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The cephalopods were once thought to have evolved from a monoplacophoran-like ancestor [8] with a curved, tapering shell, [9] and to be closely related to the gastropods (snails). [10] The similarity of the early shelled cephalopod Plectronoceras to some gastropods was used to support this view.
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. [ 144 ] [ 146 ] The Bactritida , a Devonian–Triassic group of orthocones, are widely held to be paraphyletic without the coleoids and ammonoids, that ...
Volborthella, some fossils of which predate , was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are.
Instead, nautiloids are a paraphyletic grade of various early-diverging cephalopod lineages, including the ancestors of ammonoids and coleoids. Some authors prefer a narrower definition of Nautiloidea ( Nautiloidea sensu stricto ), as a singular subclass including only those cephalopods which are closer to living nautiluses than they are to ...
The ancestors of all Coleoidea (shell-less Cephalopods) once possessed shells, and many early cephalopod species are only known from shell remains. Following the K-Pg extinction event most nautiloid species went extinct , while members of Coleoidea managed to survive.
Orthocerida, also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician) possibly to the Late Triassic 1] A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous 2] and the Eocene fossil Antarcticeras is sometimes considered a descendant of the orthocerids although this is disputed.
Dawsonoceratidae is an extinct family of orthoconic nautiloid cephalopods that lived in what would be North America and Europe from the Late Ordovician through the Middle Devonian from about 480–390 mya, existing for approximately 1]
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