Knightoconus
Encyclopedia
Knightoconus antarcticus is a Cambrian
monoplacophoran thought to represent an ancestor to the cephalopod
s. 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 starts life with an exogastric shell, that becomes endogastric as the organism grows.
Alternate taxonomy is: Tergomya, Kiringellida, Hypseloconidae.
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
monoplacophoran thought to represent an ancestor to the cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s. It had a chambered, conical shell, but lacked a siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
. 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 starts life with an exogastric shell, that becomes endogastric as the organism grows.
Alternate taxonomy is: Tergomya, Kiringellida, Hypseloconidae.
See also
- Evolutionary history of cephalopodsEvolutionary history of cephalopodsThe 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....
- The Late Cambrian fossil PlectronocerasPlectronocerasPlectronoceras is the earliest known shelly cephalopod, dating to the Upper Cambrian.Its 14 known specimens hail from the basal Fengshan Formation of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell...
is now thought to be among the earliest cephalopod fossils.