For any animal or plant to fossilize, conditions must be just right. The ideal amount of sediment has to cover the organism's remains for the fossilization process to even begin, but most plants and animals decompose long before this can take place. In addition, fossils that form must survive for millions of years, and even though fossils are set in stone, they're not nearly as solid. Over time they experience the effects of erosion, harsh weather and the dynamic changes of the Earth's surface. Add to these factors the numerous fossils paleontologists haven't even found, or ones that have been misinterpreted because of insufficient technology and you've got more than enough reasons to explain why the fossil record is not complete.
Still, new discoveries are made regularly; just because the fossil record is incomplete does not mean that it's inconclusive. Vertebrate paleontologist Michael Benton says that paleontologists have applied sophisticated mathematical techniques to fossil successions and shown that we don't know everything, but we know enough to show the progress of fossils as Darwin predicted [source: Benton].
The oldest fossils known to date are those of cyanobacteria from rocks in western Australia. They're dated nearly 3.5 billion years ago, nearly as old as the oldest known rocks to have existed (3.8 billion years ago) [source: Berkeley]. Fossil records of other life forms are often more complete and informative than the ones we have for dinosaurs, although dinosaur fossils naturally are important and exciting finds. Fossilized remains of plants and animals that were alive at the same time as dinosaurs can provide us with clues about the prehistoric climate. For example, the chemical makeup of the shells of prehistoric marine protozoans gives us an indication of water temperature at the time they lived; it also hints at the ways in which the environment changed for dinosaurs, making it impossible for them to survive.
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