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Ammonites (Ammonoids) are an extinct group of cephalopods characterized by their external shells. The earliest records of ammonites date back to the Lower Devonian period, around 400 million years ago, with their extinction occurring in the earliest Paleocene period, approximately 65 million years ago. Ammonites had ten arms and strong lower jaws, with mineralized parts of these jaws preserved as aptychi. Their shells, or phragmocones, were divided into chambers by septa. The initial chamber, known as the protoconch, was connected to the rest of the body by a structure called the siphuncle, a ligament containing nerves and blood vessels that regulated the gas and liquid content within the chambers. Although ammonite shells are predominantly spirally coiled, some evolved to have straight shells. The largest specimens reached diameters of nearly three meters. ## Keywords Paleozoic, Mesozoic, Paleocene, cephalopods, subclass