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The cephalopod radula rarely fossilizes: it has been found in around one in five ammonite genera, and is rarer still in non-ammonoid forms. Indeed, it is known from only three non-ammonoid taxa in the Palaeozoic era: Michelinoceras , Paleocadmus , and an unnamed species from the Soom Shale .
Cephalopods occupy most of the depth of the ocean, from the abyssal plains to the sea surface, and have also been found in the hadal zone. [11] Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata. [1]The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician period [2] to become diverse and dominant in the Paleozoic and Mesozoic seas.
Cephalopods are very abundant and are larger in size than those found in the Kope Formation, because of the increase in food availability for these predators following the late Cambrian. [2] Cephalopods survived well in this time period because they were able to prey on the many gastropods and mollusks that are also found in this formation.
The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian.Some, such as Aphetoceras and Estonioceras, are loosely coiled and gyroconic; others, such as Campbelloceras, Tarphyceras, and Trocholites, are tightly coiled, but evolute with all whorls showing.
Drawing of the statocyst system Statocysts (ss) and statolith (sl) inside the head of sea snail Gigantopelta chessoia. The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, [1] cnidarians, [2] ctenophorans, [3] echinoderms, [4] cephalopods, [5] [6] crustaceans, [7] and gastropods, [8] A similar structure is also found in Xenoturbella. [9]
Nautiluses are much closer to the first cephalopods that appeared about 500 million years ago than the early modern cephalopods that appeared maybe 100 million years later (ammonoids and coleoids). They have a seemingly simple brain, not the large complex brains of octopus, cuttlefish and squid, and had long been assumed to lack intelligence ...
Ammonoids are extinct spiral shelled cephalopods comprising the subclass Ammonoidea.They are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) than they are to shelled nautiloids (such as the living Nautilus). [1]