Knightoconus

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Knightoconus
Temporal range: Early Franconian
~501–488  Ma
Knightoconus reconstruction.png
artistic reconstruction of Knightoconus antarcticus
Scientific classification Red Pencil Icon.png
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]

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]

Contents

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

Related Research Articles

Ammonoidea Extinct subclass of cephalopod molluscs

Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appear during the Devonian, and the last species vanished in the Cretaceous–Paleogene extinction event.

Monoplacophora Superclass of molluscs

Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell now living at the bottom of the deep sea. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record, and were previously thought to have become extinct over 380 million years ago.

Coleoidea Subclass of cephalopods

Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less". Unlike its extant sister group, Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal cuttlebone, gladius, or shell that is used for buoyancy or support. Some species have lost their cuttlebone altogether, while in some it has been replaced by a chitinous support structure.

Nautiloid Extinct subclass of nautiloids

Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.

Evolution of molluscs The origin and diversification of molluscs through geologic time

The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.

<i>Yochelcionella</i> Extinct genus of molluscs

Yochelcionella is an extinct genus of basal molluscs which lived during the Tommotian epoch, the first epoch of the Cambrian period. This genus is often reconstructed to resemble snails.

Plectronocerida Extinct order of molluscs

Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.

The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.

<i>Scenella</i>

Scenella is an extinct genus of fossil invertebrate animal which is generally considered to be a mollusc; at various times it has been suggested that this genus belongs with the gastropods, the monoplacophorans, or the helcionellids, although no firm association with any of these classes has been established. An affinity with the hydrozoa has been considered, although some authors oppose this hypothesis. A gastropod affinity is defended on the basis of six pairs of internal muscle scars, whilst the serially-repeated nature of these scars suggests to other authors a monoplacophoran affinity. However the specimens showing this scarring have not been convincingly shown to belong to the genus Scenella. A similarity to the Ediacaran Ovatoscutum has also been drawn.

Helcionellid Extinct order of molluscs

Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about 540 to 530 million years ago in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician. These fossils are component of the small shelly fossils (SSF) assemblages.

<i>Plectronoceras</i> Genus of molluscs

Plectronoceras is the earliest known shelled cephalopod, dating to the Late Cambrian. None of the fossils are complete, and none show the apex or aperture of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a 2 centimetres (0.79 in) shell. Its shell contains transverse septa separated by about half a millimetre, with a siphuncle on its concave side. Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods.

<i>Volborthella</i> Extinct animal of uncertain classification

Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.

The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician. They are characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the lateral, with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal siphuncle with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching halfway to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic).

Orthoceratoidea Subclass of cephalopods

Orthoceratoidea is a subclass, formerly considered an infraclass or a superorder, that comprises Cephalopoda orders that have orthoconic to slightly cyrtoconic shells and central to subcentral siphuncles in which there may be internal deposits. Currently, Orthoceratoidea comprises the orders Dissidocerida, Ascocerida, Pseudorthocerida, Lituitida and Orthocerida.

The Kirengellids are a group of problematic Cambrian fossil shells of marine organisms. The shells bear a number of paired muscle scars on the inner surface of the valve.

<i>Salterella</i> Extinct genus of shelled animals

Salterella is an enigmatic Cambrian genus with a small, conical, calcareous shell that appears to be septate, but is rather filled with stratified laminar deposits. The shell contains grains of sediment, which are obtained selectively by a manner also observed in foramanifera. The genus was established by Elkanah Billings in 1861, and was named after the English palaeontologist John William Salter.

The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.

Shimanskya is a late Carboniferous fossil tentatively interpreted as an early spirulid.

Paleoceras is a genus of fossil cephalopods from the Upper Cambrian.

Minaret Formation

The Minaret Formation is a Late Cambrian limestone formation of the Heritage Group of Antarctica. The age of the formation is established to be Guzhangian to Cambrian Stage 10, dated at ranging from 500 to 488 Ma.

References

  1. 1 2 3 4 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. 1 2 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.