''Plectronoceras'' is the earliest known shelled

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

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

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.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 biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

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 Plectronocerida is a primitive order from which subsequent Cephalopoda, cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic ...

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

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References

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