1.2 - What Are Fossils?

Question 1

What are the 3 uses of the fossil record?

Answer 1

1) Only direct record of what lived where and when
2) Imperfect (has biases) but can address key questions about the past
3) Can somewhat factor in or accommodate the imperfections if we understand the processes of fossilisation and what causes bias in the fossil record

Question 2

What are the 3 types of fossils?

Answer 2

Trace fossils, Chemical Fossils, Body Fossils

Question 3

What is a trace fossil?

Answer 3

Typically no remains of the producer are present.
Seen as reworking of lithic (rock), xylic (wood) or unlithified (soft sediment) substrates by an organism.
Eg. footprints in a substrate but no person

Question 4

What is a chemical fossil?

Answer 4

Chemicals found in rocks that provide and organic signature for ancient life.

Question 5

What are molecular fossils (biomarkers)?

Answer 5

Products of cellular biosynthesis (different markers for different groups).
Original organic compounds or molecules derived from them that are stable over geological time.
Chemical fossils

Question 6

What are isotope ratios?

Answer 6

Result from metabolic processes that preferentially utilise one form of isotope over another.
Retained in the chemistry of preserved tissues.
Chemical Fossils

Question 7

Name 5 types of chemical fossils.

Answer 7

Sterols, Hopanes, Sponge biomarkers, Dicksonia, Beltanelliformis

Question 8

What are sterols?

Answer 8

Membranes of animal, plant and fungal cells (absent in prokaryotes - hopanoids)

Question 9

What are hopanes?

Answer 9

Molecular fossils derived from hopanoids

Question 10

What are steranes?

Answer 10

Molecular fossils derived from sterols

Question 11

What is the sterane:hopane ratio used to indicate?

Answer 11

Relative contributions of eukaryotic (algae and higher plants) vs prokaryotic (bacteria) organisms to the source rock
Could look at timing of evolution of eukaryotic cells based on change in the ratio

Question 12

What are the 2 sponge biomarkers, what are they chemically, what are they diagnostic of and why are the significant?

Answer 12

24-isopropylcholestane and 26-methylstigmastane
Fossil Lipids
Considered diagnostic of demosponges
Potentially significant as body fossil evidence for demosponges only in much younger rocks (biomarkers suggest earlier evolution than physical biological remains)

Question 13

What is a Dicksonia? Describe its characteristics and how it is analysed.

Answer 13

A chemical fossil.
Member of Ediacaran biota (570-541mya).
Affinities unknown/controversial.
White Sea examples preserved as organic films and geochemistry of films analysed.
Abundant cholesteroids.
Chemistry of organic residues suggests signature of animal cells (eg. cholesterol).
Can be 1m long and few mm thick and normally tissues are not preserved.

Question 14

What are Beltanelliformis? Describe its characterisitcs and what it suggests.

Answer 14

Organically preserved specimens (looks like disks).
Molecular fossils hopanes and steranes.
Different profile from adjacent macroalgal film.
geochemistry and morphology suggests large spherical colonies of cyanobacteria.
Previous interpretations had included them as resting traces of cnidarians (sea anemones - animals) so geochemistry is arguing against animal interpretation.

Question 15

What are Body Fossils?

Answer 15

Any physical remains of an organism.

Question 16

Is the body fossil record bias? How? What is the significance of this?

Answer 16

Yes, as some tissues are preserved while others are not. It reduces the fidelity of the fossil record. Some organisms lack biomineralised tissue so they have minimal/no chance of preservation under normal circumstances.

Question 17

What survives in the fossil record and what does not?

Answer 17

Survives: biomineralised tissues (bones, teeth, shells)
Does not survive or rarely survives: non-biomineralised tissues as they are less recalcitrant (less resistant to decay)
Non-biomineralised tissues vary in their recalcitrance