|MadSci Network: Evolution|
The chance of winning most state lotteries is around 1 in 7,000,000, but that is almost a "sure thing" compared to the odds that an animal will become a fossil, let alone be discovered by some future paleontologist.
Biologists estimate that there are around 1,000,000 living species of animals. Because invertebrate life appeared on earth more than 500 million years ago, paleontologists estimate that extinct species have an aggregate of at least 100 times that number.
Of the 100,000,000 extinct animal species, only around 100,000 species have been discovered and described. That means that only around 1/10 of 1% of all animal species that have ever lived have been discovered! (And remember that each species may be represented by hundreds of millions of individuals.)
I hope this answers your question. Be sure and write back if I can provide you with any additional information.
John W. Bebout
Admin Note: It's important to realize that fossils occur under very specific conditions. First, the organism needs to be swiftly buried under mud and sediment. This rapid burial prevents scavengers from consuming the deal material, and also creates an "anoxic environment" (low leves of O2 gas) which slows the rate of microbial decomposition. These conditions are more likely to occur in and around large bodies of water.
The composition of the plant or animal also plays a role in its likelihood of becoming a fossil. "Hard parts" such as bones, shells and bark are more likely to fossilize than "soft parts" such as tissues and internal organs. Thus an animals with a skeleton or hard external shell will more likely leave a "fossil record" than one with no such structure.
Then over time processes such as permineralization, or petrification need to occur. In these processes minerals in water permeating the sediemt replace the "organic" components in the buried remains
Lastly, geologic factors such as the temperature and pressure within the strata containing the buried remains will affect whether they are fossilized.
-L. Bry, MadSci Admin
Try the links in the MadSci Library for more information on Evolution.