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Written and designed by Roy Shepherd. Special thanks to my wonderful wife
Lucinda Shepherd, friend Robert Randell and various experts for their support.

What is an ammonite?

Left: A pyritised ammonite found at Charmouth. Right: A giant chalk ammonite
exposed on the foreshore at Peacehaven.



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).


Octopus
Squid
Cuttlefish
Nautilus


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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 Quantoxhead.
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 BELOW).


Living nautilus found in tropical seas around the world.


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 THEIR LIVES.


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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 OTHER PREDATORS.


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 Peacehaven. 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 CLICK HERE











 

Picture references: Octopus from www.neaq.org; squid from www.flickr.com; squid
from www.tnaqua.org; nautilus small image by from imagecache2.allposters.com;
nautilus large image from www.science.nationalgeographic.com; nautilus
cross-section from www.gallery.photo.net; nautilus attacking crab from
www.biology.unm.edu; ammonite suture (left) by Martin Rigby at
www.fossilsdirect.co.uk; ammonite suture (right) from
www.buriedtreasurefossils.com.

Safety notice: Fossil hunting can at times pose a risk to personal safety, in
particular within environments close to the coast, cliffs or in quarries and
when using the tools and equipment illustrated. Discovering Fossils provides a
free resource to inform you about this fascinating subject and does not accept
any liability for decisions made using this information. We recommend all
individuals abide by the fossil hunting guidelines available by clicking on the
icon at the top of the page.

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Roy Shepherd