Modern mucociliary creeping trails and the bodyplans of Neoproterozoic trace-makers

The bulk of Neoproterozoic trace fossils can be interpreted as horizontal creeping trails produced by minute vermiform organisms moving on or just beneath the seafloor or under algal mats. We have investigated the formation of trails by living cnidarians and platyhelminths that creep by cilia on mucus ribbons. These relatively simple metazoans produce trails that are similar in size and morphology to some Neoproterozoic traces, owing to the entrainment of sediment within their mucus trails. Thus a mucociliary locomotory system provides sufficient means to form some types of Neoproterozoic traces. It follows that the body architectures of the Neoproterozoic trace-makers may have been quite simple, though complex bodyplans are, of course, not ruled out. Thus, the use of Neoproterozoic trace fossils to constrain the time of origin of bilaterians or of any crown-group bilaterian taxon remains questionable.