3. Dominant life forms of the past: some very broad generalizations
Precambrian: prokaryotes
Cambrian-Ordovician: marine invertebrates, algae
Silurian-Devonian: fish diversify, and remain the dominant group in the seas; land colonized
Carboniferous-Permian: amphibians, insects, forests of ferns and seedless vascular plants.
Mesozoic era: reptiles (inc. dinosaurs), gymnosperms
Cenozoic era: mammals, flowering plants, new insects

4. BUT…
These are dangerously broad generalizations, and only a starting point.
Most groups originated long before they became dominant. For example:
1st eukaryotes in the Precambrian, 2.5bya
1st reptiles in Carboniferous
1st mammals in Triassic