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Right below the street sign you can find the fossils of several ammonites on the facade of Mariinsky Theater.
The ammonites ruled the seas for 300 million years and even today are easy to find as fossils, but it’s only now that we’ve discovered what ammonites ate. It turns out these “shelled squid” relied on tiny plankton, an exclusivity that could have condemned them to extinction. If so, their demise may have opened up the seas to modern plankton-feeders like the huge baleen whales.
Ammonites are an extinct group of marine invertebrate animals (subclass Ammonoidea of the class Cephalopoda). These molluscs are more closely related to living coleoids (i.e. octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.
The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder (d. 79 AD near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for "horn".
The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event. They lived in shells subdivided in chambers, and the body was housed in the last of them, near the opening. The innermost chamber was also the oldest. As the animal grew and needed more space, it would build a larger cavity in front of the opening, where it moved, and isolate a new chamber. This process was repeated as the ammonite grew. The chambers left behind served as float tanks. A delicate pipe (the siphuncle) ran through them and allowed to regulate the amount of gas or liquid inside the shell to adjust its position in the water as a submarine. By pumping air and filling the chambers with gas, the ammonite became light enough to float above the marine substrate.
Most of the ammonites had shells with the spiral in a single plane, flattened of the sides and perfectly curly, these are denominated by homomorphous amonites (equal forms). But there were also partially coiled shells, in helical form, and still straight, these ammonites, are known as heteromorphs (different forms). In addition to the previously spoken forms also emerged ornamented shells of various types: smooth, patterns of furrows, ribs and thorns.
Ammonites probably fed on small plankton, or vegetation growing on the sea floor. They may also have eaten slow-moving animals that lived on the sea bottom, such as foraminifera, ostracods, small crustaceans, young brachiopods, corals and bryozoa, as well as drifting, slow-swimming or dead sea creatures.
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs).
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