The Rarity of Old Fossils
August 29, 2008

Don Lindsay is a computer scientist who likes to debate creationists and Scientologists. He has a knack for saying things in the simplest possible way. Here he explains why we don’t have many really old fossils.

We have lots of seashells. We’re very short of jellyfish fossils. That’s not too surprising.

We have a few T. Rex fossils, but we’re short on small, fragile creatures. This is easy to explain. First, it’s just easier to find the great big fossils. Second, fragile skeletons are, well, fragile. They are more likely to be scavenged or crushed before they can form a fossil.

But there is another pattern, which is that there just aren’t very many really old fossils. Why?

There are at least four reasons. For one, the earth’s surface has been rebuilt many times. Regions have been uplifted and then eroded away. Erosion destroys rock, and destroys any fossils in that rock. The new rock that forms contains new fossils. So, much of the earth’s surface is recent, compared to the age of the planet itself. Old rocks are rare, so of course old fossils are rare too.

The second reason is that many old rocks have spent time buried. While buried, they experienced great heat and/or pressure, and are now metamorphic rocks. Their fossils have turned to smudges.

Worse again is that the best fossils are found in ocean-bottom sediments. But as the continents move, they ride over the ocean floor. Old floor is sucked down towards the center of the earth at subduction zones, never to be seen again. (Places like the North Atlantic Ridge are creating new ocean floor to replace the old.)

Continents travel at about an inch a year. So, if you look at the size of an ocean, and do some simple arithmetic, you will see that most of the world’s ocean floor should be less than 200-300 million years old. But dating methods say that animal life arose 800 million to 1000 million years ago, and it moved onto the land about 400 million years ago. So, this is a frustrating situation. The oceans have been repaved since the really interesting stuff happened. We have to make do with the very few old ocean rockbeds that escaped destruction.

And the fourth reason is that the first creatures didn’t have skeletons, and they were tiny, too. We can tell in two ways. First, we’ve been lucky, and found a few very old deposits that preserved soft things. And secondly, we’ve found tracks.

Why didn’t they have skeletons? Well, because skeletons had to be invented at some point, and that point was about 600 million years ago.

Visit Don’s website.

Seeing Through Stone
August 24, 2008

Richard Fortey loves fossils. In Trilobite! he waxes poetic about the trilobite’s stone eyes.

Trilobite eyes are made of calcite. This makes them unique in the animal kingdom.

Calcite is one of the most abundant minerals. The white cliffs of Dover are calcite… Limestones (which are calcite) have been used to build… the sublime crescents of Bath, the pyramids of Gizeh, the amphitheatres and Corinthian columns of classical times. Polished slabs composed of calcite deck the doors of Renaissance churches in Italy, still grace the interiors of Hyatt–Regency hotels, or conference halls, or wherever architects wish to suggest the dignity that only real rock seems to confer.

The purest forms of calcite are transparent. In building stones and decorative slabs it is the impurities and fine crystal masses that provide the colour and design… The dark red of the scaglio rosso so typical of Italian church doors is a deep stain of ferric iron. But when a calcite crystal grows more slowly in nature, then it may acquire its perfect crystal form, and be glassy clear…

Look into a crystal of Iceland spar and you can see the secret of the trilobite’s vision. For trilobites used clear calcite crystals to make lenses in their eyes; in this they were unique. Other arthropods have mostly developed ‘soft’ eyes, the lenses made of cuticle similar to that constructing the rest of the body.

The science of the eye demands a little explanation. It all depends on the optical properties of calcite… If you break a large piece of crystalline calcite it will fracture in a fashion related to its fine atomic structure… You are left with a regular, six-sided chunk of the mineral in your hand, termed a rhomb… The clear calcite of this not-quite-a-cube treats light in a peculiar way. If a beam of light is shone at the sides of the rhomb it splits in two; this is known as double refraction. The rays of light so produced are the ‘ordinary’ and the ‘extraordinary’ rays: their course is determined, just like the shape of the rhomb, by the stacking of the individual atoms. There is a huge specimen of Iceland spar on the first floor of London’s Natural History Museum through which you may peer to see two images of a Maltese cross, one generated by the extraordinary, and the other by the ordinary rays. But there is one direction, and one direction only, in which light is not subjected to this optical splitting… from this direction it does not split into two rays at all but passes straight through.

If to travel back to the time of the trilobite is a historical sea-change then there can be nothing stranger than the calcareous eyes of the trilobite. And pearls are chemically the same as the trilobite’s unblinking lenses, being yet another manifestation of calcium carbonate, although pearls are exquisite reflectors of light rather than transmitters of it… The trilobite saw the submarine world with eyes tessellated into a mosaic of calcified lenses… his stony eyes read the world through the medium of the living rock.

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