How do fossil footprints form?
Dr. dig responds:
Fossils do form very slowly over a very long period of time and although the fossilization process cannot be observed with the naked eye (watching a fossil form would be a bit like waiting for a pot to boil, but it would just take a lot longer...), fossils have been found in various stages of the process. This is actually not so unusual -- I have found bones that are only partially fossilized, which simply means that only part of them has been replaced with minerals from the earth. Fossilization means the mineral replacement of organic material. Some of the oldest fossils aren't completely mineralized anyway. Some ancient fossils still contain enough organic material for scientists to collect samples of DNA.
As for fossil footprints and other mud print remains. This amazes me as well, but it clearly happens. In Minnesota, there is a place where an outcrop of rosy rock is exposed in the prairie. This rock has not only been inscribed with hundreds of petroglyphs, but it also retains the ripple marks left behind by an old lake bed. One can almost imagine little fish and crabs scuttling across it.
But there it is, a snapshot from millions of years ago. Conditions must be just right for something like that to form, but they do happen. A windy day, then weeks of calm in which the minerals and salts in the water are allowed to form just enough crust to 'set' the surface of the sand -- a bit like the top of a lemon meringue pie. The ripples don't have to turn to rock -- they simply have to harden just enough so that the next layer that settles upon it doesn't mix with it. For instance, when I make a birthday cake for my children, I have to let the bottom layer cool a bit before I add the icing, otherwise it will melt into the cake and not form its own layer. This is just what happens to footprints and mudripples. The footprint has to form just enough crust to resist the next layer of sediment that forms on top of it.