nautiloid

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Illustration of an early Silurian coral-stromatoporoid community showing various labeled marine organisms, including corals, bryozoans, brachiopods, crinoids, trilobites, and a nautiloid, arranged on and around a block of seafloor substrate with a scale bar indicating centimeters.

nautiloid

cephalopod

Also known as: Nautiloida, Nautiloidea

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July 1, 2026

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Illustration of an early Silurian coral-stromatoporoid community showing various labeled marine organisms, including corals, bryozoans, brachiopods, crinoids, trilobites, and a nautiloid, arranged on and around a block of seafloor substrate with a scale bar indicating centimeters. — Silurian coral-stromatoporoid community An early Silurian coral-stromatoporoid community.

Nautiloids are an extinct group of cephalopods, a class of marine mollusks that also includes modern-day octopuses, squid, cuttlefish, and the nautilus. Nautiloids, along with ammonites, represent primitive, less specialized forms of cephalopods that likely led a sluggish existence.12

These ancient marine animals first appeared in the Late Cambrian period, over 510 million years ago, with simple, cap-like shells that evolved into more complex, coiled structures over time. Nautiloids, like other cephalopods, possessed a mantle, a head-foot with arms surrounding the mouth, and a circulatory system with three hearts and a blue respiratory pigment called hemocyanin.32

The group experienced significant diversity during the Paleozoic and Mesozoic eras but ultimately became extinct, except for the few living species of Nautilus. The end-Ordovician extinction event, which occurred around 445 million years ago, eliminated an estimated 85 percent of Ordovician species, impacting many marine groups, including nautiloids.145

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Carboniferous Period
Invertebrates
…included both straight and coiled nautiloids (early relatives of the chambered Nautilus), the ammonoids (extinct members of the same class), and the first squids. Carboniferous cephalopods were either predators or scavengers, and they swam by jet propulsion. Some of the straight nautiloids grew exceedingly large (greater than 3 metres [10…
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…included both straight and coiled nautiloids (early relatives of the chambered Nautilus), the ammonoids (extinct members of the same class), and the first squids. Carboniferous cephalopods were either predators or scavengers, and they swam by jet propulsion. Some of the straight nautiloids grew exceedingly large (greater than 3 metres [10…
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cephalopod
Form and function
The fossil nautiloids and ammonites (represented today only by Nautilus) were primitive, less-specialized forms, probably leading a rather inactive sluggish life. The modern octopuses, squids, and cuttlefishes have acquired an active, vigorous life that has led to marked departures in structure and function from the type represented…
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The fossil nautiloids and ammonites (represented today only by Nautilus) were primitive, less-specialized forms, probably leading a rather inactive sluggish life. The modern octopuses, squids, and cuttlefishes have acquired an active, vigorous life that has led to marked departures in structure and function from the type represented…
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ammonoid
…are characterized and distinguished from nautiloids by the highly crenulated and complex suture that occurs where internal partitioning walls come in contact with the outer shell wall. Ammonoids are important index fossils because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features.
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…are characterized and distinguished from nautiloids by the highly crenulated and complex suture that occurs where internal partitioning walls come in contact with the outer shell wall. Ammonoids are important index fossils because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features.
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