If you are viewing this as a single page, sliced by a search engine from the complete blog, the story begins in part 1.
Have you seen those cartoons where a dog steals and buries a dinosaur bone? The joke would be on the dog in this case, because as a rule, there are few if any atoms of bone in a fossil bone. What typically happens is that the bone is buried quickly and covered over, then ever so slowly, over many years, the atoms of the bone are replaced by other atoms that soak in.
For example, I have seen a fossilised platypus jaw bone that was an opal. That means it was made of silica, and bones just don't get made of that. It was a replacement.
Glendonite (seen here on the right) is an interesting form of calcium carbonate that only forms below 5°C, in a cold ocean. So when we find forms like this on a rock platform, we know that the rock was laid down in a cold sea, because the hole is where a glendonite crystal was later dissolved away.
In the lower shot, we see a piece of iron oxide that has formed when the crystal dissolved away, and later, iron oxide was carried in and laid down, filling the mould that was left behind.
That clue also explains some odd stones that occur on the same rock platform, which is at Ulladulla, south of Sydney. There must have been a glacier somewhere nearby, calving icebergs that floated off, melting slowly and dropping big round pebbles that plunged down into the soft mud. In a sense, those stones are fossils of a sort, and they are still the original atoms, near enough.
The Robin's Hood bay fossils in the third picture (and as seen yesterday) are both moulds and casts where every atom of the original has been faithfully replaced, perhaps many times over: see if you can work out which are casts and which moulds.
There are some plant fossils, where there is a sort of sooty imprint left behind. These may well be the same atoms that were once in the plant, but most of the solid 3D fossils have been formed in some sort of mould-and-cast way. Mind you, it takes an incredibly long time, and my aim is to make things happen as quickly as possible.
This is the Canowindra fish fossil slab from yesterday. These Devonian fish seem to have died in one of those once-in-a-thousand year droughts—of which there have been some 370,000 more since they died (do the sums!). It seems that no sooner had they foundered and flapped their way into death as the sun took away the last of their water, than the gentle rains came, followed by torrents that washed in sediment that left them buried, ready to be preserved.
These would almost certainly be replacements of some sort, and have none of their original atoms left. After all, they have had more than a third of a billion years to get to where they are today.
And now, we are ready to start our simulation, wherein we can replace all the atoms much faster. Today, I will provide the shopping list.
This is what you need:
- some shells to fossilise;
- some plaster of Paris from the hardware store;
- a few disposable plastic tubs with tapering sides;
- a couple of paint scrapers to use as spatulas;
- hydrochloric acid (sold also as "pool acid" or muriatic acid;
- safety goggles (not shown but essential);
- rubber gloves (not shown but advisable);
- a hammer and a screwdriver;
- and a small bag of grey cement from the hardware store.
In part 3, I will begin the process.