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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.
The Oncocerids also appear during this time; they are restricted to shallow water and have short exogastric conchs. [2] The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m. [2]
Furthermore, unlike the extinct ammonoids, the modern nautilus lacks an aptychus, a biomineralized plate which is proposed to act as an operculum which closes the shell to protect the body. However, aptychus-like plates are known from some extinct nautiloids, and they may be homologous to the fleshy hood of a modern nautilus. [3]
About 30 years ago, biologist Peter Ward and his colleague discovered a new species of nautilus, Allonautilus scrobiculatus, notable for the thick layer of slime and hair covering its shell.
Nautilus are unable to easily move across areas deeper than 800 metres, and most of their activity occurs at a depth of 100–300 metres deep. [4] Nautilus can occasionally be found closer to the surface than 100 metres, however, the minimum depth they can reach is determined by factors such as water temperature and season. [4]
The tentacles and arms first appear at the hind part of the body, where the foot would be in other molluscs, and only later migrate towards the head. [ 107 ] [ 130 ] The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface.
The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species.
However, the Helcionellids, which first appear over in Early Cambrian rocks from Siberia and China, [11] [12] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites. [1]