Stenothecoida is a taxon of bivalved fossils from the Early to middle Cambrian period.

They were considered by E. L. Yochelson 1968[citation needed] to belong to Mollusca, whereas Runnegar and Pojeta (1974) suggested that they might be 'bivalved monoplacophorans'.

This said, the morphology of the shell does not resemble the shell of any other class of molluscs;[4] they also look a little like brachiopods[1] It's not clear whether their two valves are each other's mirror images, as in bivalve molluscs, though they aren't identical to one another [?].

[5] As with many other Cambrian taxa,[6][7] it has been suggested that they arise through reduction of a Halkieria-like ancestor.

[2] These fossil shells have been found in Canada, China, Greenland, Kazakhstan, Kyrgyzstan, and the USA.