Zoologica Scripta 32, 56-69 (2004)
Anatomy and relationships of the Early Cambrian worm Myoscolex

Jerzy Dzik
Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland. e-mail: dzik@twarda.pan.pl

Abstract. Numerous fossil specimens of Myoscolex ateles Glaessner, 1979 from the late Early Cambrian Emu Bay shale of the Kangaroo Island, South Australia with phosphatised organic matter rich tissues show its muscular body wall penetrated by rows of rod-like structures – possible chaetae. The body wall was composed of an external layer with transverse (circular) fibres which was the thickest in lateral parts of the body and very thin dorsally. In the ventro-lateral quarter of the body circumference a belt of longitudinal fibres extended along the body. Longitudinal fibres occurred also in the dorsal region of the body. Along the venter extended a narrow longitudinal belt of probably oblique cords, crossing themselves perpendicularly. In having a virtually smooth, laterally flattened body Myoscolex closely resembles the slightly geologically younger Pikaia from the Burgess Shale of British Columbia, generally believed to be one of the oldest chordates. Being the oldest probable annelid, at least superficially similar to the opheliid polychaetes, Myoscolex may appear not too distant from the ancestor of the phylum. The lateral body flattening of Myoscolex was apparently an adaptation to swimming by undulation of the body in transverse plane, similarly to today’s errant polychaetes but without using chaetae or appendages in propulsion.