- Fossils containing original material:
- Unaltered remains. Example: frozen mammoths
- Compressions: Flattened and dehydrated, but unaltered
otherwise, with cellular details often preserved.
- Replacement fossils (with original material largely replaced):
- Permineralization:
Gradual addition of minerals by ground water, preserving
many internal details. The organic material remains, but minerals are added.
Many bones are preserved in this manner.
- Petrifaction: A common type of permineralization in which
the added mineral is either silica (SiO2) or calcite (CaCO3)
- Impregnation or embedding:
Similar to permineralization, except that the embedding material
surrounds the fossil as well as filling in any interstitial spaces.
- Carbonization: Volatile compounds lost, leaving carbonized
skeleton only.
- Mineralization: Complete replacement of original material
by minerals.
- Casts and molds: Impressions in fine-grained sediments,
preserving only surface shapes.
- Casts are solid objects. Example: endocasts of brains
- Molds are hollow, "negative" surfaces. Example: Impressions of dinosaur skin
- Trace fossils: Tracks, trails, and
other traces of activity. Examples:
- Organic material derived from biological activity:
- Amber (fossil tree sap or resin)
- Coprolites (fossil dung)
- Inorganic material indicating the "work" of an organism:
- Tracks and trails of worms, etc.
- Footprints (e.g., of dinosaurs or early humans) in fine-grained sediments
- Burrows and tubes (if filled with sediment different from surrounding material)
- Castings: sand or other sediment, often with some organic matter added,
that has passed through the body of a detritus feeder and been discarded.
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