Few fossils of the earliest animals exist because they were soft-bodied and tiny, therefore unlikely to fossilize.
Fossil is the mineralized remains and the preserved evidence of an organism (plant or animals)’s life which include shells, imprints, burrows, coprolites, and organically produced chemicals. The body parts of organisms and the type of environment in which they are mineralized plays a major role in the preservation of organisms. Hard-bodied and large animal species are much more likely to become fossilized than soft bodied and tiny animal species (such as worms or jellyfish) which usually decay before they can become fossilized or they may not be preserved at all. This is why the fossils of some soft bodied animals have never been found.