Friday, 29 January 2010

Fossils and the History of Life: Part One

So that’s that! S193 done and dusted. I’ve submitted my end of course online assessment and now await the result! I’m quietly confident of a comfortable pass, though I should never underestimate my ability to fall into the clever little traps the OU like to set for you! After doing the first 10 questions early and then going back over them towards the end, I did manage to spot one little mistake of mine, but hopefully there won’t be too many others – we’ll see!

As the Open University’s short, 10 point science courses don’t require any written assignments like the epic S104 foundation course, I thought it would be a good idea for me to summarise the course content here in ‘Holey Schist’. This will help consolidate my knowledge and maybe with a little help from you lot out there, help spot some glaring misconceptions that I may have picked up along the way? So here goes . . .

Part One: A Brief introduction to fossils

Firstly, let’s answer the obvious question: what is a fossil? Well, to quote the S193 course book, “a fossil is simply any evidence of ancient life, naturally preserved within the materials that make up the earth”. Such ‘materials’ are invariably rocks, formed from mud, silt and volcanic ash, but can also include other substances like natural resins (producing for example amber) and even ice.

Prior to doing this course I did wonder what the dividing line was between ‘dead animal’ and ‘fossil’ and the truth is that there is no strict dividing line between the two. Generally speaking though, palaeontologists would only really consider remains over 10,000 years old to be fossils.

The development of life on earth through time is recorded in successive layers of sedimentary rock – their strata. As time passed, the deposition on sediments layer upon layer effectively form ‘pages’ of the earth’s history ‘book’ and present evidence of environmental conditions at the time of deposition and a sample of life around at that time in the form of fossils.

Unfortunately, things are complicated by the fact that many pages of the ‘book’ are often missing. Periods of time when deposition didn’t occur in an area thus creates a time gap. Similarly, when layers are eroded, evidence of the time that that particular eroded rock was deposited is lost. When new deposition occurs on top, the gap in time created between the resulting strata is called an unconformity.

The study of strata and their relationships in time and space goes by the name of stratigraphy. Studying successive sequences of strata and their fossil content has enabled geologists to firstly place events in a geological perspective by dating strata relative to each other. That is, Rock ‘A’ is above Rock ‘B’ and is therefore younger. The arrival of radiometric dating enabled geologists to establish ‘absolute’ dates to rocks, subject to certain levels of uncertainty e.g. +/- 3 million years. Prior to this, dating was, with the benefit of hindsight, wildly inaccurate, but one must also bear in mind that new discoveries and advances in scientific techniques could easily spark a hasty redraft of the currently accepted stratigraphic column.

The British Geological Survey website has an excellent example of the latest Stratigraphic columm. Take a look here.

How to become fossilised:
For all of you who share my somewhat misguided ambition to be dug up ‘x’ million years from now as a beautifully preserved fossil by a future earth creature/alien visitor, just what does one have to do to become a fossil? Well, alas, it ain’t that easy! For instance, when we look at the fossil record it is important to realise that it only represents a tiny fraction of the likely total population of past life and that record is heavily biased towards creatures that lived in shallow marine environments.

The reason for that is simply due to the fact that such environments provide the best conditions for preservation to take place. As well as environmental conditions, other factors have a bearing on an organism’s ‘preservation potential’. For example, whether it has any ‘hard parts’, whether and how quickly it becomes buried in sediment and whether that sediment ultimately becomes a part of the rock record. So, if I am aiming for the best possible chance of preservation I will need to aim to be buried in suitably fine grained, oxygen starved sediment, either in a shallow marine or lake/river environment. Alternatively, I could aim for entombment in sticky tree resin as per insect fossils in Amber. Somehow this latter option doesn’t appeal quite so much – don’t ask me why!?!

It might seem odd that seemingly hard, solid substances like shell and bone can be converted into rock, but both contain miniscule pore spaces which can in the right situation, become filled with mineral rich water. In time the minerals may crystallise out thus filling the pore spaces with those minerals. This process is called ‘permineralisation’. Additionally, original bone can be replaced by minerals completely in a process called (rather unimaginatively) ‘replacement’. Together, these two processes are called ‘petrifaction’, but neither of these processes have to occur for remains of an organism to be termed a fossil. Some fossils can indeed be composed of barely altered bone or shell!

I think that will do for now.

In Part 2 we’ll look at the way that fossils are named and classified and how palaeontology fits in with evolution’. Any comments? Feel free to contribute!

Until next time!


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