Lecture 2: Origin of complexity

1. EukaryotesWhat are they?When do they evolve?When do they diversify?Evolution and the atmosphere

2. MetazoansWhat are they?Phylogenetic treeEvolution and preservation

3. The Ediacaran fauna

4. The appearance of hard parts

5. Events at the base of the Cambrian

1. Eukaryotes

Prokaryotes

Small (about 10 m),simple,lack organelles

May use oxygen

Eukaryotes

Large (about 100 m),complicatedcontain internal organelles

Obligate oxygen consumers

Mitochondria

Plastid(if plant)

Nucleus

1.1 Evolution of eukaryotes

Simple ancestor

Oxygenmetabolisingprokaryote

Spirochaetes

Blue-green alga

Eukaryoteanimal

Eukaryoteplant

Nucleus evolvesas symbiosisdevelops

Probably by endosymbiosis

1.2 When do eukaryotes evolve?

DNA evidence around 3.8 BaOldest fossil 2.1 BaAre both dates correct?

1.3 When do they radiate?

Fossils common after 1 BaFauna dominated by acritarchsDiversity peak at 700 Ma, then decline

1.4 Relationship to environment

Eukaryotes obligate aerobesFree oxygen in atmosphere and ozone shield after 2 Ba?Critical O2 level passed sometime in late Proterozoic?

Sponges

Jellyfish,corals

Annelidworms

Molluscs

ArthropodsEchinoids

Hemichordates

Chordates

BilateraliaPrebilateralia

700 My

600- 1000 My

670 - 1200 My

1100 My1100 My

940 My

Single commonancestor for allmetazoans

Figure 4. A recent classification of multicellular animals. This classification divides animals into twobroad groups, those with bilateral symmetry and those with radial symmetry. There is a large genetic gapbetween the two groups. The position of Ediacaran organisms is uncertain. Dates added to the diagramrepresent molecular estimates of time since divergence.

2.1 Metazoan origins

Metazoans are multicelled animals

2.2 Metazoan evolution and fossilisation

Molecular clocks record origin between 8-1800Ma

Fossil evidence suggests origin in late Precambrian, around 600 Ma

burrowsEdiacaran faunaCambrian provinciality

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.

3. The Ediacaran fauna

Most about 550 Ma, global distribution, phylogeny disputed.

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

5. The Precambrian/Cambrian boundary

550

510

520

530

540

d13c Phosphate

Sea level

Temperature

v…v…v…v…v

lmst

sdst

shale

Cambrianpalaeogeography

Precambrian

Cambrian

Ma

Spreading ridges

-ve +ve

The physical world in the Cambrian - conditions favoured increases in primary productivityand the development of diversity.

Sea levels rise assupercontinentbreaks up andspreading ridgesdevelop

Climate warmsworld goesfrom Icehouseto Greenhousestate

Positive d13Cexcursionand high rates ofphosphate depositionsuggest massiverise inbiologicalproductivity whichmay relate toenhanced oceaniccirculation.

Precambrian-Cambrianboundaryrelatively quiet,most importantevents occurabout 10 Ma later

Sketch boundarysection from Siberia.Note datable volcanichorizon and facieschange at boundary.

5. The Precambrian/Cambrian boundaryTrace fossils TrilobitesArchaeocyathidsSmall shelly fossils

Siliceous shells

Agglutinatedshells

Calcareousshells

Phosphaticshells

The biological world in the Cambrian - a sequence of faunas appeared over about 10-15 Ma

Trace fossilsappear inlate Precambrian,most aresimple forms

Small shelly fossilscharacterise earlyCambrian. Differentskeletal typesappear over about10 Ma.

Many of these shellsare small parts of largerorganisms whoseskeleton wasconstructed of overlappingparts like chain mail.

Archaeocyathidreefs flourishedbriefly and thendisappeared

The Geological Time ScaleP

r eca

mb r

ian

Ph a

nero

z oic

Archaean

Proterozoic

4.5 Ba

2.5 Ba

543 Ma

Cen

ozoi

cM

esoz

oic

Pa l

aeoz

o ic

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

2 Ma

65 Ma

144 Ma

213 Ma

248 Ma

286 Ma

360 Ma

408 Ma

438 Ma

505 Ma