Marissa J. Betts

Associated conchs and opercula of Triplicatella disdoma (Hyolitha) from the early Cambrian of South Australia

Internal moulds of complete shells, including conch and associated opercula, of the hyolith Triplicatella disdoma from Cambrian Series 2, Stages 3–4 of South Australia are described. The conch of T. disdoma is shown to be narrow and cone-shaped, and with a rounded triangular cross-section. The conch shows a gentle dorsal curvature in lateral view. The fossils lack evidence of helens, and the operculum was smaller than the apertural diameter of the conch and could be withdrawn a short distance into the conch. Triplicatella was probably closely related to orthothecid hyoliths.

Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia

Correlation of lower Cambrian strata is often confounded by provincialism of key fauna. The widespread occurrence of the micromollusc Watsonella crosbyi Grabau, 1900 is therefore an important biostratigraphic signpost with potential for international correlation of lower Cambrian successions. Previous correlations of W. crosbyi from Australia (Normanville Group) suggested an Atdabanian- to Botoman-equivalent age. However, in the upper part of the Mount Terrible Formation, stratigraphic ranges of W. crosbyi and Aldanella sp. cf. golubevi overlap prior to the incoming of vertically burrowed ‘piperock’, which is indicative of an age no earlier than Cambrian Stage 2. The stratigraphic range of W. crosbyi in the Normanville Group, South Australia correlates with the ranges of the taxon in China, France, Mongolia and Siberia (though not Newfoundland). The new Australian data add further support for considering the first occurrence of W. crosbyi a good potential candidate for defining the base of Cambrian Stage 2. The stratigraphic range of W. crosbyi through the lower Cambrian Normanville Group has been determined based on collections from measured sections. Although rare, W. crosbyi is part of an assemblage of micromolluscs including Bemella sp., Parailsanella sp. cf. murenica and a sinistral form of Aldanella ( A. sp. cf. A. golubevi ). Other fauna present include Australohalkieria sp., Eremactis mawsoni , chancelloriids and Cupitheca sp.