Fossilization Bias
From EvoWiki
Throughout the history of life on Earth, organisms have lived and died. The vast majority leave no tangible record of their time on this planet, but some beat the odds and become fossilized, preserved for into the future where scientists may later examine them and see the form of life past.
However, organisms are not all equally likely to fossilize and those that do fossilize are not equally likely to be discovered later. This leads to a significant amount of bias in the fossil record. Properly interpreting the fossil record requires understanding its biased nature.
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Hard vs. Soft
The first and most important bias in the fossil record is that organisms with hard parts leave fossils much more easily than those that do not. The weaker the material, the more likely it will be destroyed before burial. Decay is an especially important problem, as fossilization usually takes a bit to get going. Soft parts fossils do occur, but they are not common and are treasured by scientists when they are discovered. Most phyla have few, if any, hard parts and as such appear rarely in the fossil record. The fossil record of animals is, thus, biased towards mollusks, brachiopods, arthropods, corals, echinoderms and vertebrates. The bias towards hard parts is, in large part, responsible for the apparent Cambrian Explosion.
One thing that must be mentioned is that even within groups that have hard parts, there is bias between them based on how hard those hard parts are. Birds and pterosaurs have very light bones, hollowed out and specialized for flight. As a result, birds and pterosaurs have a much sparser fossil record than their contemporaries. Birds represent the most common type of terrestrial vertebrate on the planet today and probably have been since the fall of the dinosaurs, but their fossil record is somewhat sparse compared to that of mammals.
Burial
The word "fossil" literally means "dug up", and fossilization strongly depends on being buried to begin with. Burial does not occur evenly in all environments. There are different ways for an organism (or parts thereof) to be buried:
- Water action
- Tides
- Floods
- River deposition
- Wind action
- Deserts
- Sediment accumulation
- Lake or ocean bottoms
- Tar pits
- Chemical encrusting
- Animal action
- Homes (burrows)
- Storage
As can easily be seen, most of these depend on the presence of water. The fossil record is intensely biased towards aquatic environments, be they seashores or lakes. For example, most birds do not live near open ocean, but most bird fossils are shorebirds. In fact, the vast majority of fossils represent shallow water marine life.
As the fossil record is biased towards aquatic and marine environments, it is biased against forests. The roots of forests keep the soil largely intact, held together against the every day disturbances. There is simply very little way to get buried in a forest. As a result, forest animals are rather rarely fossilized.
The Missing Abyss
Deep sea fossils are as likely to form as those on the bottom of a lake, but are very, very unlikely to be recovered by scientists. Scientists usually look for fossils in terrestrial deposits, not with scuba gear or deep sea submersible robots. As a result, in order for a deep sea fossil to be found, it must be in an area which undergoes uplift, where tectonic forces lift the substrate up to where scientists can get at the fossil. This does happen, but unfortunately areas so uplifted are also subject to extensive erosion, rapidly destroying any fossils so contained.
As a result, the fossil record says little about the history of deep ocean species. This has led to gaps in the fossil record for "living fossils" like Coelacanth and the Monoplacophora, where the shallow water members of the groups died out and the abyssal forms persisted, unnoticed by human fossil hunters.
The Bias of Human Interest
The vast majority of of fossils are mollusk shells, but the majority of studied fossils are vertebrates. Scientists are human and simply more interested in the species that are more closely related to us. Despite making a miniscule fraction of the fossil record, far less common than any other hard-parts phyla, they are the most pursued, most collected, most studied and best documented of fossils. It is important to remember when looking for information on fossils that this favoritism is simply human bias and not an actual bias in the fossil record.
Bias and the Global Flood
How well does the creationist proposition of a global flood producing all the fossils stack up with the known nature of fossilization bias? Not very well. The flood would bury the land, essentially turning all of it, at least briefly, into a shallow water habitat, so there should be no bias in the fossil record against terrestrial forms. Burial would be rapid the world over, killing many of the animals that would contribute to decay, both aiding soft parts preservation and making it more common.
Creationism has much to explain to account for fossilization bias. Too bad for them they seem to have no interest in addressing the problem.
