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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. [ 145 ] [ 147 ] The Bactritida , a Devonian–Triassic group of orthocones, are widely held to be paraphyletic without the coleoids and ammonoids, that ...
The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the " halwaxiids " were molluscs or closely related to molluscs, [ 5 ] [ 6 ] [ 8 ] [ 9 ] debates arise about the relationships between the classes of living molluscs. [ 7 ]
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 ...
Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea.They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family Nautilidae). [1]
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]
The Cymatoceratidae is a family of Mesozoic and early Cenozoic nautiloid cephalopods and the most abundant of their kind in the Cretaceous. They are characterized by ribbed, generally involute shells of varied forms - coiled such that the outer whorl envelops the previous one, as with Nautilus, and sutures that are variably sinuous.
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.