First occurrence of a new Ocruranus-like helcionelloid mollusc from the lower Cambrian of East Gondwana

Abstract

A new cap-shaped mollusc, Emargimantus angulatus gen. et sp. nov. is described from the Arrowie Basin of South Australia. The new species is closely comparable to mollusc species from South China and North-East Greenland previously described under the generic name Ocruranus Liu, a genus recently reinterpreted as a multiplated, possibly polyplacophoran mollusc. Emargimantus is interpreted as a univalved helcionelloid mollusc and differs from Ocruranus in both morphology and function. Emargimantus angulatus represents the first discovery of Ocruranus-like helcionelloids in the lower Cambrian of eastern Gondwana and demonstrates that these molluscs had a global distribution during the early Cambrian.

Revisiting the molluscan fauna from the Cambrian (Series 2, stages 3–4) Xinji Formation of North China

“…Skovsted et al . (2012) established the genus Emargimantus to encompass a group of enigmatic orthoconic fossils that develop a high, erect shell with the subapical wall drawn out into an apron that bears a shallow median emargination. The morphologically similar Ocruranus trulliformis (Jiang, 1980), Ocruranus septentrionalis Peel & Skovsted, 2005, Ocruranus tunuensis Peel & Skovsted, 2005, and possibly Ocruranus sp.…”

“…in Peel & Skovsted (2005) were reassigned to Emargimantus , and tentatively reinterpreted as helcionelloid molluscs (Skovsted et al . 2012). In contrast, other shells referred to Ocruranus , with moderately high cap‐shaped shells and a distinctive upfolded subapical margin are likely to belong to a larger scleritome that consists of multiple skeletal elements such as Eohalobia Jiang, 1982 and some intermediate plates, and might represent a member of the chiton stem lineage (Vendrasco et al .…”

“…The new material available to us is also comparable to some specimens illustrated as Stephaconus trulliformis Jiang, 1980, considered synonymous to Ocruranus trulliformis by Qian & Bengtson (1989) and referred to E. trulliformis by Skovsted et al . (2012), from Terreneuvian strata of South China in terms of shell height and curvature (Qian & Bengtson 1989, fig. 69A, B).…”

New Data on Oikozetetes (Mollusca, Halkieriidae) from the Lower Cambrian of South Australia

“…Abbreviations: Max W¼maximum width; Max L¼maximum length; Max H¼maximum height; Rm W¼width of rostrum; Rm L¼length of rostrum; L to apex¼length to the apex. Skovsted et al (2012) noted the absence of additional multi-element components in association with the four Emargimantus species, which suggest that at least some taxa previously attributed to Ocruranus may have a closer affiliation with univalved helcionelloid mollusks. We demonstrate here that both shell morphotypes of Oikozetetes mounti n. sp.…”

An outer shelf shelly fauna from Cambrian Series 2 (Stage 4) of North Greenland (Laurentia)

“…Records of Ocruranus from the Bastion Formation (Stage 4) of North-East Greenland by Peel and Skovsted (2005) were revisited by Vendrasco et al (2009) who suggested only tentative assignment to the genus. Skovsted et al (2012) transferred the Greenland species and Ocruranus trulliformis (Jiang, 1980) to Emargimantus Skovsted, Brock andTopper, 2012. Vendrasco et al (2009, text-fig. 2) considered that Ocruranus represented the tail plate in a polyplacophoran-like mollusk, in which interpretation the emarginate, shallowly concave subapical surface lies at the anterior (right in Fig.…”