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Ammonites (Ammonoids) are an extinct group of cephalopods with an external shell. The oldest records are known from the Lower Devonian (400 Ma), and the last ones became extinct in the earliest Paleocene (65 Ma). They possessed ten arms and strong lower jaws (their mineralized parts are preserved as aptychi). The shell (phragmocone) is divided into chambers by septa. The initial chamber (protoconch) and the body are connected by the siphuncle (ligament of nerves and blood vessels), an organ responsible for regulating gas and liquid content in chambers. Although ammonite shells are predominantly spirally coiled, some evolved to have straight shells. The largest ones exceed almost three meters in diameter. ## Keywords Paleozoic, Mesozoic, Paleocene, cephalopods, subclass
