Brigg Matters Issue 61 Summer 2021

Ammonites – back in Brigg
after nearly 50 years!
Don’t get alarmed. Ammonites are extinct animals that
disappeared along with the dinosaurs (and lots of other
things) about 66 million years ago. But a collection of
small, exquisitely-preserved specimens has found its
way to me and close to the site where they were found
in 1973. Why am I excited and why are ammonites so
highly prized by geologists? Read on.
Ammonites are classified as molluscs so are distantly
related to snails, oysters, mussels and clams but their
shells were coiled and their heads had large eyes and
tentacles for catching prey. They are more closely
related to squid, octopus and cuttlefish and superficially
resembled the present day pearly nautilus. They all lived
in the seas and oceans of past worlds, first evolving at
the start of the Jurassic period about 200 million years
ago.
They varied in size from thumbnail to well over a metre
in diameter and were pelagic predators, i.e. they swam
and hunted for prey. They had an ingenious way of
controlling their buoyancy. Their shells were chambered,
each chamber, other than the one they were living in,
being sealed off but linked by a tube which could be used
to inflate or deflate the chambers in much the same way
that a scuba diver uses his or her dive suit or jacket. Not
only was this useful for finding food (or a mate at the
appropriate time!) but also for their own protection. They
shared the world’s seas and oceans with marine reptiles
such as ichthyosaurs, pliosaurs and plesiosaurs, which
hunted during the daylight hours and would be happy
to munch on a juicy ammonite. Ammonites therefore
descended to depths of up to 100m during the day but
ascended at night to hunt their prey. Those of you who
remember holidays in the Mediterranean will perhaps
have noticed that the local fishermen go out at night time
to catch squid and octopus which have a similar daily
routine. Palaeontologists (scientists who specialise in
the study of ancient life) believe that the water pressure
at depths greater than 100m (>11 bar) would have
crushed the ammonite shell.
So why are they important to geology? As fossils
they possess the four most important characteristics
that geologists need in an indicator of time. 1, they
are common. 2, they are geographically widespread
Two species of ammonite from the Oxford Clay. Scale bars indicate 10mm.
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