Early molluscan evolution and the systematic position of the sachitids
|Location||International Geological Congress,oslo 2008|
|Holding Date||21 September 2008|
The molluscs diversified in the Early Cambrian (about 530 million years ago). A distinct member of these is the sachitids in which the famous fossil Halkieria belongs. They are characterized by having mineralized sclerites arranged in morphologic zones on the dorsal side and various combinations of larger shell plates. Sachitids have also been argued to have a more general affinity as stem groups of the entire Lophotrochozoa or as stem groups of annelids and brachiopods. However, a scrutiny of the morphology is only compatible with a relationship close to the aplacophorans and polyplacophorans within the molluscs. Both forms are aragonitic and have a characteristic mineralized microstructure of longitudinal fibres. The sclerite zones are similar in growth mode and organization.
Both have a characteristic canal system of equal dimensions inside a mineralized matrix (called aesthetes in chitons). The only difference is the number of dorsal shell plates they possess, which seems to have been more plastic in the Palaeozoic than later. No single character in sachitids indicates broader lophotrochozoan affinities to for example annelids or brachiopods. The precise position of the sachitids within the molluscs remains conjectural as the phylogeny of the phylum as a whole is unresolved, but a stem group Aculiferan affinity (Aplacophora + Polyplacophora) seems possible given the current knowledge.