Knightoconus

Knightoconus antarcticus is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica.^[2] It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.^[3]

Knightoconus Temporal range: Dresbachian ~497–487 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Artistic reconstruction of Knightoconus antarcticus
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Monoplacophora
Order:	†Archinacelloidea
Genus:	†Knightoconus Yochelson et al., 1973[2]
Species:	†K. antarcticus
Binomial name
†Knightoconus antarcticus Yochelson et al., 1973[2]

Taxonomic debate

The absence of a siphuncle in K. antarcticus has been taken as evidence against its being an ancestor of cephalopods, as factors that would allow a siphuncle to penetrate preexisting septal chambers remain unknown. The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, stopping that new septum from closing completely and becoming mineralised itself.^[4]

Ten or more septa are found in mature individuals, occupying around a third of the shell – septa form very early and have been found in specimens as small as 2 mm in length.^[5] Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils.^[5] Scars from the closely related Hypseloconus have been used to determine its orientation.^[6] Knightoconus started life with an exogastric shell that became endogastric as the organism grew.^[7]

An alternate taxonomy is: Tergomya: Kiringellida: Hypseloconidae.^[1]

See also

• Evolutionary history of cephalopods
• Plectronoceras