3 - Exceptional preservation Flashcards
what is a fossil-Lagerstatten
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation
What are the two types of fossil-lagerstatten
1) concentration deposits (konzentrat-lagerstatten)where fossils occur in unusual concentrations i.e. bone bed
2) Conservation deposits (exceptional preservation) where preservation is of unusual quality
What are the 3 main types of exceptional preservation
I) preservation of organisms that are not usually preserved (i.e. embryos from pre-Cambrian preserved in phosphate rich sea)
2) Preservation of parts of organisms that are not usually preserved (icythosaur fossil found with skin preserved -> depositied in stratified sea with anoxic conditions in deep)
3) Organisms are preserved unusually articulated or in unusual configurations (ichthyosaur found fully articulated)
in terms of fossils what does articulation mean
An articulated fossil shows the bones of an animal together, connected in the rock as in life.
what permits exceptional preservation
major controls are:
1) exclusion of scavengers and bioturbators ( anoxia , rapid burial, elevated salinity) - nature normally recycles
2) Unusual chemical environment permits exceptional preservation -> often from catastrophic event such as rapid burial or deposition in an unusual chemical environment
how do microbial mats help preservation
form a “death mask” stabilising sediment and promoting early mineralisation
Give some examples of conservation traps
1) Amber fossils -> resin catches insects and then becomes fossilised
2) Ice -> cold prevents bacterial decay, I,e. preserved mammoth
3) Tar pits -> naturally formed oil from organic material of rocks, if organism falls in is preserved
4) Hot silica springs - super heated water with silicone in -> precipitate out fossils
5) Tuja -> limestone formed when carbonate minerals precipitate out of ambient temp water
6) Ash falls - pyroclastic flow consisting of hot mixture of gas and ash
How does tissue resistance to decay vary
1) Biomineralised endo/exoskeleton (v resistant)
2) Robust structural tissue (arthropod cuticle/woody tissue etc)
3) decay prone muscle etc
Decay prone tissues are not usually preserved, what conditions are required for preservation
Decay prone tissues are only preserved when they are inundated very rapidly by authigenic minerals which either:
1) precipitate around the tissue creating a replicate mold/cast
2) Precipitate on and within the tissues (can infill/ preserve actual cells e.g. tiny crystals can replace muscle tissue)
authigenic materials
Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere (allogenic) by water or wind
Examples of 3 authigenic materials and their preservation
1) Apatite - highest level of preservation, low ph
- excludes oxygen
- requires sources of phosphate
2) clay minerals - high PH, thought to happen in cold environments
3) Iron Pyrite - worst preservation of the 3, precipitates around structure then fills them in
How do fossil lagerstatten confuse us
1) they create false biodiversity peaks in areas with good preservation
2) Preferentially occur during certain periods of geological time (e.g. Cambrian)
3) Certain unusual environments are over-represented (inland lakes e.g.)
4) only show 1/3 of what existed
Cenozoic of lagerstatten
1) Messel (cenozoic , germany) - a lake believed to be volcanic in a cold era, occasionally lake produced loads of CO2 killing all the organisms.
because the lake was volcanic it had anoxic bottom waters (no scavengers) - v good for preservation with even some colour patterns of insects being preserved
Devonian example of lagerstatten
Hunsruch Slate (Devonian, Germany) -Shallow Ocean , anoxic bottom waters in stratified layers -> upper layers had trilobites, starfish etc.
- when they die they fall to bottom and become covered in pyrite (black mud)
- internal organs preserved
Case study: exceptional preservation in dinosaurs
- the Delol Biota (cretaceous, china)
- feathers preserved
- series of lakes in a volcanically active area of china
- series of layers of ash -> unusual preservation event
- plants + insects washed into water column
- fish preserved with all internal organs
- fossils found mid-action
- Pterosaurs -> flying reptiles, found with fur on them
- integumentary structures different to feathers
- shows they evolved differently to birds (diff flight structures)
- lower cretaceous, feather dinos prior to bird lineages
