Plectronoceras

Plectronoceras Temporal range: Late Cambrian ~501–488.3 Ma PreЄ Є O S D C P T J K Pg N
Plectronoceras cambria
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	†Plectronocerida
Family:	†Plectronoceridae
Genus:	†Plectronoceras Ulrich & Foerste, 1933
Species
P. cambria Walcott, 1905 P. exile Flower, 1964 P. gracile Flower, 1964 P. liaotungense Kobayashi, 1935

Plectronoceras is the earliest known shelly cephalopod, dating to the Late 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 centimetres (0.79 in) 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. Fossils of Plectronoceras have been found in the San Saba Limestone of Texas.