''Plectronoceras'' is the earliest known shelled

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

, dating to the Late Cambrian.''Plectronoceras''
at

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.org None of the fossils are complete, and none show the apex or aperture of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a shell. Its shell contains transverse

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separated by about half a millimetre, with a

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, a ...

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

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References

Prehistoric nautiloid genera Cambrian molluscs Cambrian animals of North America Cambrian United States Fossil taxa described in 1933 {{paleo-Nautiloidea-stub Cambrian genus extinctions