21.4.3: Gaps in the Fossil Record
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Learning Objectives
- Explain the gap in the fossil record
Incompleteness of the Fossil Record
Each fossil discovery represents a snapshot of the process of evolution. Because of the specialized and rare conditions required for a biological structure to fossilize, many important species or groups may never leave fossils at all. Even if they do leave fossils, humans may never find them—for example, if they are buried under hundreds of feet of ice in Antarctica. The number of species known about through the fossil record is less than 5% of the number of species alive today. Fossilized species may represent less than 1% of all the species that have ever lived.
Types of Fossils in the Fossil Record
The fossil record is very uneven and is mostly comprised of fossils of organisms with hard body parts, leaving most groups of soft-bodied organisms with little to no fossil record. Groups considered to have a good fossil record, including transitional fossils between these groups, are the vertebrates, the echinoderms, the brachiopods, and some groups of arthropods. Their hard bones and shells fossilize easily, unlike the bodies of organisms like cephalopods or jellyfish.
Romer’s Gap
Romer’s gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. These gaps represent periods from which no relevant fossils have been found. Romer’s gap is named after paleontologist Alfred Romer, who first recognized it. Romer’s gap spanned from approximately 360 to 345 million years ago, corresponding to the first 15 million years of the Carboniferous Period.
There has been much debate over why there are so few fossils from this time period. Some scientists have suggested that the geochemistry of the time period caused bad conditions for fossil formation, so few organisms were fossilized. Another theory suggests that scientists have simply not yet discovered an excavation site for these fossils, due to inaccessibility or random chance.
Key Points
- The number of species known about through fossils is less than 1% of all species that have ever lived.
- Because hard body parts are more easily preserved than soft body parts, there are more fossils of animals with hard body parts, such as vertebrates, echinoderms, brachiopods, and some groups of arthropods.
- Very few fossils have been found in the period from 360 to 345 million years ago, known as Romer’s gap. Theories to explain this include the period’s geochemistry, errors in excavation, and limited vertebrate diversity.
Key Terms
- transitional fossil : Fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.
- Romer’s gap : A period in the tetrapod fossil record (360 to 345 million years ago) from which excavators have not yet found relevant fossils.