Problems with Evaluating the Fossil Record and Extinctions

A. Data base is too small.

1. Fossil record is incomplete. The vast majority of past organisms in the biosphere were rarely preserved.

2. Fossilization process is selective, favoring certain circumstances and habitats. Inclusions in amber are biased in that they selected small organisms that frequent habitats associated with resin-producing trees. Permineralized fossils occurred in areas where sediment could have inundated them. Large bones were more apt to be preserved than small ones. Organisms with hard structures such as shells and teeth are more frequently represented than soft-bodied creatures.

3. Fossil record is not continuous, leaving large gaps and often resulting in the sudden appearance of lineages previously thought to be extinct (Lazarus effect). Very few sites are currently known to span the K/T boundary, and even these have discontinuities.

4. At fossil sites, the absence of an organism does not mean that it was not present. Nor does the presence of a fossil tell you more than that a lineage was present at a particular time and place. A single fossil provides no information about the global distribution, how long the lineage survived, or when it became extinct. The apparent absence from one location does not preclude survival elsewhere. Accordingly, it is impossible to evaluate extinctions with negative data.

5. Cretaceous fossil sites are not distributed uniformly, so determining global patterns of various lineages is very difficult.

6. Rare species may not be represented at all at some sites, but that doesn't mean they were not present.

B. Data may be flawed.

1. Dating of fossils is often indirect, so their ages and the actual timing of some extinctions may be erroneous.

2. Many fossils are redeposited from older strata into younger beds, thereby confusing the record (Zombie effect).

3. Some uncertainties with species/genera identifications exist since most fossils are fragmentary and diagnostic characters are obscured or absent. Also, there are systematic inconsistencies among taxa of different organisms.

4. As you approach extinction boundaries, the volume of rock available for sampling often decreases, thus lowering the chances of finding a particular fossil (Signor-Lipps effect).

C. Human factors.

1. There appear to be strong biases operating in the identification of fossils and the evaluation of extinctions, and with such a small database, interpretations can be motivated by prejudice.

2. Because there is no immediate urgency to resolve the issue of K/T extinctions, effort should be made to determine whether extinctions of various groups were abrupt or gradual, rather than supporting a cause.

D. Inherent scientific difficulties.

1. Interpretation of fossils is based on inferences gathered from scientific knowledge representing only a few hundred years of human experience. Mistakes are probable because our current database may be flawed and is certainly incomplete.

2. Predicting causes and effects of extinctions that occur today is often impossible. Extending predictions back 65 mya is even more problematic.

3. A seemingly small ecological change, such as a slight global warming or cooling, could have far-reaching biological consequences both regionally and globally, while a dramatic event such as a meteor impact could be significantly less important because of the unpredictability of its consequences.

4. Extinctions may be random, but random events can occur in clusters.

5. Species have different degrees of importance in biological systems. The extinction of a keystone species may have a cas cade effect and result in secondary extinctions that are a direct corollary but unrelated to the cause of the original extinction.

6. Species longevity differs. Species with short longevity spans that may have become extinct under normal circumstances cannot be distinguished from those that may have survived longer if an extinction event had not occurred. These short-lived species skew the interpretation of extinctions.

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