Left: A pyritised
Echioceras ammonite from
Charmouth. Right: Roy and Robert introduce a
Discovering Fossils public fossil hunt at
The modern use of the word 'fossil' refers to the physical evidence of
former life from a period of time prior to recorded
human history. This prehistoric evidence includes the fossilised remains of living
organisms, impressions and moulds of their physical form, and marks/traces
created in the sediment by their activities. There is no universally agreed age at which
evidence can be termed fossilised, however it's broadly understood to
encompass anything more than a few thousand years. Such a definition
our prehistoric human ancestry and the ice age fauna (e.g. mammoths) as well as more
ancient fossil groups such as the dinosaurs, ammonites and
The earliest reported fossil discoveries date from 3.5 billion years
ago, however it wasn't until approximately 600 million years ago that
complex multi-cellular life began to enter the fossil record, and for the
purposes of fossil hunting the majority of effort is directed towards
fossils of this age and younger.
Fossils occur commonly around the world although just a small proportion
of life makes it into the fossil record. Most living organisms simply decay without trace after death
as natural processes recycle their soft tissues and even hard parts such as
bone and shell.
Thus, the abundance of fossils in the geological record reflects the
frequency of favourable
conditions where preservation is possible, the immense number of organisms that
have lived, and the vast length of time over which the rocks have
How do fossils form?
The term 'fossilisation' refers to a variety of often complex processes
that enable the preservation of organic remains within the geological record.
It frequently includes the following conditions:
rapid and permanent burial/entombment - protecting the specimen from environmental or biological disturbance; oxygen
deprivation - limiting the extent of decay and also biological
continued sediment accumulation as opposed to an eroding surface - ensuring
the organism remains buried in the long-term; and the absence of excessive
heating or compression which might otherwise destroy it.
Fossil evidence is typically preserved within sediments deposited
partly because the conditions outlined above occur more frequently in these environments, and also because the majority of the
Earth's surface is covered by water (70%+). Even fossils derived
from land, including dinosaur bones and organisms
preserved within amber (fossilised tree resin) were ultimately
preserved in sediments deposited beneath water i.e. in wetlands,
lakes, rivers, estuaries or swept out to sea.
Fossilisation can also occur on land, albeit to a far lesser
extent, and includes (for example) specimens that have undergone
mummification in the sterile atmosphere of a cave or desert. However
in reality these examples are only a delay to decomposition rather
than a lasting mode of fossilisation and specimens require permanent
storage in a climate controlled environment in order to limit its
In the following example a fish is used to illustrate the
stages associated with fossilisation within off-shore marine sediments. This
is just one summarised example, in reality there are countless scenarios that create the conditions necessary for fossilisation in marine sediments.
Having reached adulthood and returned to its birth place to
spawn, this particular fish reaches the end of its life and dies.
Soon after death the body of the fish becomes water-logged and sinks to the seafloor
(note that quite often the gases produced during decomposition cause the carcass to float back to the surface, so the final resting
place may be some distance away). More often than not the carcass would be pulled
apart and scattered by scavenging crustaceans and other fish, however on this
occasion the absence of any large scavengers leaves the fish
Left: A fish returns to its birth place to spawn. Right:
Having spawned the fish dies and shortly after sinks to the seafloor.
Although fossils can be found in sediments deposited in turbulent
(high energy) environments near the coastline, complete/articulated
skeletons require undisturbed conditions. A quiet seafloor with minimal light, low oxygen levels and a soft
muddy composition are among the conditions suitable for
preserving organic remains.
Decay and burial
After several weeks the fish is partially decomposed. Despite the calm
conditions on the seafloor, several thousand meters into the
bedrock pressure is building along an active geological fault.
Suddenly the stressed rock slips, sending shockwaves
to the rock above and causing the sediment nearby to
The mobile sediment travels across the seafloor burying the fish in the process,
in what is often termed a rapid burial event.
Once entombed beneath the sediment the remaining flesh and soft
tissue are broken down by bacteria, leaving just the skeleton in the
position of burial.
Left: After several
weeks the soft body tissues have mostly decayed. Right:
Tectonic activity induces nearby sediment to mobilise, burying the fish
in the event.
Rapid burial is a common component for optimal fossilisation, as
prolonged exposure would otherwise increase the likelihood of
disturbance from scavengers and/or currents. Burial may also occur
quickly if a high volume of sediment is deposited in the area following a period of
heavy rain that delivers sediment from major rivers (for example).
Sediment accumulation and permineralisation
Over time the skeleton is
gradually buried deeper by accumulating sediment. Slowly the weight
of the sediment compacts the underlying areas, pressing the grains
together, driving excess water out, and depositing minerals in the
pores, and ultimately turning the soft sediment to hard rock - a
process known as lithification.
As this process takes place, minerals contained within the waters-saturated sediment replace the
original minerals in the skeleton and fill any voids formed as parts
of the skeleton dissolve. The process of mineral replacement is
known as permineralisation and results in a remineralised copy of the
Left: Several months
pass and all that remains of the buried fish is its skeleton.
As times passes more sediment accumulates above the fish and the
skeleton is gradually compressed and permineralised.
Uplift and exposure
Many millions of years pass and the rock
remains buried deep within the bedrock; however tectonic forces associated with
the collision between neighbouring continental plates have begun to
buckle and uplift the bedrock, raising it above sea level and
exposing it to erosion.
Gradually, the exposed rock is stripped away, until eventually the
top of the fish's skull is visible at the surface.
Left: Over time the
rock is distorted and uplifted by geological forces associated with
continental movement, raising it above sea level.
Right: The uplifted rock is exposed to weathering
and gradually erodes away, eventually exposing the tip of the
fish's skull at the surface.
Discovery and extraction
Finally, having lain beneath the ground for millions of years, the
partially exposed skull is spotted by a palaeontologist, who
undertakes a careful extraction of the skeleton. The process
requires patience and precision work to avoid damaging the specimen;
a generous amount of rock is retained around the specimen to protect
it. For more information about fossil hunting and extracting fossils
palaeontologist recognises the fish by the small area of skull
exposed and begins to carefully extract the specimen.
A Pomognathus fish from Houghton Quarry - the skull is
clearly visible, and what parts of the skeleton remain are obscured
the chalk matrix.
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Below are a selection of fossils discovered during the field trips
undertaken to produce the website. Each photo is accompanied by a brief
description of the specimen and a link to the location it was found
- as featured on Discovering Fossils.
Left: Two Perisphinctes ammonites discovered at Osmington
Right: A Pholidophorus fish
skeleton with scales from
Left: A large
theropod dinosaur footprint at Ardley Quarry in Oxfordshire.
Right: Visitors to Ardley Quarry view part of the
Venericor bivalve found at
Right: A section of Elomeryx jaw including
teeth, found at
Left: A Conulus
echinoid preserved as flint, found at
Littlehampton. Right: A Temnocidaris
echinoid in situ at
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Left: A small
trilobite tail from
Right: A split boulder containing a Lepidodendron root from
Left: A large belemnite within a split boulder in the
Brora. Right: A fossilised
Zanthopsis crab carapace, found at Warden Point.
Left: A fragment of
turtle shell from
Bay. Right: A Goniopholis(?) crocodile tooth from
Hypotodus shark tooth, found at
Herne Bay. Right: A split piece of chalk containing land
sourced plant fragments from
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