Unaltered Remains. In rare situations, an entire plant or animal, or some of their various parts (including the soft tissues), may be preserved intact by unusual conditions such as extreme cold, chemical action, or extreme dryness. Rare examples include frozen wooly mammoths and insects preserved in amber. Normally, preservation of soft tissue requires rapid but gentle burial in an oxygen-free sedimentary environment where bacterial decay cannot occur. Because such conditions are uncommon, preservation of soft tissue occurs only rarely. Much more common examples of unaltered fossils are those in which hard skeletal material has been preserved with little or no change. Many marine invertebrate fossils and microfossils are typically preserved in this manner.
This type includes most fossils because the great majority of plants and animals are changed in some way after the organism dies. These changes may take place in the following ways:
PERMINERALIZATION: This begins when an organism dies in an area such as a wetland and its body is buried rapidly under layers of sediment. Over time, more sediment is deposited, compressing the tissues. The tissues are gradually replaced with minerals from the water that percolates through the sediment. Gradually, over thousands to millions of years, the remains become petrified, or stonelike. Bones, teeth, shells, and tree trunks are generally preserved through permineralization.
REPLACEMENT: When the infiltration occurs more slowly over a longer period of time, the hard parts of the original specimen can be completely dissolved and later replaced by mineral matter.
CARBONIZATION: Soft-bodied animals and leaves and stems of plants may become a fragile carbon film after volatile body chemicals escape during decomposition, leaving behind only a thin layer of carbon.
TRACKS AND TRAILS: These include traces of activities-locomotion, resting, eating, excretion, and mating-of the extinct animals. In order for these traces to be preserved, they must be covered by a fresh layer of sediment and remain covered in order to prevent destruction.
MOLDS AND CASTS: Molds and casts can occur together in nature. Molds are formed when either part or all of a plant or animal is pressed into surrounding dirt, which later hardens into rock. This imprint is called a MOLD. Sometimes dissolved minerals fill the mold and then harden, forming a replica of the original imprint-maker. Such a replica is known as a CAST.
Fossils help geologists establish the ages of layers of rock. In this diagram, sections A and B
represent rock layers 200 miles (320 kilometers) apart.
Their ages can be established by comparing the fossils in each layer.
Fossil: geological dating . Image. Britannica School, Encyclopædia Britannica, 7 Aug. 2020. school.eb.com/levels/middle/assembly/view/190530. Accessed 3 Dec. 2020.
Formation of fossils
Cross-section model showing a stage in the formation of fossils under layers of sediment, with pair of model mammoths placed on top
Formation of fossils. Photograph. Britannica ImageQuest, Encyclopædia Britannica, 31 Aug 2017.
quest.eb.com/search/132_1504737/1/132_1504737/cite. Accessed 4 Dec 2020.
Sequential artwork of the formation of a dinosaur fossil. The process begins when the dinosaur dies and the body ends up in a location where fossilization is possible, here in sediments below water (upper left). The soft tissues decay, while minerals in the dinosaur's skeleton are gradually replaced by minerals from the mud. These minerals solidify into a form of stone or rock, preserving the shape and structure of the skeleton (upper right). The fossil may remain underground, or may eventually be exposed by erosion of the surrounding material (lower centre).
Fossil formation, diagram. Photograph. Britannica ImageQuest, Encyclopædia Britannica, 2 Mar 2017.
quest.eb.com/search/132_1428016/1/132_1428016/cite. Accessed 3 Dec 2020.