AMMONITES
As the Permian extinction swept away a great deal of life on earth, life bounced back and ushered in a new era of organisms, including the ammonites.
The Permian extinction effectively brought life back in time, a return to the Precambrian era. Where most life was wiped out, a few species did survive, such as some brachiopods, ammonoids and other marine invertebrates.
Ammonites first appeared in the fossil record about 240 million years ago, they were thought to have evolved from Orthicones, a straight shelled cephalopod that was common in the Devonian. Ammonites are so common they are used as an index fossil today which means they are used to define and identify geological periods in time.
Ammonites are iconic for their shells, every shell has a structure with suture lines. These suture lines mark where internal chambers met the inside of the shell. Suture lines became a great deal more complex as ammonoids evolved. Many fossils show suture lines if they are internal moulds of the shell. This occurs when sediment fills the inside of an ammonoid shell and hardens over time.
What is left showcases the interior features of the inside wall of the shell.
Suture lines are used to identify species as there are three major types; Goniatitic, Ceratitic, Ammonitic. Goniatitic suture patterns were characteristic of palaeozoic ammonoids, Ceratitic patterns were more common in Triassic ammonoids and Ammonitic sutures dominated the Jurassic and Cretaceous.
Ammonoids continuously built new shell as the grew, they had numerous internal chambers but only lived in the outer one. The rest of the chambers controlled the animals buoyancy as it moved through the water, to dive deeper their chambers filled with water and to raise higher they filled with gas. This method is also seen in todays nautiloids. Despite the similarity of ammonites and nautiloids, ammonites are more closely related to octopuses.
Ammonites have fascinated humans since antiquity, the name derives from the Egyptian god Amun or the Greek god Ammon, both had rams horns on their head. Fossilised ammonites were also once believed to be snakes turned to stone as a punishment from God.
Although ammonites spread throughout the ancient seas and evolved to be hugely successful, they died out 65 million years ago, along with the dinosaurs.
