Biology 1001A Chapter Notes - Chapter 20: Gastrolith, Stromatolite, Cyanobacteria
29 Apr 2018
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20.1 The Geological Time Scale
Sedimentation and Geological Strata
• Many fossils are found in sedimentary rock
• Sediments found in any one place form distinctive strata (layers)
• If undisturbed, strata are arranged in order in which they are formed, youngest at
the top - thus useful in dating fossils
• Fossils in a particular sedimentary stratum represent organisms that lived and died
at roughly same time
• Use sequence of strata to establish geologic time scale and history of life
20.2 The Fossil Record
20.2a How Fossils Were Formed
• Fossil skeletons - dissolved minerals enter spaces within bones and then solidified
• Amber - preserve organisms in tree resin and show fine details
• Moulds or impressions - show shape of organism (ex. Impression of fern preserved
in coal)
• Footprint - fossilized when animal walks across mud
• Coprolites - fossilized droppings
• Gastroliths - stones in stomach used to grind food (ex. Found in birds)
• Frozen or mummified humans
• Petrified forests - trees fossilized by minerals
• Fossil record is incomplete
o Few organisms fossilize completely
o Some organisms are more likely to fossilize than others
o Natural processes destroy many fossils
X-Ray Tomography and 3D Structure of Fossils
• Tomography uses thin sections cut from fossil to reconstruct an organism's
appearance and observe in fine details
• Applying tomography to X-ray views allows reconstruction of fine details of
structures (ex. Surfaces of eggs or nuclei)
20.2b Early Fossils
• Stromatolite: laminated, usually mounded sedimentary fossil formed from layers
of cyanobacteria, CaCO3 and trapped sediment
• Stromatolites are the first fossil evidence of life
• Prokaryotes dated 3.5 bya
• Unicellular eukaryotes dated 2 bya
• Multicellular eukaryotes dated 1.2 bya
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Document Summary
20. 1 the geological time scale: many fossils are found in sedimentary rock, sediments found in any one place form distinctive strata (layers) 20. 2 the fossil record: fossil skeletons - dissolved minerals enter spaces within bones and then solidified, amber - preserve organisms in tree resin and show fine details, moulds or impressions - show shape of organism (ex. Impression of fern preserved in coal: footprint - fossilized when animal walks across mud, coprolites - fossilized droppings, gastroliths - stones in stomach used to grind food (ex. Found in birds: frozen or mummified humans, petrified forests - trees fossilized by minerals, fossil record is incomplete, few organisms fossilize completely, some organisms are more likely to fossilize than others, natural processes destroy many fossils. X-ray tomography and 3d structure of fossils: tomography uses thin sections cut from fossil to reconstruct an organism"s appearance and observe in fine details, applying tomography to x-ray views allows reconstruction of fine details of structures (ex.
