Molecular data suggest that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains unresolved.

The Gadilids, on the other hand, are much smaller, have a shell whose widest portion is slightly posterior to its aperture, and have a foot which is disk-like and fringed with tentacles which inverts into itself when retracted (in this state resembling a pucker rather than a disk).

According to the World Register of Marine Species: There is a good fossil record of scaphopods from the Mississippian onwards,[5] making them the youngest molluscan class.

The Ordovician Rhytiodentalium kentuckyensis has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from ribeirioid rostroconch molluscs such as Pinnocaris.

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.

[10] More recent research, including the sequenced genome of tusk shells, support the Diasoma model with bivalves as the sister group.

According to Shimek and Steiner, "[t]he apex of the shell and mantle are anatomically dorsal, and the large aperture is ventral and anterior.

The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting upward.

Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.

A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food, which they then convey to the mouth.

The massive radula of the scaphopods is the largest such organ relative to body size of any mollusc (among whom, except for the bivalves, the presence of which is a defining characteristic).

These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.

Dentalium shells were also used to make belts and headdresses by the Natufian culture of the Middle East, and are a possible indicator of early social stratification.