Our look at those fossils that commonly bought rather than collected has so far looked at the fossil remains of animals, whether shells, teeth or whatever. But this time we’re looking at a trace fossil; that is, a fossil produced by a living organism but not actually part of that organism. Trace fossils include such things as burrows and footprints, and are of huge importance in telling us about the behaviour of animals in ways their skeletal remains might not. The trace fossils we’re looking at today are fossilised faeces, also known as coprolites.
It might be a surprise to know that faeces fossilise at all, but essentially the same principles apply to them as to any other type of organic material. Provided they are quickly buried in some type of sediment lacking in oxygen, there’s a chance they’ll become fossilised. In some cases the faeces eventually decompose away to nothing, but not before leaving a mould of some sort in the surrounding sediment, so what the geologist uncovers will be a cast of the original droppings, composed of some type of mineral, such as marcasite (iron pyrites).
But often some elements of the original faeces remain, usually the harder and more durable parts that persisted long enough to become mineralised, such as fish scales, fragments of bone, even certain types of plant material. These coprolites are of very great value because if the palaeontologists can determine what types of animals produced those coprolites, they can then say something about what sort of foods those animals were eating.