Knightoconus

Knightoconus Temporal range: Early Franconian ~501–488 Ma PreꞒ Ꞓ O S D C P T J K Pg N [1]
artistic reconstruction of Knightoconus antarcticus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Monoplacophora
Order:	†Archinacelloidea
Genus:	†Knightoconus Yochelson et al., 1973[1]
Species:	†K. antarcticus
Binomial name
†Knightoconus antarcticus Yochelson et al., 1973[1]

Knightoconus antarcticus is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica.^[1] It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.^[2]

Taxonomic debate

The absence of a siphuncle in K. antarcticus has been taken as evidence against cephalopod ancestry, 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.^[3]

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.^[4] Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils.^[4] Scars from the closely related Hypseloconus have been used to determine its orientation.^[5] Knightoconus started life with an exogastric shell, that becomes endogastric as the organism grew.^[6]

An alternate taxonomy is: Tergomya: Kiringellida: Hypseloconidae.^[7]

See also

• Evolutionary history of cephalopods
• Plectronoceras

References

1. Yochelson, E.L.; Flower, R.H.; Webers, G.F. (1973). "The bearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda". Lethaia. 6 (3): 275–309. doi:10.1111/j.1502-3931.1973.tb01199.x.
2. Boyle (2005). "Chapter 3: Origin and Evolution". Cephalopods - ecology and fisheries. p. 36. doi:10.1002/9780470995310.ch3. ISBN 9780632060481.
3. Holland, C.H. (1987) [1986]. "The nautiloid cephalopods: A strange success". Journal of the Geological Society. President's anniversary address. 144 (1): 1–15. Bibcode:1987JGSoc.144....1H. doi:10.1144/gsjgs.144.1.0001. S2CID 128629737.
4. Webers, G.F.; Yochelson, E.L. (1989). "Late Cambrian molluscan faunas and the origin of the Cephalopoda". In Crame, J.A. (ed.). Origins and Evolution of the Antarctic Biota. Special Publications. Vol. 47. London, UK: Geological Society. p. 29. doi:10.1144/GSL.SP.1989.047.01.04. S2CID 128682549.
5. Peel, J. S. (1991). "Functional Morphology of the Class Helcionelloida Nov., and the Early Evolution of the Mollusca". In Simonetta, A. M.; Conway Morris, S (eds.). The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge University Press. pp. 157–177. ISBN 978-0-521-40242-2.
6. Webers, G.F.; Pojeta, J. Jr.; Yochelson, E.L. (1992). "Cambrian Mollusca from the Minaret Formation, Ellsworth Mountains, West Antarctica". Geology and Paleontology of the Ellsworth Mountains, West Antarctica. pp. 181–248. ISBN 978-0-8137-1170-6.
7. "The Paleobiology Database". Retrieved 6 August 2009.