Wikipedia
Plectronoceras
Plectronoceras is the earliest known shelly cephalopod, dating to the Upper Cambrian. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a 2 cm shell. Its shell contains transverse septa separated by about half a millimetre, with a siphuncle on its concave side. Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods.
Plectronoceras is the type genus of the family Plectronoceratidae.