Fossils: The physical traces of past life.

Or, more fully, a fossil is any remain of an ancient organism or its behavior preserved in the rock record.

Fossils are the only direct evidence of past life, although indirect evidence exists in the form of the phylogenetic and biogeographic distribution of modern organisms.

The study of the preservation of fossils is called taphonomy.

Two major types of fossils:

• Trace fossils: the record of organisms' behavior preserved in rock.
• Body fossils: the physical remains of an organism preserved in rock.

Trace fossils are, essentially, biologically-generated sedimentary structures. They include: They include:

• Footprints and trackways
• Burrows
• Feeding marks
• Coprolites (fossilized feces)
• Nests
• And more
• Represent activities of the animal while alive, rather than part of the dead creature

Preservation of trace fossils is just like other sedimentary structures: must have rapid burial, and preserved by lithification of the rock itself.

Body fossils: can be preserved in a variety of ways.

In general, only organisms with hard parts can be preserved: shells, bones & teeth, wood, etc.

Main hard part mineralogies/biochemistries:

• Carbonate (calcite & aragonite): found in coccoliths, some multicellular algae, forams, some sponges, corals, brachiopods, bryozoans, mollusks, trilobites & other calcified arthropods, echinoderms, etc.: very easy for organisms to generate from sea-water.
• Silica (normally cryptocrystalline quartz): diatoms, radiolarians, some sponges
• Calcium phosphates (including hydroxylapatite): some brachiopods, vertebrate bones & teeth
• And various complex organics, such as:
  • Cellulose: plants, some algae
  • Chitin: arthropods
  • Spongin: some sponges

But many organisms have NO hard parts, and are only preserved in rare instances.

Most hard parts are not solid material, but porous. Pore space is occupied by organic material in life. Upon death, organic material begins to decay.

Modes of preservation:

• Unaltered: simple burial, some weathering. Becomes rarer (for stochastic reasons) further back in fossil record.
• Permineralized: very common mode.
  • Pore space is filled in with ground water: some dissolved minerals precipitate in pores (probably some contribution by bacterial activity).
  • Common minerals found in permineralized fossils: silica; calcite.
  • Original hard parts remain, but extra material added to pores.
  • "Petrified" wood is actually permineralized wood
  • In some cases, soft tissues can be permineralized, but this seems to be very rare
• Recrystallization: very common in calcitic fossils. After burial, calcite crystals reorder and grow into each other. Original mineralogy remains, but structure is lost.
• Replacement: grades from permineralization.
  • Partial to complete replacement of crystals of one mineralogy with another, controlled by hard part material and by dissolved material in ground water.
    • Bone (hydroxylapatite) can be replaced with uranium-bearing minerals, for instance.
  • Common forms of replacement: silicification; pyritization; phosphatization.
• Carbonization: organic material is "distilled" under pressure. Many volatiles lost: carbon film left behind.
  • Mode of preservation of coal
  • Also preserves soft tissues of some animals (like the feathers of some dinosaurs or the body outline of ichthyosaurs) and plants
  • Bacterially controlled

Different organisms have different potential for fossilization.

• Hard parts vs. no hard parts
• Single hard parts (e.g., gastropods & cephalopods) vs. two hard parts (e.g., brachiopods & bivalves) vs. many well-connected parts (e.g., arthropods & echinoderms) vs. many parts connected only by soft tissue (e.g., vertebrates)
• Microscopic to sediment-sized to immense
• Lived in erosive environments (e.g., mountains) vs. depositional environments
• Lived in accessible vs. inaccessible environments (e.g., continental shelves vs. oceanic basins)

Plants a special case: different organs (leaves, stems, trunks, fruit, flowers, seeds, pollen, etc.) are only very rarely preserved together. Each part generally given its own species name!

Some general ways of describing organisms' life habits:
Trophic Level: where is the organism in the food web?

• Producers: photosynthesizers (use sunlight) or chemosynthesizers (use chemicals)
• Consumers: 1rst order (“herbivores”) vs. 2nd, 3rd, etc. order (“carnivores”)
• Detritivores: suck up broken down organics in sediment
• Filter feeders: suck up whole organisms and broken down organics in water column
• Decayers: break down organic material

Mobility: does it move?

• Sessile: immobile: can be attached or sediment sitter
• Motile: movable

If in water, where in the water/sediment column is it?

• Plantkon: floating on or near surface, does not move far under own power
• Nekton: free swimming in water column
• Benthic: bottom dweller.
  • Epifaunal: lives on water-sediment interface
  • Infaunal: lives in sediment

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Last modified: 10 January 2008