Part D: FossilsFossils are the remains or traces of prehistoric life. Why are they important to science today?-Reveal information about past environments (depositional environments): water salinity, temperature, turbidity, depth, sediment type and can be used for dating purposes.

Fossils are grouped into two categories:a) Body fossils: Evidence of tissue (hard or soft)b) Trace fossils: Evidence of an organism’s activities, but not the organism itself.

Have all organisms that lived on Earth been fossilized? Why or why not?-No, fossilization requires:a) Hard body parts: Physically and chemically durable. Soft body parts often decompose (only preserved with rapid burial and low O 2 levels). b) Quick burial: Preserve organic remains from scavengers/chemical breakdown.

Soft Body Part Preservation Methods*All of the methods below involve quick burial and low O2 levels which isolate the remains from scavengers and microbes (decomposers).1. Unaltered Soft Body Part Preservation-Very rare, only occurs in amber, peat bogs, tar pits, permafrost, deserts.

2. Altered Soft Body Part Preservationa) Carbonization:- Tissue preserved as a carbon film after the loss of hydrogen, oxygen and nitrogen and deposition of clay/mica minerals on the tissue surfaces.-Burgess Shale fossils.

b) Imprints: Impression of organism in sediments. Typical of Pre- Cambrian organisms, such as the Jellyfish to the left.

Hard Body Part Preservation Methods:

Geology 12: Sedimentary Rock Page 1

1. Unaltered Hard Body Part Preservation- Hard body parts made up of very stable materials can survive unchanged, such as shells and sharks’ teeth.

2. Altered Hard Body Part Preservationa) Permineralization (Petrifaction):-The pores in hard parts are filled by minerals that precipitate from surrounding water (which is found in the pore space).-Very common in porous material, such as wood or bone.

b) Replacement:-Substitution of original material by another substance atom by atom. -The original structure is accurately preserved.-For example, calcium carbonate shell replaced by pyrite or silica.

c) External Mold:

Geology 12: Sedimentary Rock Page 2

-The sediments surrounding an organism take on its external shape.-For example, a periwinkle shell in soft sand.-Negative relief

d) Internal Mold:-The sediments take on the internal shape of an organism.-For example, the soft sand manages to fill the shell’s empty interior.

e) Cast:-The shell dissolves away and leaves an empty space, an external mold and an internal mold.-Empty space is filled with minerals = a replica of the exterior of the original shell. -Positive relief.

Geology 12: Sedimentary Rock Page 3