MUSCLE SCARS OF THE CAMBRIAN UNIVALVED MOLLUSKS AND THEIR SIGNIFICANCE FOR SYSTEMATICS

Abstract

The structure of the shell muscles among the Cambrian mollusks has been a stumbling block for malacologists studying the morphology and phylogeny of the most ancient members of the phylum Mollusca. Apart from several dubious finds of the muscle scars among very few forms of the helcionelloideans (Runnegar, 1981; Vassiljeva, 1990; 1998, Geyer, 1994), the reliable position of the muscles within their shells remains unknown, and this fact has been repeatedly noted in the literature (Runnegar and Pojeta, 1974; Pojeta and Run-negar, 1976; Yochelson, 1978; Peel, 1991). This cir-cumstance coincides with the fact that, to date, we have obtained extensive data on the helcionelloid mollusks: about 500 of nominal species are already described (personal estimation). It is really surprising that the shells or internal moulds of helcionelloideans have sometimes perfect preservation even with visible ele-ments of the wall microstructure (Runnegar, 1983; Bengtson et al. , 1990; Kouchinsky, 2000) but, at the same time, lack the muscle scars, which can be rather distinctly preserved among some other Cambrian mol-lusks (bivalves, bellerophontids, etc.). To explain such a riddle, I speculated (Parkhaev, 2000) that the absence of the muscle scars on the shell of helcionelloideans could have been caused by a spe-cific position of the muscle attachment zone. Probably, the muscles were attached to the shell on the parietal wall in the subapical area. Therefore, it is really difficult to find the scars in this place due to its minor area, not very common preservation, or overlapping by the sub-sequent whorl (in coiled shells). Recent investigations support this speculation, at least for the spirally coiled forms. Also, the scars of the shell muscles were found on the internal moulds of cap-shaped helcionellids. The result of this study is presented in the paper. MATERIAL The extensive collection (about 5000 specimens) from the Lower Cambrian of South Australia (housed at