Burren and Cliffs of Moher UNESCO Global Geopark - Geology

Cephalopod fossils; underwater
swimming predators without fins or
tails.
Cephalopods are a type of mollusc, so are related to our
garden snails and slugs as well as the osyters and mussels
that live along our coast. Cephalopods have existed on
our planet for over 500 million years and are represented
in our oceans today by octopus, squid, cuttlefish and the
iconic coiled-shell Nautilus.
Cephalopods were more abundant and diverse in the past.
Many had a shell (unlike modern octopus and squid),
and over time the shell evolved into a variety of straight,
curved and spiral forms, incuding some bizarre contorted
shapes. The most well known fossil cephalopods are
the coiled ammonites, the spiral shape of which is often
used as an icon image for fossils or geology; some of the
ammonites reached over 1m in diameter.
Cephalopods today are predators; they have good eyesight
for targeting prey, tentacles for grasping prey and tough
beaks for biting and ripping apart prey. 320 million years
ago they were doing the same thing, swimming and
hunting in the seas that eventually formed the rocks of the
Burren and Cliffs of Moher. Fortunately, some of them are
preserved in those rocks as fossils.
We find two main types of fossil cephalopods in our rocks
here; thin, conical, straight-shelled forms and tightly
coiled spiral forms. When these shells are well-preserved
you can see they have a series of internal chambers,
the same as in modern Nautilus shells. These chambers
were connected by a tube which was able to regulate the
amount of fluid and possibly gas inside the chambers,
this enabled the cephalopod to adjust it’s position up or
down in the water column. In addition to this marvelous
mechanism the cephalopods also deposited layers of
shell inside the chambers to act as ballast as well as to
strenghten the shell to protect it from predators.The
animals actually only inhabited the final open section of
the shell.
The cephalopods propel themselves by pumping water
through a funnel known as a hyponome and squirting it
out. This acts like jet propulsion and gives them the speed
to catch their prey or escape predators, many also use an
ink sac to squirt out clouds of dark ink to confuse their
predators and remarkably there are fossil examples of
these ink sacs preserved in exceptional conditions.
Being skilled hunters, the cephalopods would have been
close to the top of the food chain. For a long time it was
thought they were the apex predators here. However,
these cephalopods were likely preyed upon by larger
predators such as sharks. I am currently hunting for
more of these fossils to expand our knowledge of these
fascinating ancient predators and the intricate web of life
that existed 320 million years ago.
Figure caption. Two types of fossil cephalopods from the rocks of the Burren. Both were common here 320 million years ago.