Acaenoplax — polychaete or mollusc?

Abstract

Palaeozoic invertebrate fossils may pose severe problems in assigning them to Recent taxa. Sutton et al.¹ describe the beautifully preserved and illustrated Silurian fossil Acaenoplax hayae as a “plated aplacophoran” mollusc, interpreting its polychaete-like characters as convergent features. In our opinion, it is more parsimonious to place this organism in the Polychaeta, as the molluscan similarities are limited to serial valve-like structures, suggesting polyplacophoran affinities. It is unlikely that Acaenoplax represents a primitive organization that is neither molluscan nor polychaete — instead, it appears to represent a highly derived, specialized line of invertebrate evolution.