Temporal range: Early Franconian
Yochelson et al., 1973
Yochelson et al., 1973
Knightoconus antarcticus is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle. The absence of this siphuncle has been taken as evidence against cephalopod ancestry - how, it is argued, could a siphuncle evolve to penetrate existing septa? 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, producing an obstacle to the complete closure of the septum and becoming mineralised itself. 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. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils. Scars from the closely related Hypseloconus have been used to determine its orientation. Knightoconus started life with an exogastric shell, that becomes endogastric as the organism grew.
- Evolutionary history of cephalopods
- The Late Cambrian fossil Plectronoceras is now thought to be among the earliest cephalopod fossils.
- 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
- "Origin and Evolution". Cephalopods - ecology and fisheries. 2005. p. 36. doi:10.1002/9780470995310.ch3.
- Holland, C. H. (1987). "The nautiloid cephalopods: a strange success: President's anniversary address 1986". Journal of the Geological Society 144: 1–0. doi:10.1144/gsjgs.144.1.0001.
- Webers, G. F.; Yochelson, E. L. (1989). "Late Cambrian molluscan faunas and the origin of the Cephalopoda". In Crame, J. A. Origins and Evolution of the Antarctic Biota 47. Geological Society, London: Special Publications. p. 29. doi:10.1144/GSL.SP.1989.047.01.04.
- 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. The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge U. Pr. pp. 157–177. ISBN 978-0-521-40242-2.
- Webers, G. F.; Pojeta Jr, J.; 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.
- The Paleobiology Database. Accessed 6 August 2009.
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