Anatomy and relationships of the Early Cambrian wormMyoscolex

Abstract

Dzik, J. (2004). Anatomy and relationships of the Early Cambrian wormMyoscolex. —Zoologica Scripta, 33, 57–69.Numerous fossil specimens ofMyoscolex atelesGlaessner, 1979 from the late Early Cambrian Emu Bay shale of Kangaroo Island, South Australia with phosphatized 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. This layer was 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 also occurred 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,Myoscolexclosely resembles the slightly geologically youngerPikaiafrom 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,Myoscolexmay appear not too distant from the ancestor of the phylum. The lateral body flattening ofMyoscolexwas apparently an adaptation to swimming by undulation of the body in transverse plane, similar to today's errant polychaetes but without using chaetae or appendages in propulsion.