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 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
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 O2 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 8-1800Ma Fossil evidence suggests origin in late Precambrian, around 600 Ma burrows Ediacaran fauna Cambrian provinciality
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.
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
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