The hard parts of an organism may, however, remain unaltered. We find this in the teeth of sharks from the last few tens of millions of years. The teeth are so much tougher than the rest of the skeleton that they survive for a long period. The bones of Ice Age mammals trapped in the tar pools of Los Angeles are another example. Again, no dinosaurs are preserved like this. Sometimes only the original carbon of rhe organic substance remains.
This is seen in the black shapes of leaves that are sometimes seen in plant fossils. Taken to an extreme, this process gives us coal.(walking dinosaur costume)
Over long periods of time, the cellular structure of the original may be replaced by a totally different substance. Silica in ground-water passing through rocks may replace the original carbon, molecule by molecule, and give a fossil that shows the original microscopic structure, hut made of silica. Petrified wood is a good example. It is also seen in some Australian plesiosaurs, in which the bone has been replaced by opal.(Animatronic dinosaur)
Groundwater percolating through the rock may also dissolve away all traces of the original organism. The result is a hole in the rock called a “mould’ in exactly the same shape as the original. Permian reptiles from the desert sandstones of Elgin，in Scotland, occur in this way, and pouring latex into the moulds produces casts of the original bones. This casting is sometimes done naturally, with dissolved groundwater minerals filling the moulds. Sea urchins in chalk are sometimes found replaced by flint. Then there arc fossils which contain no part or impression of the original animal at all. They are called trace fossils and encompass footprints and trackways, coprolites and eggs.