Ammonite (mollusk)

Ammonite (mollusk), common name for a group of extinct marine mollusks with a coiled shell made up of a series of chambers. Ammonites are members of the cephalopod class, which includes nautilus, squid, octopus, and cuttlefish.

They first appeared during the Silurian Period (435 million to 410 million years ago) and were abundant and widespread in the seas of the Jurassic and Cretaceous periods (175 million to 65 million years ago). Ammonite fossils are found in sedimentary rock layers of limestone and clay and in clay mineral iron ore. Ammonites are important index fossils—that is, they often link the rock layer in which they are found to specific geological time periods.

Ammonite shells are very similar in design to those of the nautilus, the only living cephalopod that retains an external shell. The ammonite shell is generally coiled in a flat spiral. Within the coil is a series of progressively larger chambers divided by thin walls called septa(singular, septum). Only the last and largest chamber was occupied by the living animal. Passing through the septa was a thin living tube, called a siphuncle, that extended from the ammonite’s body into the empty shell chambers. The ammonite secreted gas into the shell chambers, enabling it to control the buoyancy of the shell.

Mature ammonites typically fell into two distinct size classes: macroconchs, the larger female forms, and microconchs, the smaller males. Ammonites varied greatly in size. The largest known had a diameter of 1.7 m (5.6 ft) while other species were as small as 2 cm (0.75 in) in diameter. During the Jurassic and Cretaceous periods, ammonites evolved more streamlined shells for swimming and the structure of the shell became stronger. Different shell shapes emerged as well, such as snail-like or uncoiled. Some ammonites secreted an outer covering of mother-of-pearl that is sometimes preserved in fossils. In other cases of fossilization, the original shell is replaced by minerals such as iron pyrite. The junction where the septum and the shell wall met left visible seams, called suture lines, that are well preserved in fossils. The ammonite suture lines are zigzagged while those of the nautilus are straight. This is one of the characteristics that differentiate the ammonites from nautiluses.

Ammonites probably lived for one to six years, with the majority living two to four years. They fed on plankton (tiny free-floating organisms), sea lilies, and smaller ammonites. Although many fed off the ocean floor, others may have caught plankton while floating or swimming. Ammonites moved by jet propulsion, expelling water through a funnel-like opening to propel themselves in the opposite direction.

The fertilized egg developed into a juvenile ammonite known as a larval protoconch, which drifted freely as plankton. As the ammonite grew and extended its coiled shell, it moved forward within the shell, sealing off chambers behind it. Ammonites slowly became extinct throughout the Cretaceous Period when most of the modern ocean animals evolved. It is likely that the floating larval stages were vulnerable to the many newly evolving predators.

Scientific classification: Ammonites make up the order Ammonoidea. They belong to the class Cephalopoda of the phylum Mollusca. The largest known ammonite is classified as Pachydiscus sepenradensis.