Knightoconus

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]

Knightoconus
Temporal range: Early Franconian
~501–488 Ma [1]
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
(unranked): Monoplacophora
Order: †Archinacelloidea
Genus: †Knightoconus
Yochelson et al., 1973[1]
Species:
†K. antarcticus
Binomial name
†Knightoconus antarcticus
Yochelson et al., 1973[1]

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

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.
  3. Holland, C.H. (1987) [1986]. "The nautiloid cephalopods: A strange success". Journal of the Geological Society. President's anniversary address. 144: 1–15. doi:10.1144/gsjgs.144.1.0001.
  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. 47. London, UK: Geological Society. p. 29. doi:10.1144/GSL.SP.1989.047.01.04.
  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.CS1 maint: multiple names: authors list (link)
  7. "The Paleobiology Database". Retrieved 6 August 2009.
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