Machaeridian locomotion

The discovery that machaeridians (class Machaeridia Withers, 1926) are annelidsallows their mode of locomotion to be interpreted in the context of the body plan of this phylum. The Plumulitidae were errant epibenthic forms, moving with parapodia.The body of Turrilepadidae and Lepidocoleidae, however, was enclosed largely withinthe mineralized plates that make up the skeleton. Articulated specimens indicate thatthese machaeridians were able to burrow like other annelids using peristaltic locomo-tion. A lepidocoleid specimen indicates that multiple waves of shortened and con-tracted regions moved over the body. This is in contrast to the mode of locomotion inearthworms and most polychaetes, but similar to peristaltic progression in Polyphysia (Scalibregmidae). Either the rugose sculpture (turrilepadids) and/or the margins of theoverlapping shell plates functioned as a burrowing sculpture, allowing forward move-ment but preventing backwards slipping. A trace from the Devonian Hunsru¨ck Slateassociated with a lepidocoleid indicates that considerable flexing of the skeleton waspossible, but this is an escape trace and does not represent normal locomotion. Featuresof the skeleton of machaeridians are convergent on those of molluscs where the shellslikewise function in protection and burrowing.