1.3 - Body Fossils: Preservation Potential and Origin of Bias in the Fossil Record

Question 1

What is recalcitrance?

Answer 1

The resistance to decay

Question 2

List the soft parts of organisms.

Answer 2

Muscle, internal organs, labile, decay prone tissues

Question 3

List the hard parts of organisms.

Answer 3

Shells, bones, teeth -> biomineralised tissues

Question 4

What is important in soft tissue for preservation potential?

Answer 4

2 forms of carbon - refractories and volatiles

Question 5

What are the two forms of carbon important in soft tissues preservation?

Answer 5

Refractories and volatiles

Question 6

What are refractories?

Answer 6

Higher preservation carbon eg. cuticles

Question 7

What are volatiles?

Answer 7

Lower preservation carbon eg. tentacles

Question 8

What are the 3 processes involved in decay?

Answer 8

Autolysis, microbial degradation, scavenging

Question 9

What is scavenging?

Answer 9

The destruction of tissue by macroscopic organisms

Question 10

What is autolysis?

Answer 10

The self-breakdown of cells (as cells die they use enzymes to self-destroy/split apart - chemical agents of destruction).

Question 11

What is microbial degradation?

Answer 11

Microbes are present everywhere (except sterile environments) and they can recycle tissues after death

Question 12

Are refractories or volatiles rare?

Answer 12

Both are rare as soft tissue preservation is rare in the fossil record - but volatiles are rarer than refractories

Question 13

What makes up the bulk of the fossil record?

Answer 13

Biomineralised tissues

Question 14

What are exceptional biotas?

Answer 14

Rare cases where tissues that are not normally preserved are sampled

Question 15

What are Clam Shrimp? What age are they? Where are they found? What was preserved on the surface? What is it’s biology like?

Answer 15

A type of Crustacean.
Carboniferous.
Found in cores in Castlecomer, Kilkenny.
Thin carbon film preserved on surface.
2 appendages at front and a carapace (shell-like structure) closing the body.

Question 16

What were the results of decaying a clam shrimp in a lab?

Answer 16

Volatile carbon tissues (internal organs and musculature) decay very quickly (8 days) and refractory tissue has a higher preservation potential.
There is relative recalcitrance of different tissues.
There is relative recalcitrance of the same tissues (cuticle not uniform).
There is an impact of disturbance on pre-decayed specimens.

Question 17

Why is the preservation of Clam Shrimp important?

Answer 17

They give us insights we wouldn’t otherwise (as they have soft tissue preserved) have due to the exceptional preservation circumstances.

Question 18

What are the 2 principle preservation pathways for non-biomineralised tissues? Describe each

Answer 18

1) Preserved as organic reamains (typically occurs with very recalcitrant tissues such as arthropod cuticles and graptolite periderms - rarer preservation of labile tissues as they tend to decay quickly).
2) Replicate tissue structure in a mineral (more common method of preservation for labile tissue - often in calcium phosphate and excellent detail - looking for an environment where calcium phosphate precipitates out).

Question 19

What is the phylum for a graptolite and what is its sister phylum?

Answer 19

Hemichordate.

Sister to chordates (us).

Question 20

Give an example of an animal that was preserved through replicating tissue structure in a mineral.

Answer 20

Tardigrade (water bear) - Middle Cambrian 520 mya

Question 21

What are the 3 types of minerals that biomineralised tissues are often made of?

Answer 21

Calcium carbonate, Calcium phosphate, Silicon dioxide (silica)

Question 22

What forms does calcium carbonate come in? Give 2 examples of organisms with these minerals.

Answer 22

Calcite and Aragonite (polymorphs)

Ammonites (aragonite) and coccolithophores (calcite)

Question 23

Why is aragonite different to calcite?

Answer 23

It has less preservation potential than calcite.
It dissolves and turns to calcite.
Has limited stability when it is buried.

Question 24

Where is calcium phosphate found?

Answer 24

vertebrates - bones and teeth

Question 25

What is silicon dioxide found in?

Answer 25

Diatoms (unicellular algae) and phytoliths (secreted inside plant cells)

Question 26

What are the 4 methods of preservation for shells?

Answer 26

Retained, recrystallised, replaced, dissolved

Question 27

What happens when a shell is retained?

Answer 27

the shell is kept and its chemistry is intact (could retain chemical signature of fluids from which minerals precipitated - eg isotope ratios)

Question 28

What happens when a shell is recrystallised?

Answer 28

It changes from one biomineral to another (eg aragonite inverts to calcite during burial or dissolves)

Question 29

What happens when a shell is replaced?

Answer 29

Biomineralised tissues can be replaced by other minerals (eg. pyrite)

Question 30

What happens when a shell dissolves?

Answer 30

The biomineral dissolves which leaves space in the rock where the shell originally was (moulds and casts)

Question 31

When shells are retained, they can be in 3D and 2D forms. What occurs for each of these things to happen?

Answer 31

3D: implies infill (by sediment or mineral) of internal chambers before compaction during burial).

2D: no infill of internal chambers before compaction during burial

Question 32

What is an ammonite? What does the preservation fo its shell show?

Answer 32

A cephalopod
Presevation of aptychi of an ammonite was made of calcite but the shell was made of aragonite which dissolved - shows different preservation potential of tissues

Question 33

What occurs that create a mould?

Answer 33

Skeletal hard parts dissolve resulting in a void in the rock. Internal moulds preserve internal structures while external moulds preserve external structures. Both can occur in the same fossil (called steinkern)

Question 34

What occurs to create a cast?

Answer 34

New material fills in natural moulds forming a replica of the original skeleton.

Question 35

Why is timing important for moulds and casts?

Answer 35

If you dissolve the shell before the rock is lithified, you will lose all trace of it

Question 36

What is the bias in shells preserved?

Answer 36

Aragonite dissolves where calcite is preserved so there is a bias in the types of shells preserved.