Ammonites are an extinct group of cephalopod molluscs that lived in
marine environments. They first appeared in the Late Silurian
or Early Devonian (about 400 million years ago), and went extinct
at the end of the
Cretaceous period
(about 65 million years ago), during the
Cretaceous-Tertiary mass extinction.
Although they outwardly physically resemble the modern Nautilus, Ammonites
are actually believed to be more closely related to cuttlefish, octopi,
and squid.
Ammonite
fossils are often (but not
always) a distinctive flattened spiral coil with visible ribs, although there
are species with unusual coil shapes, partially coiled shells, and even
uncoiled shells.
Most Ammonites are quite small, perhaps 9 inches (23 centimeters) or less
in diameter - there are however some larger types, for example
Parapuzosia seppenradensis which is found in
Cretaceous
rocks from
Germany
is sometimes upto 6½ feet (2 meters) in diameter,
Fossilized ammonites preserved in rock:
Ammonite fossils have been known
since ancient times:
The Roman author, Pliny the Elder (23 CE to 79 CE) called the
fossils,
the "horns of Ammon", because he thought that they resembled
the ram's horns which the Egyptian god Ammon was depicted with.
In medieval times, Ammonites were often thought to be petrified snakes,
and were often called "snake stones" or
"serpent stones". The supposed petrification was usually ascribed to
the actions of saints such as Saint Patrick (who was said to have driven the snakes
out of Ireland).
In India, Ammonites were sometimes regarded as symbols of the god Vishnu,
and were kept in temples.
Today, modern scientists often use Ammonites and Amminoids
as "index fossils"
(fossils which are used to help
date the rocks that they are found in), particularly for
Mesozoic Era rocks. This
is possible because these fossils
are relatively abundant in rocks,
are widely distributed, and because different species evolved and went
extinct in rapid succession - thus the presence of a particular species
is indicative of rocks being formed in a particular time period.
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