EXTENSION 2: THE FOSSIL RECORD
FOSSIL
evidence of past life preserved in rock
PALAEONTOLOGY
the study of ancient life through its fossil remains or the
traces of its activity as recorded by ancient sediments.
ORGANIC MATERIAL LIKELY TO BE FOSSILIZED AS BODY FOSSILS
Most fossils = preserved hard parts of organism = body fossils
Vertebrates
Invertebrates
Bone
COLLAGEN (scleroprotein)
hardened by CALCIUM PHOSPHATE
Teeth
Denser structure than bone
hardened by calcium phosphate
ENAMEL - very hard
= almost pure CALCIUM PHOSPHATE
Horn/Claw
KERATIN (scleroprotein)
Shell
ARAGONITE- most molluscs
CALCITE
- echinoids + crinoids
Exoskeleton CHITIN (Complex Polysaccharide C, N, H, O atoms
joined in chains to form long molecules
- trilobites
KERATIN + COLLAGEN (scleroproteins)
- graptolites
Spicular skeletons SILICA - sponges
Plants
CELLULOSE - fibrous polysaccharide forming cell walls
LIGNIN
- complex polymer binding cellulose fibres
SPOROPOLLENIN - coats spores + pollen; very resistant
Plant fossils - IMPRESSIONS : contains no actual plant material
COMPRESSIONS: contain original organic matter C
PETRIFICATIONS: impregnation with mineral salts
eg MgCO or CaCO , FeS, SiO
FOSSILIZATION PROCESSES
HARD PARTS
CARBONISATION - scleroprotein, chitin, cellulose, lignin
- relative carbon content of organic materials increased by
liberation of volatiles
- outline + occasionally details of soft parts preserved as
carbon residue
PETRIFICATION
IMPREGNATION
REPLACEMENT
- 'turning to stone' by impregnation or replacement
- infilling of gaps left in a bone or shell after organic decay.
- substitution of different mineral for original mineral of
hard parts; may destroy internal structure.
Calcification
- impregnation of calcareous shells by calcite
- replacement of aragonite by calcite destroys internal structure
Silicification
- silica replaces calcite, chitin, wood
amorphous silica (opal) may preserve micro-structure
Pyritisation
- replace calcareous shells, graptolites
tend to oxidise + disintegrate once exposed to air
MOULDS
1 Decay of soft parts leaves hollow = INTERNAL MOULD
2 Sediment infills shell interior
(if not, = void- crushed during compaction
- filled by calcite - GEOPETAL structure
3 Sediment surrounding shell consolidated - takes impression of
external surface = EXTERNAL MOULD
CASTS
- Cavity between internal + external mould filled
- Commonly a coarse mosaic of crystals
nb Handout sheet exercise.
ACTUAL REMAINS - preservation of soft parts
Special conditions needed for preservation of complete organism (ie including soft parts):
a) ICE
- woolly mammoth + woolly rhinoceros preserved in Siberia
+ Alaska 45,000 (edible meat!)
b) AMBER
- fossilized tree resin; Mesozoic + Cainozoic insects
c) MUMMIFICATION- dehydration in hot dry climates eg sloths in New Mexico
preservation only while conditions persist
d) TAR
- residue left by evaporation of oil seeps
when covered with water, attracts animals to drink
animals get stuck eg Pleistocene mammals in California
e) PEAT
- anaerobic, antiseptic conditions
soft tissues preserved by rapid tanning by humic acids
bones decalcified by acid water - soft + flexible
eg Irish Elk, Pete Marsh (Staffordshire), Tollund Man
f) EXCEPTIONAL ROCKS - Jurassic Solnhofen Limestone: Archaeopteryx
- Cambrian Burgess Shale: Trilobites + soft bodied
TRACE FOSSILS
- often the only evidence of soft-bodied organisms
- interpretation largely based on studies of modern organisms
Locomotion
Resting + dwelling
Feeding
- footprints + crawling trails
- burrows + borings
- tooth marks, grazing furrows, gastroliths, coprolites, faecal pellets
THE BIASED AND INCOMPLETE NATURE OF THE FOSSIL RECORD
NATURE OF ORGANISM
- Fossilization favours organisms with hard parts
- fossilization favours small abundant organisms with
few components in skeleton/exoskeleton.
