Lecture #15 | Fossils

Fossils

Remains of organisms preserved in sedimentary rocks or amber

• actual body parts or and indication of activity

Fossilization of original matter

Preserved specimen that is intact

Amber

Often includes original material

• Polymeric glass served from the exuding sap or resin of pine trees

• An initial trap for various plants or animals

Alteration of original material

After hard body parts are buried, the structure if affected by percolating water and mineralization so that the original materials are often reformed or replaced

Impregnation

Hard parts are penetrated water, resulting in the disposition of minerals that solidify in all internal cavities (petrified wood)

Impressions

Percolating water may have completely dissolved the remains of a fossil leaving only a detailed impression in the rock where the structure occurred

Relative Dating (Rocks)

How old is a rock or event compared to other rocks

Discovered by James Hutton

• • Father of modern geology

  • Rock cycling

  • Credited with concept of deep time

• Sterno’s law of superposition

• Principle of cross-cuting relationships

Sterno’s Ale of Superposition

in an undisturbed sequence of sedimentary rock layers, the oldest layers are at the bottom and the youngest are on top

Principle of cross-cutting relationships

Younger features cut across older features

Absolute dating

What is the actual age of a rock event

• Uses half-life to determine age

  • time required for half of the parent compound to decay into the daughter compound

• Most rocks used are igneous rocks

• Dates from igneous rocks are useful for bracketing gas of sedimentary rocks

Molecular clocks

Used mutation rates to estimate divergence times

• idea that sequence divergence is proportional to the time since divergence

• Strict clocks” they assume constant rates of change across all branches of the tree

Note: Molecular clocks and rocks do not always agree

Relaxed molecular clock

Constant rate of change cannot be assumed

• mutations known to not occur at a constant rate even for closely related taxa

Estimate divergence time with ought assuming a constant rate

• Impacts on mutation rate

  • generation time (reproduction length)

  • Pop size

  • Evolutionary history

  • Changes in intensity of natural selection

Calibrations

Must calibrate molecular clocks

• without fossil studies, we must rely on other sources: geological events, substitution rates

• Most commonly fossils are used to calibrate the molecular clock

Node calibrations

Fossils used to specify constraints on a specific node (oldest fossil for group gives minimum age)

• Disadvantage: clades are usually older than their oldest fossil- how much is unknown

Tip calibration

• Fossils are treated as taxa and places at tips of the tree

• Allows many fossils to inform dates

• Disadvantage= our understanding of fossil relationships may be tenuous due to convergent evolution of morphologies

Disadvantages/biases to fossils

Not everything that dies becomes a fossil

• things with bodies of hard material are likely to turn

• soft bodied organisms are difficult

Location → needs lots of sediment, be rapidly buried, not tectonically/volcanicaly active areas