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Left: A pyritised ammonite
found at Charmouth.
A giant chalk ammonite exposed on the foreshore at
Ammonites are perhaps the most widely known fossil, possessing the
typically ribbed spiral-form shell as pictured above. These creatures lived
in the seas between 240 - 65 million years ago, when they became extinct
along with the dinosaurs. The name 'ammonite' (usually lower-case) originates from the Greek
Ram-horned god called Ammon. Ammonites belong to a group of predators
known as cephalopods, which includes their living relatives the octopus,
squid, cuttlefish and nautilus (see pictures below).
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How did ammonites evolve?
These sea creatures first appeared 415 million years ago in the
form of a small, straight shelled creature, known as Bacrites. They
quickly evolved into a variety of shapes and sizes including some
shaped like hairpins. During their evolution the ammonites faced no
less than three catastrophic events that would eventually lead to
their extinction. The first event occurred during the Permian (250
million years ago), where only 10% survived. These surviving
species went on to flourish throughout the Triassic, however at the
end of this period (206 million years ago) they faced near
extinction, when all but one species survived. This event marked
the end of the Triassic and the beginning of the Jurassic, during
which time the number of ammonite species grew once more. The final
catastrophe occurred at the end of the Cretaceous period when all
species were annihilated and the ammonites became extinct. This
event apparently coincided with the death of the dinosaurs.
How did ammonites live?
Ammonites began life as tiny planktonic creatures less than 1mm
in diameter. In their infancy they would have been vulnerable to
attack from other predators, including fish; however, they quickly
assumed a strong protective outer shell that shielded their soft
interior from damage. Evidence suggests that they gained in size
rapidly, with females growing up to 400% larger than the males.
Left: Roy holds a
fantastic ammonite, found on the foreshore at
Right: A participant on a Discovering Fossils event holds
a small ammonite.
Ammonites moved by jet propulsion, expelling water through a
funnel-like opening to propel themselves in the opposite direction.
They typically lived for two years, although some species survived
beyond this and grew very large as pictured above. Evidence of
their short lives is estimated by looking at their living relatives
- the nautilus. These creatures exist within modern day seas and
possess many characteristics similar to ammonites (see picture
Living nautilus found in tropical seas around
Nautilus shells comprise of individual chambers, each growing in
size as the creature grows. These chambers are secreted by the
creature at a rate of one every four weeks, equal to 13 each year.
Using this as a guide an ammonite shell containing 26 chambers could
be assumed to have housed the creature for two years. Like the
nautilus, ammonites retained their original shell throughout their
life. However it's worth noting that in comparison to modern day
nautili which live in cold, deep water, ammonites preferred warm
shallow waters and may have had a higher metabolism. Consequently,
it's possible that ammonites could reach larger sizes far quicker
than modern day nautili.
Ammonites were the predators of their time, feeding on most
living marine creatures including molluscs, fish and even other
cephalopods. By analogy to modern cephalopods, their method of
attack probably comprised of silently stalking their prey, then
rapidly extending their tentacles to grasp the target. Once caught
the prey would be devoured by the ammonite's powerful jaws, located
at the base of the tentacles, between the eyes.
Much of the ammonite's life was spent in shallow waters. The
evidence to support this includes their diet, which could be found
in the greatest volumes in the warm shallows. It is also unlikely
that their shells could withstand the high pressures present in deep
water (over 100 meters). Other theories based around their social
behaviour suggest their shells were decorated by an array of
patterns, indicating that colour/good light played a large part in
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What does the shell reveal about the ammonite?
Most ammonite shells are coiled, and all contain a series of
linked chambers. The body of the ammonite was contained within the
large final, open-ended section called the living or head
chamber, from which the tentacles were extended to catch prey. The
opening of the shell - called the aperture - was possibly covered by
a protective shield that could shut to protect the ammonite from
Left: A modern-day
nautilus preys upon an crab. Right: The internal
chambers visible in the cross-section of a nautilus shell.
As the animal grew, new chambers were added behind the head
chamber. The chambered interior of the shell is referred to as the
phragmocone, and in life this contained gasses which enabled the
ammonite to regulate its buoyancy within the water column. A small
tube called the Siphuncle links the chambers.
Suture marks visible on the outer
surface of the fossil ammonite shells.
Some ammonite fossils bear intricate patterned details on their
outer surface called Sutures. These are located beneath the
external shell wall, and are often visible if the fossil has been
subject to weathering or artificial polishing. These patterns mark
where the walls of the chambers, Septum, meet the outer wall of
the ammonite shell. The bulk of the septum is relatively flat, but
becomes folded where it meets the outer shell. This method of
construction is thought to have provided strength to the shell when
diving to deeper depths. Suture patterns are very useful for
distinguishing different species of ammonite.
Join us on a fossil hunt
Left: A birthday party with
a twist - fossil hunting at
Right: A family hold their prized ammonite at Beachy Head.
Discovering Fossils guided fossil hunts reveal evidence of life that existed
millions of years ago. Whether it's your first time fossil hunting or you're
looking to expand your subject knowledge, our fossil hunts provide an
enjoyable and educational experience for all. To find out more