On the phylogenetic relationships of scaphopod molluscs – What else is new?

The phylogenetic relationships of Scaphopoda with the other major conchiferan taxa remain unclear, although their systematic position is pivotal for our understanding of conchiferan evolution.The ambiguous information from morphology and palaeontology has been supplemented by inconclusive molecular and phylogenomic studies. Scaphopod anatomy shows some resemblance to that of bivalves, e.g. lateral mantle elongation, a burrowing foot, and the organisation of the nervous system. This is likely due to convergent adaptation to a burrowing lifestyle, and the bivalve-scaphopod sister-group relationship is not supported by molecular analyses. Similarities with the cephalopod bauplan include the pronounced and elongated dorsoventral body axis and a single, annular attachment of the shell muscle. A close phylogenetic affinity of Scaphopoda and Cephalopoda is also indicated by mitochondrial and nuclear DNA sequences and developmental gene expression data, although the signal for a sister-group relationship is not robust. However, the absence of unambiguous scaphopods in the fossil record before the Carboniferous remains unexplained. A divergence time analysis of DNA sequences attempts to shed light on the origin of the Scaphopoda and their major subtaxa.