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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 .
Palcephalopoda is meant to correspond to groups which are closer to living nautilus, while Neocephalopoda is meant to correspond to groups closer to living coleoids. One issue which this scheme is the necessity of establishing a firm ancestry for nautilus, to contextualize which cephalopods are closer to which of the two living end members.
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 have been noted to exhibit an extensive range of depth, close to 500 metres, however, they were demonstrated to be at risk of implosion when exceeding their depth and pressure limits. Depending on the species, the shells of live Nautilus will collapse at depths of 750 metres or deeper. [18] [4]
Early cephalopods are thought to have produced jets by drawing their body into their shells, as Nautilus does today. [74] Nautilus is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water. [74] When motionless, Nautilus can only extract 20% of oxygen from ...
Volborthella, some fossils of which predate , was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are.
Pliny the Elder, in the first century CE, did not believe in lyngurium and called the gemstone a belemnite for the first time—though not recognizing it as a fossil. [34] The name is from Ancient Greek βέλεμνον bélemnon meaning dart for the guard's shape. [35] [11] Subsequent authors either considered it to be lyngurium or amber.
They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, [26] where most species, save for lampreys, became extinct. Unusually for an agnathan, anaspids did not possess a bony shield or armour. The head is instead covered in an array of smaller, weakly mineralised scales.