Figure 1. The relative rates of decay (vertical axis) and mineralization (horizontal axis) determine the kinds of tissues that may be preserved. At minimum decay rate and with very early mineralization, highly labile muscle tissues may be preserved. When decay has gone to a maximum, and when mineralization occurs late, all that is left are the nonorganic tissues such as shells.

Figure 2. The conditions for exceptional preservation. (a) Rate of burial and organic content are key controls on the nature of mineralization of organic matter in fossils. Pyritization (high rate of burial; low organic content) may preserve entirely soft-bodied worms, as in an example of the starfish Loriolaster (b) from the Early Devonian Hunsrückschiefer of Germany. Phosphatization (low rate of burial; high organic content) may preserve feathers as in (c), an unidentified bird from the Eocene of Germany. Soft parts may be preserved in carbonate (high rate of burial; high organic content), such as the limbs and antennae of a shrimp (d), from the Carboniferous of Scotland. If decay never starts, small animals may be preserved organically and without loss of material, as in a centipede in amber from the Early Tertiary of the Baltic region (e).

Figure 3. Processes of breakage and diagenesis of fossils. Dead organisms may be disarticulated (a) or fragmented (b) by scavenging or transport, abraded (c) by physical movement, bioeroded (d) by borers, or corroded and dissolved (e) by solutions in the sediment. After burial, specimens may be flattened (f) by the weight of sediment above, or various forms of chemical diagenesis, such as the replacement of aragonite by calcite (g) may take place.