1.3 - Body Fossils: Preservation Potential and Origin of Bias in the Fossil Record Flashcards
What is recalcitrance?
The resistance to decay
List the soft parts of organisms.
Muscle, internal organs, labile, decay prone tissues
List the hard parts of organisms.
Shells, bones, teeth -> biomineralised tissues
What is important in soft tissue for preservation potential?
2 forms of carbon - refractories and volatiles
What are the two forms of carbon important in soft tissues preservation?
Refractories and volatiles
What are refractories?
Higher preservation carbon eg. cuticles
What are volatiles?
Lower preservation carbon eg. tentacles
What are the 3 processes involved in decay?
Autolysis, microbial degradation, scavenging
What is scavenging?
The destruction of tissue by macroscopic organisms
What is autolysis?
The self-breakdown of cells (as cells die they use enzymes to self-destroy/split apart - chemical agents of destruction).
What is microbial degradation?
Microbes are present everywhere (except sterile environments) and they can recycle tissues after death
Are refractories or volatiles rare?
Both are rare as soft tissue preservation is rare in the fossil record - but volatiles are rarer than refractories
What makes up the bulk of the fossil record?
Biomineralised tissues
What are exceptional biotas?
Rare cases where tissues that are not normally preserved are sampled
What are Clam Shrimp? What age are they? Where are they found? What was preserved on the surface? What is it’s biology like?
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.
What were the results of decaying a clam shrimp in a lab?
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.
Why is the preservation of Clam Shrimp important?
They give us insights we wouldn’t otherwise (as they have soft tissue preserved) have due to the exceptional preservation circumstances.
What are the 2 principle preservation pathways for non-biomineralised tissues? Describe each
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).
What is the phylum for a graptolite and what is its sister phylum?
Hemichordate.
Sister to chordates (us).
Give an example of an animal that was preserved through replicating tissue structure in a mineral.
Tardigrade (water bear) - Middle Cambrian 520 mya
What are the 3 types of minerals that biomineralised tissues are often made of?
Calcium carbonate, Calcium phosphate, Silicon dioxide (silica)
What forms does calcium carbonate come in? Give 2 examples of organisms with these minerals.
Calcite and Aragonite (polymorphs)
Ammonites (aragonite) and coccolithophores (calcite)
Why is aragonite different to calcite?
It has less preservation potential than calcite.
It dissolves and turns to calcite.
Has limited stability when it is buried.
Where is calcium phosphate found?
vertebrates - bones and teeth
What is silicon dioxide found in?
Diatoms (unicellular algae) and phytoliths (secreted inside plant cells)
What are the 4 methods of preservation for shells?
Retained, recrystallised, replaced, dissolved
What happens when a shell is retained?
the shell is kept and its chemistry is intact (could retain chemical signature of fluids from which minerals precipitated - eg isotope ratios)
What happens when a shell is recrystallised?
It changes from one biomineral to another (eg aragonite inverts to calcite during burial or dissolves)
What happens when a shell is replaced?
Biomineralised tissues can be replaced by other minerals (eg. pyrite)
What happens when a shell dissolves?
The biomineral dissolves which leaves space in the rock where the shell originally was (moulds and casts)
When shells are retained, they can be in 3D and 2D forms. What occurs for each of these things to happen?
3D: implies infill (by sediment or mineral) of internal chambers before compaction during burial).
2D: no infill of internal chambers before compaction during burial
What is an ammonite? What does the preservation fo its shell show?
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
What occurs that create a mould?
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)
What occurs to create a cast?
New material fills in natural moulds forming a replica of the original skeleton.
Why is timing important for moulds and casts?
If you dissolve the shell before the rock is lithified, you will lose all trace of it
What is the bias in shells preserved?
Aragonite dissolves where calcite is preserved so there is a bias in the types of shells preserved.
