{{Short description|Extinct genus of molluscs}} {{Speciesbox | fossil_range = Dresbachian<br />~{{fossil range|497|487|ref=<ref name=PaleobiologyDB/>}} | image = Knightoconus_reconstruction.png | image_caption = Artistic reconstruction of ''Knightoconus antarcticus'' | parent_authority = Yochelson et al., 1973<ref name="Yochelson1973"/> | taxon = Knightoconus antarcticus | authority = Yochelson et al., 1973<ref name="Yochelson1973"/> }}

'''''Knightoconus antarcticus''''' is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica.<ref name=Yochelson1973>{{cite journal |first1=E.L. |last1=Yochelson |first2=R.H. |last2=Flower |first3=G.F. |last3=Webers |name-list-style=amp |year=1973 |title=The bearing of the new Late Cambrian monoplacophoran genus ''Knightoconus'' upon the origin of the Cephalopoda |journal=Lethaia |volume=6 |issue=3 |pages=275–309 |doi=10.1111/j.1502-3931.1973.tb01199.x|bibcode=1973Letha...6..275Y }}</ref> It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.<ref name='Boyle3'>{{cite book |author=Boyle |chapter=Chapter 3: Origin and Evolution |title=Cephalopods – ecology and fisheries |url=https://archive.org/details/cephalopods00boyl |url-access=limited |year=2005 |page=[https://archive.org/details/cephalopods00boyl/page/n49 36] |doi=10.1002/9780470995310.ch3|isbn=9780632060481 }}</ref>

==Taxonomic debate== The absence of a siphuncle in ''K.&nbsp;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.<ref name=Holland1986>{{cite journal |last1=Holland |first1=C.H. |year=1987 |title=The nautiloid cephalopods: A strange success |series=President's anniversary address |orig-year=1986 |journal=Journal of the Geological Society |volume=144 |issue=1 |pages=1–15 |doi=10.1144/gsjgs.144.1.0001|bibcode=1987JGSoc.144....1H |s2cid=128629737 }}</ref>

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&nbsp;mm in length.<ref name='Webers1989'>{{cite book |last1=Webers |first1=G.F. |last2=Yochelson |first2=E.L. |name-list-style=amp |chapter=Late Cambrian molluscan faunas and the origin of the Cephalopoda |year=1989 |title=Origins and Evolution of the Antarctic Biota |editor-last=Crame |editor-first=J.A. |publisher=Geological Society |place=London, UK |series=Special Publications |volume=47 |page=29 |doi=10.1144/GSL.SP.1989.047.01.04|s2cid=128682549 }}</ref> Unlike monoplacophoran fossils, there is no evidence of muscle scarring in ''Knightoconus'' fossils.<ref name='Webers1989'/> Scars from the closely related ''Hypseloconus'' have been used to determine its orientation.<ref name=Peel1991>{{Peel 1991}}</ref> ''Knightoconus'' started life with an exogastric shell that became endogastric as the organism grew.<ref>{{cite book |author1=Webers, G.F. |author2=Pojeta, J. Jr. |author3=Yochelson, E.L. |name-list-style=amp |year=1992 |title=Geology and Paleontology of the Ellsworth Mountains, West Antarctica |pages=181–248 |chapter=Cambrian Mollusca from the Minaret Formation, Ellsworth Mountains, West Antarctica |publisher=Geological Society of America |chapter-url=https://books.google.com/books?id=zIe00bn_KiUC&pg=PA181 |isbn=978-0-8137-1170-6}}</ref>

An alternate taxonomy is: Tergomya: Kiringellida: Hypseloconidae.<ref name=PaleobiologyDB/>

== See also == * Evolutionary history of cephalopods * ''Plectronoceras''

== References == {{reflist|25em|refs= <ref name=PaleobiologyDB>{{cite web |url=https://paleobiodb.org/classic/basicTaxonInfo?taxon_no=7843 |title=†''Knightoconus'' Yochelson et al. 1973 |work=Paleobiology Database |access-date=1 July 2025}}</ref> }}

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Category:Prehistoric monoplacophorans Category:Cambrian molluscs Category:Paleozoic Antarctica Category:Prehistoric animals of Antarctica Category:Fossil taxa described in 1973