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, 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 water, wind, glacial ice, etc.
  • Sediments are deposited in deserts, flood plains, rivers, lakes, swamps, coastlines, continental shelves, etc.: depositional environments
  • Sedimentary rocks form where animals and plants lived and died

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

• Quiet water (lagoons, lakes, etc): 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
  • Ripple marks
  • Raindrop marks
  • Coal beds
  • Others as well (desert sand dunes, landslides, etc.)

Fossils are divided into two primary categories:

• Body fossils: part of the original organism
• Trace fossils: physical mark of the behavior of the organism

For vertebrates (such as dinosaurs), 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:

• Animal must die (in the case of bones) or lose teeth
• Body must be buried by sediment before decay, weathering, scavengers, etc. destroy the remains
• Remains must be preserved by one of several modes
  • Unaltered
  • 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

Relatively new discovery: mineralized soft tissues

Trace fossils:

• Some (footprints and trackways, burrows, skin impressions) are essentially sedimentary structures
• Others (coprolites, eggs) more similar to body fossils
• Represent activities of the animal while alive, rather than part of the dead creature

Dinosaur fossils have been weathering out of the rock since LONG before humans evolved, yet "Dinosauria" was not recognized until 1842. Why did it take this long?

Before scientists could recognize the existence of dinosaurs, they had to recognize that fossils were the remains of dead (not alive), unknown, extinct (not living anywhere) organisms.

Baron Georges Cuvier (France) "father of comparative anatomy" examined many fossils in the late 1700s/early 1800s:

• Recognized fossils were remains of unknown organisms
• Explained them being unknown because they were extinct organisms

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Last modified: 14 July 2006