What are fossils and how do they form?
Hello and welcome to the Natural History Podcast. It has been a few weeks since I last made an episode, a big sorry to my 4 listeners. I began writing a follow up to the last extinction episode and became bogged down in research. Matters where made much worse when some building work on my house began and since then have discovered an 8 foot deep sink hole below the kitchen floor.
So as I said a follow up episode is on its way, as I feel extinctions, may have raised more questions than answered. But in the mean time, to break up those long days staring out of a window just hoping for the next natural history podcast will be released soon. Here is something to keep you occupied.
Lets begin, Fossil, what are they? Well that’s easy. They are the remains of animals long gone… correct?
and indeed you would be right. For the Cambridge dictionary describes fossils as “ an old person, especially one who will not accept new ideas”
Oh wait, hold on. Let me scroll down… Ok! lets try again. A fossil as describes by the Cambridge dictionary is
And that is a great description, but take note of the wording “part of, or its shape” because fossils don’t just have to be the actual animal.
For, there are five different types of fossil. Body fossils are only one type.
we also have foot prints which are also known as trace fossils. These can be burrows and tracks as well. They can often point to to animal which palaeontologists have not yet found, and only known through the tracks and foot prints they leave.
Next we have moulds, this is where an impression of the body is left. Not the actual fossil. If you split a rock open and find a fossil, one half will often have the body and the other half will be the mould of the body.
Fourth up, petrifaction. It is the process of turning the fossil in to minerals. As the body decays minerals replace the body and that give us the colourful shiny fossils we see. If they are dull, they are simple body fossils or moulds. If they have crystals, you are looking at a petrifaction.
Finally we have coprolites. Fossilised dung. These are also found in the stone formate by the time they reach us (rather fortunateley) and they are a wealth of information, as they often shed light on feeding habits.
Now we know what a fossil is, how do we make one? The French writer Voltaire, thought they where the remains of left over picnics, with pilgrims slinging aside old fish remains.
This method could work eventually more by luck than design and it would take a few million years not hundred years to fossilise a fish. No, instead imagine an ammonite, jetting along under the sea. Then suddenly it died, its eyes turn to crosses and it sinks to the sea bed. Now depending upon where it lands, depends on if it will fossilise or not. As if this ammonite is coastal and lands on a nice calm sea bed, it will be devoured by anything that can land its fins on it.
However if our ammonite lands in a delta, the area where a river meets the sea, or somewhere else that a lot of sediment moves very fast. Swamps and rivers for instance, then it has a chance of becoming a fossil. The reason we need fast moving mud is to bury the body. The burial will keep it from being exposed to oxygen. which is key. For oxygen is a funny element. It is high in energy, but also very destructive. It assists in the decomposition process after death. However if we take that oxygen away, the ammonite body will not break down in the usual manner. Instead it will begin to fossilise.
The Burgess shale in Canada is a famous example of rapid burial. It was an underwater landslide that buried a whole eco-system alive. It happened during the Cambrian period, about 500 million years a go. We are quite lucky because of this, we know so much about the life back then and the fossils it produces are the most weird and wonderful. Not so lucky for the Cambrian fauna…
There are other methods of fossilisation, than rapid burial. Being frozen is the obvious one. There we completely stop decomposition and that is how we find perfectly preserved wholly mammoths.
You can dry the body out. Think of dried fruit. There we have artificially kick started the preservation process. That is how you can keep raisins in a jar for years, where as a jar of grapes would turn to mush fairly quickly.
However, to actually become a fossil is a very rare thing. Less that 1% of everything that was once alive has made it in to fossil record. The other 99% of life, well, we have no idea. Whole species that we will never know about because they are not fossilised.
Oil, natural gas and coal, which we burn in our cars and home are too a type of fossil.
Oil and gas are originally made up of the most unsuspecting living being of all. Plankton, algae and even fish. As they die their bodies fall to the sea bed, where bacteria will start to make light work decomposing the plankton and similar organisms. A layer of sand or mud will then bury the dead organisms along with the hungry bacteria. This prevents oxygen from reaching the bacteria and without it, the bacteria soon perish.
With the plankton unable to decompose it will slowly turn in to oil and gas.
Which at some point will start to try and seep up to the surface. It can move up through porous rocks like as sandstone which is filled with gaps. Eventually it will slowly seep out to the surface. Such as Kimmeridge bay in Dorset or it will hit a solid wall of rock and become trapped. Then the Oil companies come along crack the top open and extract the oil and gas.