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The vagaries of the fossil record

The fossil record is hopelessly patchy. Unfortunately it’s also the main source of information for palaeontologists as they try to piece together ancient events. You could have two populations of identical beasts roaming around in two different areas. One we know about, and the other we don’t. One left a good fossil record. The other left little, or even none at all.

This is why palaeontologists are constantly reminding each other that “absence of evidence is not evidence of absence”.

The primitive mammals that came through the K-T extinction (more) found an almost empty environment just waiting for them to exploit more. Most of what we know about these animals comes from North America. Very few fossils from the time have been found in Europe. And yet palaeontologists are persuaded that actually they were equally important over here.

Another gap in the record covers the period between the Ediacaran and Cambrian periods. Not one of the Ediacara creatures, or even descendents thereof, seemed to have survived into the Cambrian period. This led to theories of a mass extinction to end all mass extinctions. Now however a jellyfish-like creature has been found from well into the Cambrian, which palaeontologists are persuaded is a genuine descendent from an Ediacara creature. Not only that but it appears to be a genuine ancestor of modern comb jellies. Apparently comb jellies are an entire ‘phylum’ of animals, so this is big news.

Where to find fossils

The place to look for good fossil sites is the middle of a large continent. Many of the most exciting finds have been from Australia, Africa and Canada. But nothing is simple in this game. As we saw on the home page, the oldest real estate ever found is in Greenland. And it was preserved by being buried deep under another layer (just as parts of northern India are busily burying themselves under Asia today more).

An area subject to plate tectonic activity, such as Europe, is likely to be much less productive – because so much has been destroyed.

How fossils are formed

Most fossils are bones which have been turned into stone. This needs fairly special conditions and most bones simply rot away. I’ve not so far found a succinct description of how fossilisation works. This may be because there are many different types of fossil, and many different ways in which they can be made.

So the following is the best I can do. To get fossilised, a bone needs to be buried in gravel or some such. Then there needs to be more similar material piled on top of it, until the layer with the fossil in becomes compressed into sedimentary rock. The bone needs to have mineral-rich water trickling over it. This is quite easy. As the rainwater trickles down, it dissolves minerals from the pebbles and such that it passes over. As the mineral-rich water seeps into the pores in the bones, some gets left behind. The bony material gradually gets dissolved away at the same time. This leaves more pores to get filled with mineral. And eventually the entire bone becomes stone. The process may take 100,000 years or so.

The conditions for producing this kind of fossil is actually quite common. So fossilised bones are found all over the place, and are the mainstay of palaeontology.

But a fossil skeleton can only tell you so much. Fortunately conditions very occasionally occur, in which much more is preserved. For this the entire corpse needs to be buried in a mudslide or a bog, so that all air is instantly excluded. If all the air remains excluded for long enough, then the complete body can be preserved. It may be squashed, and it may have been swept away from where it died. But you can get the most intricate details preserved for hundreds or even thousands of millions of years.

These conditions are extremely rare. The early finds were entirely accidental. First there was the Burgess Shale, made famous by Stephen Jay Gould more. Next came the Ediacara Fauna, which came from shortly before the Cambrian Explosion.

But then palaeontologists started to learn where to look for such cornucopias. And now all sorts of goodies are being found, such dinosaur feathers, dinosaur lungs and many others exciting things.