The most common kind of animal fossils are those resulting from the mineralization of shells or bones. Other parts of the animal’s body are usually recycled by decomposers and never make it to the fossil stage. Only about 1/10 of the species living today are made up of tissues and live in habitats that make it possible for fossilization to occur. Animal fossils are only formed in areas with unusually good conditions for preservation that allow the more biodegradable parts to be preserved and represent an accurate depiction of the variety of species alive during that time.
Conditions that improve the chances of the formation of a good animal fossil include early silicification of sediments, impregnation of fossil material with calcium phosphate, forest fires, amber, shale and mudstone, and course-grained sediments. The oldest animal fossils on earth resemble bacteria and have been dated 3.5 billion years back. These were the result of silicification of sediments in the Achaean and Proterozoic eons.
Impregnation with calcium phosphate is another way excellent animal fossils are formed and happens in between when the sediments are deposited and when they solidify. Small arthropods and the early embryonic stages of small animals have been preserved so well by this process that even the cells in animal fossils can be viewed. Forest fires are another way to preserve fossils through coalification, but most of these fossils are plant fossils rather than animal fossils. Amber, or fossilized resin from trees, has been found with small animals and plants captured for the rest of time.
Another, less known way for animals to become preserved is by freezing. Recently, there has been incredible discoveries of preserved soft-tissue from mammoths. The reason why there aren’t many discoveries like this is that the temperatures have to stay low and never rise for a very long time. Shale and mudstone have yielded many very well preserved animal fossils. Animal fossils are preserved by shale and mudstone when the rock above compresses the remains below. The shaley deposits have been found to preserve many of the larger animals. Trace fossils come from the tracks of animals and coprolites are fossilized feces. These types of animal fossils are less informative about animal anatomy, but still important in the quest to understand how the animals behaved or fit into the ecosystem. |
