Another process that makes interpreting the fossil record difficult is that species, rather than going extinct, sometimes simply abandon an area in favor of another one nearby. If today we were to search only locally, we might mistakenly conclude that the species had become extinct. Yet a broader search finds it on a nearby island or in an adjacent region. It did not expire, it moved. Migration causes us to under estimate the true ranges of species in the fossil record and to think they became extinct before they actually did. (In Chapter 9 we see a good exampl e in the ammonites.)
Fossil material is usually composed of carbonate or phosphate, chemicals that can dissolve in groundwater. Thus even though an organism becomes fossilized, its remains may later be dissolved away and disappear. This causes us to underestimate the true range of a fossil species, or in the extreme, to miss it entirely.
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