Fossils give us a glimpse into biological world before humans existed on Earth. They allow us to follow the twisted paths and understand the steps that species have taken to evolve as they are today. When most people think of fossils they think of dinosaur bones perfectly arranged in a museum, sometimes in action poses. The reality is that the vast majorities of fossils found are not perfect and require some interpretation to tell the story of that organism. This article will discuss ways that some organisms find themselves in a display case at a natural history museum millions of years after they have died.
How to become a fossil
Not everything that lives becomes a fossil. In fact, very few organisms are lucky enough to be preserved as well as most of the fossils we find. All fossils are found in sedimentary rocks. These are rocks that are formed from loose particles like sand or clay that over time turned to stone. The easiest example of this would be sandstone, which forms when sandy areas are compressed over time so much that the grains chemically fuse together to form one big conglomerate.
So, the first thing you have to do to become a fossil is to die in an area where there is a lot of sediment deposition, like the mouth of a river, and quickly become buried by sediment. Next, you are not allowed to decompose very fast. If you’ve chosen an area with a lot of oxygen for decomposers, then your body will be broken down too quickly and nothing will be left to find! The best fossils are found in areas that used to be anoxic (without Oxygen), where decomposers are not able to break down the organism.
Recrystallization and Permineralization
The most common form of fossilization is called recrystallization. This occurs when minerals that make up the hard parts of an organism are enlarged or changed slightly without changing the shape of the organism. For example, ‘mother of pearl’ is a mineral that is made by many different types of shelled organisms, and the mineral that makes up the pearls we wear. Over time, and with pressure, mother of pearl can be changed into another mineral called calcium carbonate (limestone). All the same molecules are there, they just changed positions.
A similar process is called replacement. This happens when the original mineral, in a bone for example, is gradually replaced with a different mineral. Most often the replacement mineral is quartz or calcium carbonate (limestone).
Permineralization occurs when the hollow spaces in an organism are filled with water and dissolved minerals. The dissolved minerals then turn to crystal. This is what geologists call a precipitate. Think of it this way: if you are holding a piece of petrified wood, you are not holding a piece of wood; you are holding a piece of quartz in the exact same shape of the wood it replaced.
Molds, Casts and Trace fossils
Sometimes, a bone or shell can completely dissolve, like salt grains in water, without replacing the lost minerals. When this happens a mold can be formed, like when you make a footprint in the sand, that footprint is a mold of the bottom of your foot. The fossil can be left like that, or the mold can be filled in with material and harden to form a cast. If you put plaster of paris in your footprint and let it harden, you would then have a cast of the bottom of your foot.
Sometimes, in anoxic sediments, the organic soft parts of the organism will decay and leave a black imprint on the side of the mold. These are excellent fossils, because they allow paleontologists to see detail they wouldn’t otherwise be able to see.
Trace fossils are things that the organism left behind, like a dinosaur footprint or a tunnel dug out by an insect. When these are fossilized and later discovered by scientists, they can tell a lot about the habitat that the organism lived in as well as how that organism lived in the habitat.
Mummies are like gold to a paleontologist. They are a rare find, but when they are found, they can give a huge amount of information because normally the soft parts of the organism (muscle, skin, hair) are well preserved. This happens when the organism is dried out quickly before it is buried. Many decomposers can’t survive in dry environments as well as wet environments, so when the organism is dried out it won’t decompose as fast. A lot of times this happens in very cold environments in a process called freeze drying. Many important human fossils and mammoth fossils have been preserved this way. The Ancient Egyptians began mummifying their dead by simply placing them in the hot sand and letting their body dry out. Later Ancient Egyptians began mummifying their dead by embalming them to replicate natural mummification.
There are certain environments that preserve an animal better than anything else, because very few decomposers can survive where the animal is deposited. The La Brea tar pits in California, or various bogs all around the world have produced some of the best fossils found for this reason. The movie Jurassic Park depicted paleontologists taking DNA from well preserved insects found in fossilized tree sap (amber). While taking DNA from preserved insects is very difficult, a large amount of insect fossils have been in amber mines.