone and KLM, in addition to the Ramsay, Stansbury and Coobowie limestones, Yorke Peninsula; WPC clasts, Kangaroo Island.