Burrowing armored worms: Machaeridians

Machaeridians are a group of armoured worms that is known from the Early Ordovician to the Carboniferous. They comprise three groups: The plumulitids, turrilepadids and lepidocoleids. It has recently been demonstrated that this group belongs to the annelids and much information is now available with respect to the organization of their exoskeletal armour in relation to the body. Turrilepadids and lepidocoleids have been suggested to be infaunal in contrast to the plumulitids which seem to have been epibenthic. They could have burrowed much like other annelids, using peristaltic movements. In this way, the body regions could be elongated and shortened combined with lateral expansion and contraction. The expanded segments would have anchored the body and allowed posterior regions to contract and anterior regions to propagate. Fossil evidence from exquisitely preserved lepidocoleids indicates that multiple peristaltic waves propagated over the body simultaneously like in modern annelids. In stark contrast to any other annelid however, they had mineralized shell plates that were directly used in the burrowing. The way that the shell plates were used is similar to the way bivalve mollusks use their skeleton and therefore represents an excellent example of functional convergence. The skeleton is ratchetted with posteriorly oriented terraces either by the mode of the overlapping skeleton or by surface ornamentation, which is seen in some infaunal bivalves (Seilacher 1984, Palaeontology). This prevents slipping in one direction, but allows propagation in another. The realization that machaeridians were peristaltic burrowers indicates the potential of recognizing their trace fossils among Palaeozoic trace fossil assemblages as they posses a combination of unique features: Turrilepadids are perfectly square in transverse outline, whereas lepidocoleids are oval or heart shaped with some forms possessing a functional dorsal hinge that could leave a dorsal groove. The traces would also potentially leave periodic imprints of the ratcheted armour laterally. Presumably the traces are horizontal and would resemble Planolites or Arthrophycus type ichnofossils.