A SUMMARY OF THE FOSSIL RECORD
THE ORIGIN OF LIFE
Origin of Earth 4600mya
First 1100my (ie 2x length of Cambrian)
Hostile environment: - crust very mobile + subject to meteor bombardment
- entire atmosphere stripped away + slowly restored
Early atmosphere rich in: Carbon Dioxide, Carbon
Ammonia,
Hydrogen, Nitrogen
No protection from ultraviolet radiation of sun
Monoxide,
Methane,
Lightning discharges (ie strong electric charges) acted on primordial atmosphere
Created complex molecules - long carbon atom chains
- = compounds upon which the chemistry of life
depends
- Amino acids; raw materials of proteins
- unstable in today's atmosphere
But ancient atmosphere had no free oxygen
so molecules could survive
Hot dry conditions nreeded to allow amino acids to form proteins
Such conditions found near volcanoes - also site of torrential rain + electric
discharges
Primitive proteins quenched with water formed primitive cells
DNA probably formed in similar haphazard fashion
First life - primitive bacteria @3500mya
PRECAMBRIAN FOSSILS
1
First record of PROCARYOTES @ 3500mya
2
First record of UNICELLULAR EUCARYOTES @ 1400mya
3
First record of METAZOANS @ 630mya
PROCARYOTES
Unicellular - no organised nucleus or organelles
1
CYANOBACTERIA ('BLUE-GREEN ALGAE')
eg STROMATOLITES
- bun shaped mounds few cm across
- laminated structure
- algal mats trap calcareous mud
(+?precipitate)
- modern in warm, intertidal, shallow water
- 3500mya W Australia Warrawoona Group
(chert)
- common by 2500mya
First Cyanobacteria probably anaerobic
Evolution of PHOTOSYNTHESIS - oxygen releasing cyanobacteria
- Oxygen very reactive; initially immeadiately combined with other elements eg
Iron
- Recorded in pre-Cambrian BANDED IRONSTONES @3200mya
-Later led to build up of oxygen in atmosphere
- essential in evolution of Metazoa
- Solar ultraviolet radiation changed O to O = OZONE
- provided protective screen against solar radiation
2
BACTERIA
Diverse, cells occurring singly or as aggregates
Many mobile - flagella
Some AUTOTROPHS - synthesise organic constituents from simple inorganic
materials
Some HETEROTROPHS - live on organic matter, alive or dead
Abundant and ubiquitous on land, in fresh water and in the sea
Bacterial activity in sediments commonly leads to chemical + mineralogical
changes
eg
1 Organic matter decomposed by bacteria produces methane
2 Ferric hydroxide reduced to ferrous form
Sulphate reduced to hydrogen sulphide
3 React to form ferrous sulphide - eventually pyrite
also Calcium Sulphate to Calcium Carbonate
Calcium Sulphate to Sulphur
Direct precipitation of Ferric Hydroxide ('Bog Iron Ore' formed this way)
Fossils extremely rare
- W Australia Warrawoona Group 3500mya
- Canada Gunflint Chert 200mya
EUCARYOTES
Plant-like 'algae' AUTOTROPHS (Diatoms, Seaweeds) - PHOTOSYNTHESISE
FOOD
Animal-like protozoans HETEROTROPHS (Foraminifera) - Feed on other
organisms
Microscopic, one-celled (except seaweeds)
Aquatic
Skeleton organic, calcareous or siliceous
Common as fossils
nb Foraminifera very
palaeoenvironment
useful
in
oil
industry
-
zoning,
correlation,
METAZOA
No body fossils before 630mya - just trace fossils
Oldest metazoans (MULTI-CELLED) 630mya:
- Impressions of soft-bodied animals EDIACARAN FAUNA
- Exceptional preservation; rare preservation of ?
widespread fauna
- Late Pre-Cambrian: S Australia, N Russia
- Over 30 forms recognised
- ? Jellyfish (medusoids), sea-pens, annelid worms
CAMBRIAN FOSSILS
CAMBRIAN RADIATION: sigmoid curve of species plotted against time
(fig2.3 Palaeontology, an introduction Nield + Tucker
- logarithmic increase in numbers
- signifies sudden rise in preservation potential rather than in life itself
combined with explosive radiation of organisms with hard parts
- Cambrian to present = PHANEROZOIC 'Evident life'
- Invertebrate body fossils first known in Lower Cambrian 630mya
- Clearly differentiated - must have existed before 630mya ? just not
preserved
- Mostly BENTHIC
1st stage Lower Cambrian: Sponges, Gastropods, Inarticulate Brachiopods
Also marine 'plants' - calcareous algae
2nd/3rd stages:
Trilobites, Echinoderms, Articulate Brachiopods,
Bivalves
Mid Cambrian:
Dendroid Graptolites
Upper Cambrian:
Nautiloids, vertebrates
Lower Ordovician:
Tabulate Corals
* During 80 million year period all major groups of skeletal invertebrates
appeared + diversified
* Radiation of organisms after appearance of Ediacaran metazoans until upper
Cambrian appearance of verebrates = @ 150my: 1/4 of Phanerozoic time
* Development of rigid skeletons = great advance in invertebrate body plan
1
Protection
2
Base for muscle attachment
3
Improved locomotion
4
Improved feeding
5
Adaptation to variety of habitats
* Multiple ADAPTIVE RADIATIONS within all skeletal invertebrate groups
Led to displacement of soft-bodied forms - could not compete