Evidence of Moulting in Palaeoscolecid Worms from Chengjiang  

The palaeoscolecidians are a group of fossil priapulid-like worms which bear a variety of trunk ornaments, including sclerites, plates, and platelets with several central nodes.  They have a wide distribution and are known from the Early Cambrian to the Late Silurian; they were a significant component of benthic and endobenthic Cambrian communities.  Morpohological data supports the placement of the group as an Ecdysozoan clade, basal to the lobopods and arthropods.  While there is abundant evidence of moulting in lobopods and arthropods, direct evidence of moulting in palaeoscolecidians is lacking.  The fossil record of palaeoscolecidians predominantly consists of trunk fragments and discrete sclerites, which cannot preserve evidence of moulting.  However, soft tissues are rather abundant in the Early Cambrian Chengjiang biota. 

There are two methods used in recognizing moults.  The first is to identify new sclerites located beneath the older ones, which we contend can be recognized in some well preserved Cricocosmia specimens, as well as in other palaeoscolecids.  A similar record of moulting has been reported in the Chengjiang lobopod Microdictyon. The other approach considers the preservation status of worms in event beds: moults are usually strongly twisted, and lack a proboscis and guts, which clearly differentiates them from poorly-preserved body fossils.  However, we have yet to observe palaeoscolecid moults preserved within their burrows.  We suggest that moulting style differs in various worms, and that breakage of the cuticle, perhaps caused by the scalids, occurred between the trunk and the pharynx.