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The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago. We know this because the orthocerids were the first known representatives of the neocephalopoda, [ 40 ] were ultimately the ancestors of ammonoids and ...
Download as PDF; Printable version; ... For color change to have evolved as the result of social selection the environment of cephalopods' ancestors would have to fit ...
Plectronoceras is the earliest known shelled cephalopod, dating to the Late Cambrian. [ 1 ] [ 2 ] [ 3 ] None of the fossils are complete, and none show the apex or aperture of the shell. [ 3 ] Approximately half of its shell was filled with septa; 7 were recorded in a 2 centimetres (0.79 in) shell. [ 4 ]
The absence of a siphuncle in K. 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 ...
Baculitidae is a family of extinct ammonoid cephalopods that lived mostly during the Late Cretaceous, and often included in the suborder Ancyloceratina. [1]Baculitid genera are characterized by a small to minute initial coil of about two whorls followed by a long straight or slightly curved shaft.
Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids (Orthocerida) early in the Devonian, which in turn gave rise to the Belemnites. Aulococerids are characterized by a straight, slim, conical shell with a large living chamber, simple aperture.
These names were applied by later workers running cladistic analyses on the cephalopods. Berthold & Engeser (1987) adopted the name Angusteradulata for the clade comprising ammonoids and coleoids. Later, Engeser (1996) included not only the coleoids, ammonoids, and bactritids, but also some orthocerid families: Michelinoceratidae ...
Orthoceratidae is an extinct family of actively mobile carnivorous cephalopods, subclass Nautiloidea, that lived in what would be North America, Europe, Asia, Africa, and Australia from the Ordovician through Triassic from 490—203.7 mya, existing for approximately