A molecular palaeobiological perspective on aculiferan evolution

Recent advances in fossil, developmental and molecular studies show that aplacophorans are derived molluscs that evolved from chiton-like ancestors presumably in the Ordovician period (488–440 Ma), thereby forming the clade Aculifera along with extant polyplacophorans. Other major molluscan groups (i.e. bivalves and gastropods) diversified in the Early Cambrian along with a suite of distinct sclerite-bearing forms, such as Halkieria Poulsen, 1967 and other sachitids. These extend the aculiferan stem lineage into the Cambrian. Both fossil data and molecular divergence estimates find that modern chitons (the crown group) originated in the Carboniferous period (about 340 Ma) and that the major group of chitons with slitted insertion plates (Chitonida) originated in the Permian (about 260–270 Ma). A distinct and peculiar lineage of stem group chitons is the 17-plated multiplacophorans, which diversified prior to the Silurian, Wenlock (about 426 Ma) and evolved a number of characteristics similar to certain crown group chitons convergently. Multiplacophorans likely inhabited more exposed substrates, which demanded a tougher and integrated armour as observed in some modern chitons.