Human earth shapers/ETHS101/Rock Record/Fossils
Fossils
- Fossil
- a fossil is any recognisable trace of a past living organism that is preserved in the geological record.
That means that all of these are fossils:
- The bones of woolly mammoths
- Petrified wood
- Mummified leaves
- Pollen grains preserved in rocks
- Oil
- Dinosaur footprints
Fossils can tell us a lot about the past. They help us understand how many species there were in the past, and when and where they lived. They tell us about what species have gone extinct. They provide us with information about what the world was like in the past (what the environment was like). They also can tell us how old rocks are.
But before you can use fossils to do any of these things, you really need to know how fossils are formed. One of the most important ideas about fossils is that the fossil record is incomplete – not every organism leaves a fossil. In fact, we have fossils of only a small proportion of all the species that have ever lived. This means that many species have gone extinct without us knowing they ever existed. Here is a mini-lecture explaining how and where fossils are formed.
How man species were in the past? When did things go extinct?
Although it’s hard to work out how many species there were in the past, and how many have gone extinct, it isn't quite impossible. The main way of answering this question is to interrogate the fossil record, but this runs up against the problem highlighted in the lecture – that the fossil record is incomplete. Palaeontologists often get around this complication by focusing on fossils of types of organisms and types of environment for which the record is strong.
By far the most complete fossil record is of shell-forming animals living in shallow parts of the ocean. We can assume that the record of these animals is almost complete, and some of our best knowledge of when major extinction events happened is based on these animals. Other things with good fossil records are the pollen and spores of plants, and the bones of marine vertebrates like fish and whales. These are also used to look at extinctions. Some other organisms like dinosaurs, birds and land mammals have reasonably good fossil records in some parts of the world that have been studied intensely. To account for the incompleteness of the fossil record, palaeontologists also use statistics that adjust the numbers of extinctions to allow for the chance of a kind of organism becoming a fossil and for the number of fossil records available for a given period.
How do we use fossils to tell us about what the world was like in the past?
This is the idea of using proxy evidence – fossils aren’t thermometers and aren’t photographs of vegetation. But if we make some uniformitarian assumptions we can use fossils to tell us about what the part of the world the original living organism came from was like when that organism became fossilised. So the fossils provide proxies for the environment. Here are two examples:
Brachiopod fossil.
Petrified wood
The image of the Brachiopod fossil shells (a big one right and a smaller one to the left) tell us about the environment at the time. They are brachiopods. Living (present today) brachiopods now live in muddy sediments in the sea. So, if we assume that the ancient brachiopods function the same way as modern brachiopods ( the principle of Uniformitarianism), then these came from a seafloor.
In this piece of petrified wood, you can see the rings. It came from a large tree (more than 1 m wide and more than 20 m tall) that was related to modern Norfolk Island Pines and Kauris. The rings are about 1 cm wide. If we assume that, like the rings in modern trees, each ring represents one year of growth and compare the rings to the living trees (especially the living relatives of the fossils) we can say some things about the climate when that tree grew. 1 cm is wide for tree rings (especially trees like Kauris and Norfolk Island Pines) and that means that the tree grew fast. That, in turn, means conditions were really good for tree growth – the climate was warm enough and there was enough rain. You can go even further - on the right of the picture you can see the individual cells in another piece of the same wood, you can even see the holes (pits) in the wall of individual cells (see the red arrow near the bottom left of the picture). These cells can tell you more still about the environment.
These examples show just a couple of the many ways that fossils can be used to tell us about the past. They can tell us about climate, about vegetation, about other organisms (for instance fossil teeth tell us about what kind of food an animal ate) and many other things.
