Pronounced AM-uh-nites. From Ammon's Stone, named after the Egyptian god Ammon, it is the name used for an extinct group of Cephalopods (which means 'head-foot'). 

The ammonites were present on the Earth for a period spanning over 140 million years. They first appeared on Earth some 208 million years ago, during the Carboniferous period. They evolved during the Mesozoic era into a variety of forms and were very abundant and widespread. The rapid evolution of the ammonoids and their widespread distribution, makes them a valuable tool when studying both the Paleozoic and Mesozoic periods. Large specimens were commonly up to one metre across and weighed up to 100 kg. It is currently believed that they evolved from the same ancestors as the belemnites.  

The ammonites became extinct about the same time that the majority of the dinosaurs disappeared. This is believed to be at the end of the Cretaceous Period, some 65 million years ago, known in paleontology as the time of the K-T mass-extinction.

They were a form of fast moving marine invertebrate, closely related to the modern-day squid, octopus and nautilus.

Most ammonoids had a coiled shell, which internally, was partitioned into small chambers. They lived in a shelled chamber, occupying the largest and most recently formed chamber, all combining to form a spiral. 

Once the ammonite grew too large for its living chamber, it formed a new one next door and then moved into it, in the process creating the characteristic spiral shape of the shell. The unused chambers were capable of being filled with gas to provide buoyancy at different depths in the oceans. It is these unused chambers which tend to be preserved in different coloured calcite.

Photographs

A calcite Promicroceras planicosta ammonite as found in a limestone nodule.

A partially fossilised Androgynoceras lataecosta ammonite in a mudstone nodule. Found in the Green Ammonite Bed at Wear Cliffs.

A partially crushed Oxynoticeras oxynotum ammonite fossil. Found on the beach under Golden Cap.

Two Androgynoceras lataecosta ammonites partially fossilised in mudstone. Found in the Green Ammonite Bed at Wear Cliffs.

A partially fossilised Tragophylloceras loscombi mudstone ammonite. The surface has very fine striations. Found in the Green Ammonite Bed at Wear Cliffs.

Another partially fossilised Dactylioceras mudstone ammonite. Found in the Green Ammonite Bed at Wear Cliffs.