Answer: Gaps in the fossil record. Because not all animals have bodies that easily fossilize, the fossil record is considered incomplete.
Explanation:
The fossil record is in fact incomplete, preserved fossils represent a biased sample, fossil discovery is not random and those found reveal only partial information about the original organisms. Nucleic acids, for example, have a very limited survival time.
Under normal conditions of fossilization, only about 15% of organisms are probably conserved. The fossil record is influenced in favor of those animals and plants with hard, mineralized shells, skeletons or cuticles, and towards those that live in marine environments.