RATE OF DEPOSITION
- Rapid deposition favours fossilization
- less chance of predation/scavengers + abrasion
TYPE OF SEDIMENT
- Limestones, Clays, Shales; ie fine grained
nb Nodules; good preservation,uncrushed
ENVIRONMENT
- Shallow marine best - abundant organisms + best
chance of rapid, permanent burial of BENTHOS
- also good; Estuarine, Lacustrine, Fluvial
- Land fossils rare; Flash floods, bogs, tar pits
BIOLOGICAL ATTACK
- Crushing during eating (nb angular fragments)
- Boring organisms
- Anaerobic conditions reduce chance of predation,
scavenging + bacterial decay
PHYSICAL ATTACK
- Least likely in low energy environments
- Transport leads to preferred orientation
- Abrasion during transport leads to rounding
- Transport leads to fragmentation
dense exoskeletons more resistant
eg thick shelled gastropods
SEDIMENTARY COMPACTION- thin shells crushed flat or distorted
- seen most in muds + clays
CHEMICAL ATTACK
- Aragonite/Calcite destroyed by dissolution
very limited in shallow water because saturated
Important in deep water; Hi pressure/temperature
leads to increased dissolution
No calcium carbonate deposited below CCD
3000m - 5000m, depending on temperature
- In sandstone, circulating pore water commonly
removes calcite in solution, leaving moulds
METAMORPHISM
- Fossils distorted + eventually destroyed by
metamorphism
- Amount of distortion can be used to calculate
amount of strain rocks have suffered
WEATHERING + EROSION
- Many fossils destroyed
FOSSIL ASSEMBLAGES
LIFE ASSEMBLAGES
- an assemblage of fossils preserved in life position by
catastrophic burial eg storm, turbidity current,
or death due to: upwelling of anoxic water,
'Red tides'; algal blooms produce toxins
- allow accurate determination of palaeoecology
- little time for physical damage/disarticulation
DEATH ASSEMBLAGES - an assemblage of fossils formed because they were brought
together after death by sedimentary processes rather than
because they had all shared the same habitat during life
eg shell bank
PRE-FOSSILIZATION
- partial burial followed by exhumation + transport
eg bones:
pre-fossilization involves filling of pore spaces with
phosphate: very resistant to abrasion if exhumed
BONE BED results during periods of reduced deposition
eg Rhaetic bone bed between triassic + Jurassic
DERIVED FOSSILS
- Fossils found in rocks younger than the time of fossilization
- Broken, worn, size-sorted, aligned, mix of environments,
mix of ages.
- 'Recycled fossils'
FOSSIL CLASSIFICATION
Hierarchical system of classification:
SUPERKINGDOM, KINGDOM, PHYLUM, CLASS, ORDER, GENUS, SPECIES
SUPERKINGDOM
Procaryotes - Primitive cells with no nucleus + DNA
dispersed in protoplasm.
Eucaryotes - DNA in strands (chromosomes)
inside a membrane bounded nucleus
KINGDOM
Monera
processes
Protocista
Plantae
Animalia
-
Bacteria important in geological
but
rare
as
fossils
- Algae + foraminifera, abundant as
fossils
- Plants
- Animals - multi celled
PHYLUM
KINGDOM PLANTAE
Bryophyta
Tracheophyta
- Mosses
- Vascular plants
KINGDOM ANIMALIA
Porifera
Cnidaria
Brachiopoda
Mollusca
- Sponges
- Corals + Jellyfish
- Brachiopods
- Ammonites, Belemnites,
Bivalves, Gastropods
- Trilobites, Crustaceans, etc
- Echinoids, Crinoids, Starfish
- Graptolites
- Vertebrates: Mammals, Birds,
Reptiles, Amphibians, Fish
Arthropoda
Echinodermata
Hemichordata
Chordata
BINOMIAL system: each organism has a name consisting of 2 parts
SPECIFIC name
denotes the species (group of similar organisms that can breed - not
possible to be sure of this with fossils - take species to be group of
organisms with only very minor differences)
GENERIC name
denotes the genus to which the species belongs (a number of species
with similar features + which are closely related)
TYPE SPECIMEN
When a fossil is first named a specimen which is a good example
of the species is chosen and ideally stored in a museum for
reference.