Why is it difficult to determine different species in the fossil record?

Organisms that were very rare in their environment might never have been fossilized simply because the odds of preservation favor more numerous organisms. Animals with small and delicate bones, such as small birds and amphibians, would be less likely to be preserved - and discovered - than larger organisms with tougher bones. Soft-bodied organisms like worms are even more poorly represented in the fossil record because they had no hard parts that could resist decay. We usually learn of their existence only if we find fossilized burrows, or trackways or impressions left by their bodies in soft sediments.