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

1. Eukaryotes Prokaryotes Small (about 10  m), simple, lack organelles May use oxygen Eukaryotes Large (about 100  m), complicated contain internal organelles Obligate oxygen consumers Mitochondria Plastid (if plant) Nucleus

1.1 Evolution of eukaryotes Probably by endosymbiosis

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 O 2 level passed sometime in late Proterozoic?

2.1 Metazoan origins Metazoans are multicelled animals

2.2 Metazoan evolution and fossilisation Molecular clocks record origin between Ma Fossil evidence suggests origin in late Precambrian, around 600 Ma burrows Ediacaran fauna Cambrian 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

The Geological Time Scale Precambrian Phanerozoic Archaean Proterozoic 4.5 Ba 2.5 Ba 543 Ma Cenozoic Mesozoic Palaeozoic 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