Scaphopod

Phylogeny

The scaphopods are largely agreed to be members of the Conchifera, however their phylogenetic relationship with the other members of this subphylum remains contentious. The Diasoma concept proposes a clade of scaphopods and bivalves based on their shared infaunal lifestyle, burrowing foot, and possession of a mantle and shell. Pojeta and Runnegar proposed the extinct Rostroconchia as the stem group of the Diasoma. An alternative hypothesis proposes the cephalopods and gastropods as sister to the scaphopods with helcionellids as the stem group. A review of deep molluscan phylogeny in 2014 found more support for the scaphopods, gastropods, or cephalopods than for scaphopods and bivalves, thus the shared body features of scaphopods and bivalves may be convergent adaptations due to similar lifestyles. Analysis of the scaphopod nervous system demonstrated that both scaphopods and cephalopods share a similar nervous system structure, with ventrally shifted pedal nerves and lateral nerves that extend dorsally. These similarities led to the conclusion that scaphopods are sister to the cephalopods with gastropods as sister to them both. More recent studies, including the sequenced genome of tusk shells and a concordant genome-based phylogeny with fossils and morphology, support the Diasoma model with bivalves as the sister group.