How long does it take for animals to turn into fossils?
Hi James. You have asked a very good question, and you might be suprised to learn that generally speaking we know very little about the fossilisation process (how the remains of animals or plants are replaced by minerals and literally turned to stone). As you are probably aware the chances of being fossilised are extremely small (we have very few fossils compared to the total number of animals that have ever lived). This is largely because the fossilisation process can take so long that usually decay or scavenging by other animals will get there first and there will be nothing left to fossilise!
One way of answering your question would be to do actual experiments - taking dead animals and literally waiting to see what happens. This is known as "actuopalaeontology" and has been helpful in telling us about which environments are most likely to preserve the corpse of an animal for long enough to allow fossilisation to occur. However, we have never been able to observe the fossilisation process itself because it takes so long (probably hundreds to thousands of years). Over that length of time the hard parts of an animal (such as the bones of dinosaurs or the shells of molluscs) are far more likely to remain than the softer parts (such as muscles) which decay very quickly (in days to weeks).
Despite this we do have fossils with soft parts - even rare animals such as dinosaurs (see http://news.nationalgeographic.com/news mmy.html). More impressive (to us biologists at least) are fossils from the Cambrian (a period of time when the first animals arose). We are very lucky to have multiple sites across the globe (from Canada to China) that preserve whole communitites of animals - including those with no hard parts at all. Strangely this global, high-quality preservation is not seen again for 500 million years. Despite some very good guesses by palaeontologists no one has yet come up with an explanation for this that fits all the facts. Perhaps one day you will be the one to figure it out!
Graeme is absolutely right when he says that we don't tend to get the soft parts of larger animals preserved, because the process of fossilisation is so slow that the flesh is often eaten by scavengers, and rots away before the material can be preserved. However, this is all a matter of scale. If you are a tiny egg or embryo (less than half a millimetre in diameter), of a worm-like marine organism from the Cambrian (over 500 Million years ago) you will have been preserved because the process of mineralisation is actually relatively fast! The conditions need to be right for this to happen, but it is not impossible to occur naturally (we have the fossils), and it has been repeated in the laboratory with some limited success in a matter of weeks (using the eggs of lobsters). We believe that if these tiny organisms are covered over by mud, and the conditions prevent them from rotting away (which can happen in a matter of a few hours if the embryo is in oxygenated, normal sea water) the relative speed of mineralisation (one of the first stages of fossilisation, replacing organic material of dead organisms, with a mineral) is quick enough for the tiny organism to be preserved for over 500 million years. In fact it seems that if you do not start to mineralise in the first 2 weeks of burial, these tiny organisms will not ever become part of the fossil record. So size really is an issue when it comes to chance of an organism fossilising. The reason that this is of such interest to evolutionary biologists is that the way an animal develops, from the egg to an adult organism, is important to how we understand its relationships to other animals. As Graeme said above, the Cambrian is when all the animal groups we see today first appeared in the fossil record. In having embryonic stages of animals from the same aged rocks, we can really start to understand how animals developed and how this has changed over the last half a billion years.
Last edited by Neil Gostling (16th Feb 2007 12:34:04)
Experts: login to post a reply