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Zoologica
Scripta 32,
56-69 (2004) 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. |