Plectronoceras

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.^[3] Approximately half of its shell was filled with septa; 7 were recorded in a 2 centimetres (0.79 in) 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.^[3]

Plectronoceras Temporal range: Late Cambrian ~501–488.3 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Life reconstruction of P. cambria
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Order:	†Plectronocerida
Family:	†Plectronoceridae
Genus:	†Plectronoceras Ulrich & Foerste, 1933
Species
P. cambria Walcott, 1905 P. exile Flower, 1964 P. gracile Flower, 1964 P. huaibeiense Chen & Qi, 1979 P. liaotungense Kobayashi, 1935

Plectronoceras is the type genus of the family Plectronoceratidae. Fossils of Plectronoceras have been found in the San Saba Limestone of Texas.^[2]