Plectronoceras is the earliest known shelled cephalopod, dating to the Late Cambrian.

[1][2][3] None of the fossils are complete, and none show the apex or aperture of the shell.

[4] Its shell contains transverse septa separated by about half a millimetre, with a siphuncle on its concave side.

[3] Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods.

Fossils of Plectronoceras have been found in the San Saba Limestone of Texas.