Question

By definition, our knowledge of the fossil record will always be incomplete, but this does not mean that scientists are unable to test hypotheses regarding the evolution of our own and other species. Summarize the process (and often fortuitous suite of factors) by which an organism’s remains become fossilized. What other post-depositional factors influence fossil discoveries and our knowledge of the fossil record?

Answer 1

Fossilization is the process by which a plant or animal becomes a fossil. This process is extremely rare and only a small fraction of the plants and animals that have lived in the past 600 million years are preserved as fossils.

• First, the soft tissue that exists during life decays leaving behind only the "hard parts" (bone, shell, teeth).
• Second, hard parts may be transported and broken. This causes the fossilized remains to be incomplete representations of the living animal. It is much more common to find a fragment of shell or bone than it is to find a complete skeleton.
• Third and most important, hard tissues become buried and altered. In most cases this involves destroying the original material from which the hard parts were made as minerals are slowly dissolved and replaced by new ones. Often times a hard part is dissolved without being replaced by new material, leaving behind only an impression or mold of the original animal. If this mold is filled with sediment that is later cemented into rock it will make a cast of the original animal.

Factors influence fossil discoveries:

Environments such as rainforests teeming with life and in a hot and moist climate are also poor places for fossils to form as a carcass can decay quickly and not have time to be buried. Similarly a rocky mountain top is a poor place for fossils to form with no fine sediments being laid down. A nice floodplain or stagnant lake or a coastal lagoon is great, however – not too much decay and lots of mud or sand swirling around in the water – and deserts too can be good.

The organisms themselves will have an effect too, and not just in terms of their anatomy or where they lived. Those with large populations that ranged far and wide would have many more individuals which could be buried and preserved, but a small group restricted to a small area less so. Similarly, those species that lasted for millions of years stand a better chance of hanging around for us to find compared to those that came and went in the blink of a geological eye.

Time can also play a different role in making fossils unavailable. Earth's geological processes are mostly slow, but the tectonic plates do move over time and will eventually shift to the point that what is currently on the surface will be covered or destroyed. That means the older a fossil is, the more likely it is to have been shunted deep underground or lost forever and thus older fossils tend to be more rare.

Finding things have biases too. We can't dig for fossils where no rocks of the right age are exposed, so while Montana and Mongolia are great, the rainforests of the Congo or the volcanic beds of Japan are useless .

Big fossils are easier to find than small ones so that helps, and the types of rock can have an effect too – very soft stuff that erodes quickly could mean that bones are destroyed before palaeontologists could find them, or that they are so damaged they can't be excavated and saved. Very hard stuff may take so long to work on that palaeontologists with limited budgets can't afford to work there.