This week I have been sieving mud! But it’s not as boring as it sounds, far from it, as the mud contains thousands of tiny microfossils. When larger fossils (such as tetrapods) are hard to find, the sediment itself can house thousands of tiny fossil fragments that can give you a great deal of information. The sample I examined contained tiny plant fossils (roots, stem, leaves and spores), ostracods, arthropod cuticle, fish teeth and bones and vertebrate bones (that may have come from a tetrapod, hard to say when they are so small). The pieces ranged in size from 2mm to 0.1mm, but the finer your sieve, the smaller the fossils you see.
So how do you turn a mud rock into microfossils? These are the basics.
Step 1. Break up your mudrock into small pieces with your fingers. If the rock does not break up easily you should seek advice on how to break it down with acid, for which you will need professional safety kit (fume cupboard etc.), so you will need to work in a certified laboratory such as at a university. But if you have soft muds you can just use water and do it at home.
Step 2. Sieve the pieces with water through at least 2 sieves stacked on top of each other. Put the largest sieve holes at the top, I like to use the following sizes as a minimum: 0.125mm, 0.425mm and 1mm to catch any larger rock bits that remain. Make sure you sieve them into a sink where muddy grit drainage will not cause a problem!
Step 3. When the water runs clear from the sieve you can collect the residue. Put what remains in each sieve into filter paper or small containers (keeping each size fraction separate). This is a tricky bit, as you need to tilt each sieve to wash the pieces into a heap before you can tip them into the container. A small hose pipe attached to a tap is very useful here. Leave the samples to dry.
Step 4. Pick through the residues under a microscope. Put a small amount of residue on a picking tray and examine it at a high enough magnification so you can see what the pieces are made of. Below is me picking under the microscope and a close up of the picking tray and slide equipment that I use.
Step5. Using a paintbrush pick out specimens you are interested in and put them in a slide for further study. Below are close-up photos of the cells of the slide, containing 1-2) vertebrate microfossils and 3-4) plant microfossils.
If you want to try this yourself you can purchase sieves from a geological supply company (online) and use a magnifying lens (or hand lens) instead of a microscope.
Want to know more about micropalaeontology?
The Natural History Museum in London has an excellent blog written by their Curator of Micropalaeontology Dr Giles Miller.
Until next time