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Nautilus (from Latin nautilus 'paper nautilus', from Ancient Greek ναυτίλος nautílos 'little sailor') [3] are the ancient pelagic marine mollusc species of the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina .
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]
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards 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]
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. Competitive pressure from fish is thought to have forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell ...
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]
The current classification of the Nautilida, in prevalent use, [1] is that of Bernhard Kummel (Kummel 1964) in the Treatise which divides the Nautilida into five superfamilies, the Aipocerataceae, Clydonautilaceae, Tainocerataceae, and Trigonocerataceae, mostly of the Paleozoic, and the later Nautilaceae.
Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian (circa 409 million years ago (Mya)) and became extinct shortly after Cretaceous (66 Mya). The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers.