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Nautilids are marine mollusks that begin growing their shells before emerging from their eggs and continue to add shell segments throughout their lifetimes. The walls between their shells' chambers are called septa, and the deeper the water in which a septum forms, the greater the concentration of the isotope oxygen-18 the septum will contain. Since temperature falls as depth increases, if the area of ocean a nautilid inhabits is known, this isotopic signature can reveal the temperature at which its septa formed. Paleontologist Amane Tajika and colleagues examined each of the septa in two nautilid shells and concluded that septum sample M23 formed at a temperature of 14.0°C whereas sample F02 formed at 20.8°C.