Lecture 2: Origin of complexity
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Lecture 2: Origin of complexity. 1. Eukaryotes What are they? When do they evolve? When do they diversify? Evolution and the atmosphere 2. Metazoans What are they? Phylogenetic tree Evolution and preservation 3. The Ediacaran fauna 4. The appearance of hard parts

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Lecture 2: Origin of complexity

1. Eukaryotes

What are they?

When do they evolve?

When do they diversify?

Evolution and the atmosphere

2. Metazoans

What are they?

Phylogenetic tree

Evolution and preservation

3. The Ediacaran fauna

4. The appearance of hard parts

5. Events at the base of the Cambrian


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1. Eukaryotes

Mitochondria

Plastid

(if plant)

Nucleus

Eukaryotes

Large (about 100 mm),

complicated

contain internal

organelles

Obligate oxygen

consumers

Prokaryotes

Small (about 10 mm),

simple,

lack organelles

May use oxygen


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1.1 Evolution of eukaryotes

Probably by endosymbiosis


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1.2 When do eukaryotes evolve?

DNA evidence around 3.8 Ba

Oldest fossil 2.1 Ba

Are both dates correct?

1.3 When do they radiate?

Fossils common after 1 Ba

Fauna dominated by acritarchs

Diversity peak at 700 Ma, then decline

1.4 Relationship to environment

Eukaryotes obligate aerobes

Free oxygen in atmosphere and ozone shield after 2 Ba

?Critical O2 level passed sometime in late Proterozoic?


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2.1 Metazoan origins

Metazoans are multicelled animals


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2.2 Metazoan evolution and fossilisation

Molecular clocks record origin

between 8-1800Ma

Fossil evidence suggests origin in late

Precambrian, around 600 Ma

burrows

Ediacaran fauna

Cambrian provinciality


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3. The Ediacaran fauna

Most about 550 Ma, global distribution,

phylogeny disputed.

Cyclomedusa – a probable jellyfish with a pelagic mode of life.

Spriggina –

Probably an ancestral arthropod, with a rudimentary head.

Tribrachidium –

A problematic organism with three-fold symmetry not seen in modern animals.

Dickinsonia –

Depending on your perspective, this is a worm, a soft coral or a completely extinct representative of a group of quilt-like animals, the Vendozoa.


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4. Appearance of hard parts

Happened at or near 543 Ma

Defines the Precambrian/Phanerozoic boundary

Increases preservation potential many fold,

and hence quality of fossil record.

Diversity appears to increase here -

is this real or an artefact?

Skeletons are useful for

- increasing potential size

- increasing potential speed

- protection from predation

- predation




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The Geological Time Scale

Quaternary

2 Ma

Cenozoic

Tertiary

65 Ma

Cretaceous

144 Ma

Jurassic

Mesozoic

213 Ma

Triassic

248 Ma

Permian

286 Ma

Carboniferous

Phanerozoic

360 Ma

Devonian

Palaeozoic

408 Ma

Silurian

438 Ma

Ordovician

505 Ma

Cambrian

543 Ma

Proterozoic

2.5 Ba

Precambrian

Archaean

4.5 Ba


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