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Fossilised Stone

GGiANT AMMONITES (GONIOCERAS)

FROM THE ATLAS MOUNTAINS, MOROCCO

Dated: Devonian – 350 million years old

Ammonite History

Ammonites are extinct members of the Cephalopod class.  Modern members include Nautilus, Squid and Octopus.  They first appeared during the Silurian period (410 to 435 million years ago) and were abundant and widespread in the seas of the Jurassic and Cretaceous periods (65 to 175) million years ago).  Ammonites are important index fossils – that is, they often link the rock layer in which they are found to the specific geological time periods.

 Ammonites vary greatly in size.  The largest known had a diameter of 1.7 m (5.6) ft while other species were as small as 2cm (.075in) 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.

 The shells of ammonites had hollow chambers separated by walls called septa.  A tube called the siphuncle, connected the body with the chambers allowing the animal to fill them with water or air, changing it’s buoyancy in order to rise or drop in the ocean.  The living animal occupied only the last and largest chamber.

 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 orthoceras.  Although many fed off the ocean floor, others may have caught plankton while floating or swimming via jet propulsion, expelling water through a funnel-like opening to propel themselves in the opposite direction.

 Furthermore because ammonites lived exclusively in marine environments, their presence also indicates the location of prehistoric seas. 

ORTHOCERAS FOSSIL

Found: Atlas Mountains, Morocco

Orthoceras (meaning “straight horn”) were primitive types of Nautiloids that looked like squid with a long, straight (or slightly curved), conical, chambered and furrowed shell.  The shell ranged from a few centimetres to over 9 feet long.  The Orthoceras lived in the larges chamber in the shell.  Orthoceras lived from the middle Ordovician period to the Devonian period, from about 360 to 470 million years ago.  Fossils have been found on many continents including the Americas, Africa, Europe and Asia.