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Fossil Question

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Anonymous

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I was reading an article in the latest FT when a question arose - how do we get fossil footprints? Actually, more specifically, how do footprints last long enough to become fossils? Wouldn't they get washed away, covered up or otherwise destroyed?

And the age of fossils. Whenever an object is found in strata too old to contain it (or a similar scenario when it's carbon dated etc and found to be older than it supposedly could be) how come there never appears to be a suggestion that the actual dating process itself could be inaccurate, rather than the usual two choices of a) we have to rethink current theories or b) it's a fake?

Probably both rather silly questions, with an obvious answer, but I can't find it at the bottom of a bottle of Baileys;)
 
Fossil footprints are IIRC generally found in fine grained strata, like volcanic ash and mud. I think the usual scenario is that hominid/dinosaur/alien walks through either mud or wet ash which is then covered by another layer of mud/ash.

As for the carbon dating thing. I generally hear that there is generally a date range and the older the sample the greater the range. It also depends onwhether someone is using dating evidence to back or discredit a theory. If someone's pet theory is that people were around earlier than generally supposed they will use the older date from the range. A debunker will use the younger age given.
 
Don't forget that the clay sediments that have footprints in them can get quite hard in the sun before they get covered up- almost like terracotta if they are left for several days-
then the water comes back,
often rapidly with a burden of particles with a different size to the original sediment-
the interface between the two layers then gets nice and distinct-
because of the different particles.
Oh and sometimes, like at Laetoli, the deposition of particles is from airborne ash- and the different size of the particles corresponds to different phases of the eruption.
Coupled with a quick shower of rain between eruptions to turn the footprints into near cement, the footprints at Laetoi have been beautifully preserved.
Fossils of the present found in years to come might be concrete pavements that read 'Bart was here' and so on.
 
Yup, I thought of those. Just somehow they don't seem convincing at the moment. I don't really know why. Maybe the old brain cells are firing incorrectly or something;) I was just thinking of the likelihood of, say, my leaving footprints in my back garden, leaving the soil undisturbed, then coming back even a month later and still find them.

Dating methods are such a pain, though, aren't they? It's the typical thing of using them as irrefutable evidence when it suits a theory, but ignoring them when the date they provide doesn't fit the 'evidence'. Maybe I'm a complete Luddite, but I don't altogether trust carbon dating etc. There seem to be too many variables.
 
Helen said:
Dating methods are such a pain, though, aren't they? It's the typical thing of using them as irrefutable evidence when it suits a theory, but ignoring them when the date they provide doesn't fit the 'evidence'. Maybe I'm a complete Luddite, but I don't altogether trust carbon dating etc. There seem to be too many variables.
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Carbon-14 has a half life of about 5568 years. Carbon dating isn't accurate after 40 to 60 thousand years, after that you're on to exotic things like stratigraphy, fossil comparison and tracking the decay of rare isotopes like uranium 235.

Fossil tracks have 10's of millions of years and thousands of different environments to form in. Something always gets overlooked by the passage of time. :)
 
My favoriteof the dating methods is Varves.
they are little layers in shales and stuff- each one representing a yearly flood (or sometimes a more frequent event)
trouble with these is you don't always know how often they are deposited, and the errors soon build up.
nice word tho'
 
A fossil footprint, a form of trace fossil, is usually created in soft muds and sands. They tend to wash away, yes. What we are left with are those that did not wash away. This is the state of play with all fossils. Most are gone but some remain. Trace fossils are fossils that are a by-product of a living creature, including humans. Coprolites, tools, nests, tracks, prints, tooth marks, etc, etc......

There are two different ways that a fossil can be created.

The first is actually very similar to the casts that you may buy from a dealer of replicas. The creature has died and become covered with sediment but the sediment is not air-tight and is not subject to much pressure and so it allows the creature to decompose quite rapidly. The remaining cavity in the sedimentary rock can lie for quite a while, completely devoid of creature remains. Then, due to numerous actions, the cavity is filled with a different or simply younger sediment. The result is an ancient fossil cast.

The second, mineralisation, is much more impressive, expensive (if you are a dealer or collector) and mind-blowing. The creature is killed and is covered with sediment but this time the sediment is completely air-tight and becomes subject to huge pressures. Over time, the molecules in the creature give way to mineral replacement. It tends to be different levels of quartz and occasionaly pyrite. This mineral replacement allows us to open the interior of the fossil and study the internal and external structures properly. The fact that it is made of much harder minerals than sand and mud means that it lasts longer than cast fossils.

......but in answer to your other questions:

Yes, most get destroyed and washed away.

Yes, the dating systems are questioned all the time.

Carbon dating is useless for this stuff, as androman said.

It's the typical thing of using them as irrefutable evidence when it suits a theory, but ignoring them when the date they provide doesn't fit the 'evidence'.
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Spot on, Babe! :D
 
Carbon Dating

Nearly forgot. In order for carbon dating to be possible, there has to be organic carbon present. Living things take up carbon-14, the unstable form of carbon, whilst alive. At death the amount of C-14 becomes fixed and then slowly decays, radioactively, with time.

Footprints aren't a good candidate for carbon dating, charcoal, from fires, is.
 
Nobody really trusts carbon dating; living things have a horrible tendency to move about and get into places that you'd rather they didn't and make things that are humungously old come out the test as being made last week.
 
Natural casts of footprints are also found, when the layer in which the footprints were made has eroded away, leaving the material that filled them.

I photographed these (looking like dome-shaped bumps on the underside of an overhanging ledge) near Canon City, Colorado.
They are thought to have been made by an ankylosaur, 107 million years ago.

The story of how they were discovered.
 
There are some nice trace fossils of Tree Fern roots at South Queen's Ferry, they look rather like offroad bicycle tracks in the stone.
 
Thank you all for the information, the pictures and the links. All interesting stuff. I know it was rather a simple question, but sometimes I find it handy to break things down to a basic level, rather than blithely accept. Although I'm still amazed at fossil footprints - I mean, what's the likelihood, eh?
 
It's fairly amazing that any fossils are created. Many environments just aren't conducive to fossil formation.
 
I took my geology A level while I was living in Cumbria,it has to be one of the best places in the world to study geology!Anyway,on a field trip half way up some fell , my teacher was hacking away at a rockface to see if he could find any fossils, he found a really lovely fossil of a sponge type creature-then informed us that it shouldn't be there as the rocks were supposedly too old for that type of fossil! He laughed it off and never mentioned it again-I would have gone nuts til I found the answer! (if I had been a proper geologist that is,I was a teenager at the time !)
 
Eburacum45 said:
My favoriteof the dating methods is Varves.
they are little layers in shales and stuff- each one representing a yearly flood (or sometimes a more frequent event)
trouble with these is you don't always know how often they are deposited, and the errors soon build up.
nice word tho'
Click to expand...

A Varve as my geology studies have lead me to understand is the the yearly deposits in glacial lakes, when spring comes melt water brings sediements down in to the lake where they are deposited, in winter the run off stops so a definate yearly build up can be seen.

Carbon dating isnt used in the dating of fossils, statigraphy and radiometric dating to some extent is used. However more often than not fossils are used as Zone fossils to date the rocks they are found in.
 
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