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Ammonites (Ammonoids) are extinct group of cephalopods with the external shell. The oldest recors are known from the Lower Devonian (400 Ma) and the last ones became extinct in the earliest Paleocene (65 Ma). They possessed ten arms, strong lower jaws (their mineralized parts are preserved as aptychii). The shell (phragmocone) is divided into chambers by septa. The initial chamber (protoconch) and the body is connected by siphuncle (ligament of nerves and blood-vessels), an organ responsible for the regulation of gas and liquid content in chambers. Shells are predominantly spirally coiled, but they should be also evolved to straight. The largest ones exceed almost 3 meters in diameter.
## Keywords
Paleozoic, Mesozoic, Paleocene, cephalopods, subclass
