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A ghost lineage is an evolutionary lineage for which no fossil record exists, but the presence of which is inferred from related organisms separated by vast stretches of time. For example, marsupial and placental mammals are known from the early Cretaceous, but until recently, monotremes were only known from 80 million years after. The obvious fact that monotremes are derived from a common ancestor with placentals required the postulation of a then unknown, 80 million year long fossil lineage. Another interesting case are the lineages of chimpanzees and gorillas. While plenty of fossils of human ancestors and close relatives have been found for the last 4-5 million years, no comparable ones for chimps and gorillas have been found.

Creationists are likely to claim "Aha! That lack of fossil record means that those ancestors did not really exist, and their descendants had been specially created relatively recently!" However, such claims are often easily disproved by the discovery of appropriate intermediate fossils.

And there are some good reasons for the existence of ghost lineages:

• We have not been looking in the right places for fossils. Animals with basal Gondwanan distributions, for example, might be overlooked as the majority of paleontology is conducted in Laurasian continents. In similar fashion, we would have a much poorer picture of the evolution of humanity's recent ancestors if paleontologists had never searched in eastern Africa in rocks formed over the last 4-5 million years.
• The members of those ghost lineages had escaped fossilization, with subcategories
  • Those members had lived in places that form few fossils, like upland areas. This can explain the absence of those ancestral chimp and gorilla fossils.
  • Those members do not fossilize very well. This explains why several animal groups, notably flatworms (Platyhelminthes) and "aschelminths" (various tiny worms), have extremely poor fossil records; none of their members have easily-fossilizable parts.