The bearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda

Abstract

Yochelson, E. L., Flower, R. H. & Webers, G. F.: The hearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda. Knightoconus, a new genus of the Hypseloconidae (Mollusca: Monoplacophora) from rocks of early Franconian age in Antarctica, is multiseptate. The multiple septa are a criticàl feature to be expected in a form ancestral to cephalopods. Fossil cephalopods, however, invariably have a siphuncle as well as septa; some gastropods, some hyolithids, and some monoplacophorans also have septa but lack a siphuncle. Therefore, only the siphuncle can be considered a unique and particularly significant feature of the cephalopod shell. Hypothetical reconstructions of molluscan anatomy support the notion that cephalopods may have been derived directly from a hypseloconid having a high, slightly curved, multiseptate, bilaterally symmetrical shell, by the subsequent development of a siphuncle.

Two Palaeozoic Hydrothermal Vent Communities from the Southern Ural Mountains, Russia

“…Furthermore, it is not laterally compressed and coiled like Romaniella, and has a smaller apertural expansion angle than Kornoutella. Septa have been recorded in two other late Cambrian and late Cambrian-early Ordovician high-conical genera: Knightoconus Yochelson, Flower and Webers, 1973and Hypseloconus Berkey, 1898(Webers et al 1992. However, neither of these taxa has straight-sided shells like T. shadlunae, being loosely coiled, and both also lack strong concentric ornamentation.…”

“…However, neither of these taxa has straight-sided shells like T. shadlunae, being loosely coiled, and both also lack strong concentric ornamentation. The occasional septation in T. shadluane does not imply a relationship with the cephalopods, because there is no evidence for a siphuncle, or any particular phylogenetic significance within the molluscs, because septa are very common in some euophalacean gastropod genera (Gubanov et al 1995) and occur sporadically in a wide range of other fossil and Recent gastropod taxa (Yochelson et al 1973). The higher taxonomy of conical Palaeozoic 'monoplacophoran' molluscs is in a state of some flux at present (see Peel 1991 for discussion) with most arguments centring around the interpretation of muscle scars and their use in placing these molluscs into the classes Gastropoda, Monoplacophora, Tergomya or Helcionelloida.…”

Review of the geology and paleontology of the Ellsworth Mountains, Antarctica

“…The Springer Peak monoplacs also contain a number of high-coned forms. We believe one of these, Knightoconus (Yochelson et al, 1973) to be representative of a group that is ancestral to the cephalopods. It is a high-coned, multiseptate species but lacks the siphuncle of the cephalopods (Figure 3, no.6).…”

The ancestory of the Cephalopoda. From Helcionella to Plectronoceras.

“…Starting from septate Helcionella, Cephalopoda derivation from Monoplacophora in 1958 was iterated lately by Yochelson (1973), Runnegar and Pojeta (1974) and Runnegar and Jell (1976).…”

“…In erecting Knightoconus with a multiseptate apex in the Hypselconidae, recently Yochelson, Flower and Webers (1973) Runnegar and Jell (1976).…”