Fossilization processes
– Permineralization: mineral-rich groundwater fills empty spaces within an organism, producing detailed fossils
– Casts and molds: original remains dissolve, leaving organism-shaped holes in the rock that can be filled with sediment to create casts
– Authigenic mineralization: minerals precipitate around an organism, preserving fine three-dimensional morphological detail
– Replacement and recrystallization: shell or tissue is replaced with another mineral, preserving microstructural features
– Adpression: chemical reduction of organic molecules results in compression fossils or impression fossils
Soft tissue, cell and molecular preservation
– Soft tissue has been discovered in dinosaur fossils, including blood vessels and proteins
– Iron particles associated with soft tissues enhance stability and preservation
– Collagen fibrils have been preserved in fossils ranging from the Devonian to the Jurassic
– Quality of preservation depends on the arrangement of collagen fibers
– Geological age does not necessarily correlate with the quality of preservation
Carbonization and coalification
– Carbonization or coalification can occur in fossils
– Fossils that are carbonized or coalified have undergone chemical changes
– Carbonization involves the reduction of organic molecules
– Coalification results in the formation of coal from plant material
– Carbonized or coalified fossils can provide insights into ancient plant life
Types of fossils
– Index fossils are used to define and identify geologic periods
– They are fossils of species that have a short time range and are common and easy to identify
– Index fossils are valuable for correlating different sediments
– Trace fossils consist of tracks, burrows, coprolites, and feeding marks
– They provide evidence of animal behaviors and can date back earlier than body fossils
Dating and limitations of the fossil record
– Geochronology and relative dating are used to estimate fossil ages
– Radiometric dating is the only method for absolute dating of rocks older than 50 million years
– Fossil record is biased towards organisms with hard parts
– Only a small percentage of life-forms are preserved as fossils
– Transitional fossils provide evidence of evolutionary transitions
