Fossils:

• Physical remains or evidence of behavior of ancient organisms preserved in the rock record
• Only direct data for knowledge of past life

Rocks (naturally occurring cohesive solids comprised of one or more minerals or mineraloids) are generated in one of three primary manners (basis of rock classification):

• Igneous
  • Formed by the cooling of molten material
  • Because initial conditions are hundreds or more degrees C, no fossils will be found
• Metamorphic
  • Formed by transformation of previously existing rocks due to intense heat and/or pressure
  • Any fossils previously existing in the rocks will likely be obliterated by metamorphism
• Sedimentary
  • Formed by accumulation and lithification of bits of previously existing rock and/or organic matter
    • Bits of previous rock and/or organic matter are called sediment
    • Detrital (siliciclastic) sedimentary rocks
      • Sediment is grains of various sizes weathered from previously existing rock
      • The sediment grains are cemented together by minerals in the ground water
    • Carbonate sedimentary rocks: sediment is grains of carbonate minerals, including shells
    • Other types as well (coal, volcanoclastic, evaporates, etc.)
  • Sediment is transported by various agents (water, wind, glacial ice, etc.)
  • Sediments are deposited in the same places (deserts, flood plains, rivers, lakes, swamps, coastlines, continental shelves, etc.) in which animals and plants live and die
  • Sedimentary rocks are where fossils are found!

The different environments of deposition of sedimentary rocks can preserve different sorts of information:

• Quiet water (lagoons, lakes, etc) have very fine grained sediments, preserve small details
• Faster moving water, wind, etc. deposit large amounts of sediment quickly, more likely to bury large objects (such as large dinosaur bodies)
• Environment of deposition often indicated by sedimentary structures:
  • Mud cracks: indicate periods of wetting and drying (lake shores, shore lines, etc.)
  • Ripple marks: indicate moving water (streams, beaches and lake shores, etc.)
  • Raindrop marks: indicate was above water level for at least some time
  • Coal beds: layers of vegetation, indicating wetness and lots of growth (swamps, marshes, etc.)
  • Others as well (desert sand dunes, landslides, etc.)

Fossils are divided into two primary categories:

• Body fossils: part of the original organism (shell, bones and teeth, leaves, twigs, pollen, etc.)
• Trace fossils: physical mark of the behavior of the organism (footprints, skin impressions, feeding marks, burrows, coprolites (fossilized dung), etc.)
  • Eggs are special case: trace fossils of the mother, body fossils (of a sort) of the next generation

For vertebrate animals such as dinosaurs, the body fossils are primarily bones and teeth

Bone:

• A structural unit of vertebrate anatomy: humerus, scapula, metatarsal, etc.
• A composite material: calcium phosphate (hydroxylapatite) grains in a protein (collagen) matrix
• A living tissue: store for various nutrients (calcium, phosphate, etc.), modified and reused throughout life

In order for bones and teeth to become fossilized:

• The animal must die (in the case of bones) or lose teeth
• The body must be buried by sediment before decay, weathering, scavengers, etc. destroy the remains
• The remains must be preserved by one of several modes
  • Unaltered: organic material may or may not have decayed, but original minerals all remain
    • Very rare for Mesozoic and older fossils, commoner in young fossils
  • Permineralized: organic material decays, original minerals remain, pore spaces in bones and teeth filled in over time by minerals in the ground water
    • MOST COMMON mode of preservation for dinosaur fossils
    • Essentially the same process as cementation in detrital sediments
    • Differences in minerals in ground water result in different colored bones and teeth
  • Replacement: organics decayed, original minerals replaced by other minerals
    • Once considered main form of fossilization
    • Very rare for vertebrate fossils

Relatively new discovery: mineralized soft tissues

• Formed by replacement of tissue with calcite or other carbonate mineral
• Highly dependant on the precise conditions inside the carcass
• Seem to have been controlled by bacteria
• In dinosaurs, some skin, muscle tissue, intestines, trachea, and possible heart have been preserved

• How soon was it buried after death? (Sooner means less time for weathering and scavenging, so more complete)
• How rapidly was it buried? (Very rapid deposition can cover bigger objects; slow rate of deposition might preserve small items while larger objects are destroyed)
• How soon was the fossil discovered after exposure? (If fossil found while still mostly in the rock, can recover most of it; if it has been exposed for years, modern weathering will destroy much of it)

Trace fossils:

• Some (footprints and trackways, burrows, skin impressions) are essentially sedimentary structures
  • Preserved in same way as other sed. structures: burial as new layer of sediment is formed
• Others (coprolites, eggs) more similar to body fossils
  • Preserved by same modes as body fossils (unaltered, Permineralized, replaced)
• Represent activities of the animal while alive, rather than part of the dead creature

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