The ammonites are members of the Class Cephalopoda, whose members include squids and octopi, and the Sub-Class Nautiloidea, whose members also include the extant (still living) Nautilus. Cephalopoda means "head foot", referring to the fact that their mouths are surrounded by their tentacles, so their head and "feet" are both in the same place. (See my article "Biological Nomenclature" for more information about how biologists divide animals into groups.)
The Nautiloidea are mollusks with univalve shells. This means that their shell is all one piece; unlike the bivalve clams and oysters, whose shells are in two pieces, held closed by the muscles of the animal. The shells of the Nautiloidea are divided internally into a series of chambers (called camerae) by thin walls called septae. The septae are secreted by the animal: as it grows, it moves forward in the shell, secreting the septa behind it and separating off a new chamber. The modern Nautilus is free-swimming. Its chambers are filled with air at atmospheric pressure, making it buoyant. The Nautilus swims by squirting water through a siphon in its body and can move quite rapidly. Extinct ammonites probably swam in the same manner.
Where the walls of the septa come into contact with the outer shell, they produce a complicated pattern called the septal suture line. This, the shape of the shell, and any ornamentation are used to group the Nautiloidea into families and genera. The modern Nautilus has mostly straight septal suture lines, while ammonites have suture lines with extremely complex designs.
Most ammonites have a shell that is coiled in a spiral that lies in all one plane. However, some are completely straight, others are partially coiled and partially straight, while still others are coiled helically (like a spring).
The Nautiloidea are found in the fossil record as far back as the Cambrian Period (570 to 500 million years ago). They reached the peak of their development during the Paleozoic Era (70 to 230 million years ago). The Ammonoid group first appeared in the Early Devonian Period (395 to 345 million years ago).
The Nautiloidea are wholly marine (ocean-living) animals. They had a worldwide distribution until the mid-Cenozoic Era (about 36 million years ago). Their numbers and diversity then declined to a single genus (Nautilus), which contains four genera. Their geographic distribution was reduced to the seas of the central western Pacific Ocean from Samoa to the Philippines and the Indian Ocean off southern Australia.
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