6 The Cambrian Explosion Flashcards
What characteristics do Metazoans share?
1) Multicellular body formed from different kinds of cells
2) Ability to manufacture the protein collagen (cells not constrained by cell walls)
3) Reproductive cycle with gametes produced by meiosis
4) Nervous system composed of neurones (not in sponges)
How has the problem of the origins of metazoans been addressed?
1) New fossil finds
2) Phylogenetic analysis of anatomical and molecular data
3) Molecular clock studies - map fossil occurrence, work out when animals diversified
4) Molecular genetics of animal development
What has been dated to the Late Precambrian 635-542 Mya?
The Ediacaran biota
Trace fossils of triploblastic organisms - excreted organic matter (so had a gut) which fossilised
Egg cases containing embryos
What are Ediacaran biota?
Compressions of these fossils
Up to 1m in size
Have similar characteristics in the way they are preserved and their symmetry
The organisms had no mouth, gut or anus
How did Ediacaran organisms feed?
Took in substance through the body wall - particulate food, dissolved organic matter
Chemosymbiosis (utilise sulphide oxidising bacteria) - deep sea vents
What have Ediacaran organisms been interpreted as?
Simple ancestors of several modern phyla (sponges, jellyfish etc)
Diploblastic animals- show range in variation not seen in living examples
Entirely separate attempt at multicellular life that failed
Geochemical evidence to suggest some are indeed animals
What is the benefit of phosphate rich environments?
Small organisms can be perfectly preserved even if composed of soft tissues that would normally rot very quickly
Some large Acritarchs contain phosphatised embryos
What has been dated to the Early Cambrian 542-530 Mya?
Small shelly fossils
- Are probably disarticulated elements of a skeletal covering
- Yet to evolve into a large discrete cell coving whole organism
- Adaptation against predation
- Probably represent an extinct phylum close to Mollusca or a form of annelid
Why did shells need to evolve?
A predator had evolved
- The others before were unable to adapt so went extinct
What happened in the Middle Cambrian 530-520 Mya?
The Cambrian explosion - fossilised hard parts appear including representatives of all modern phyla
Exceptional preservation as no worms present
Very rarely also find some soft parts preserved in exceptionally preserved biotas
What does the Cambrian explosion represent?
Is it?
1) The appearance of fossilisable parts
- Long Precambrian history of animals but only fossils once they developed armour
2) True evolutionary burst to large size and greatly increased anatomical variety
- Animal diversity exploded rapidly with little subsequent change
How could the environment have caused the Cambrian explosion?
- Large supercontinent that dominated Precambrian starts to break apart
- Two extensive glaciations causing snowball Earth scenarios
- Isotope records - very low C-isotope values in latest Precambrian, coincides with extinction of skeletal fossils from microbial reefs
- Atmospheric O2 levels rose dramatically at Precambrian/Cambrian boundary
How could ecology have caused the Cambrian explosion?
Late Precambrian
- Ediacaran animals inhabit sea floor unprotected but with no predators
Early-Middle Cambrian
- Triploblastic predators with teeth evolve, most Ediacara become extinct, others evolve armour to protect themselves or burrow
Late Cambrian
- Predators become more efficient (eyes), multicellular organisms evolve better armour and exploit more burrowing niches
