[Europan Ocean (OP)]

What kind of plants and especially animals, lived on the surface of the Earth during the times before an Oxygen atmosphere?

[Bored chemist]

To answer the second part (about animals); None.
The oxygenation event happened before there were any animals.
It had to; animals need oxygen.

https://www.nationalgeographic.com/science/article/890-million-year-old-sponge-fossil-may-be-the-earliest-animal-known


"The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, and the Oxygen Crisis, was a time interval when the Earth's atmosphere and the shallow ocean first experienced a rise in the amount of oxygen. This occurred approximately 2.4–2.0 Ga (billion years) "
From
https://en.wikipedia.org/wiki/Great_Oxidation_Event

[evan_au]

The plants producing the oxygen are thought to be microscopic cyanobacteria, floating near the water surface, or building up mounds like stromatolites.
- Stromatolites peak in the fossil record about 1.25 billion years ago; their decline is thought to be associated with the development of animals which were able to eat the cyanobacteria.
https://en.wikipedia.org/wiki/Stromatolite#Fossil_record

Some theories suggest that life may also have occurred around deep ocean vents (like the black smokers of today's mid-Atlantic ridge).
- There is no light there, so there would not be plants.
- Microbes in these regions metabolize chemicals like H₂S to gain energy, and process CO₂ & CH₄ to produce organic chemicals.
- Would you call anything that doesn't photosynthesize "animals"?
- Probably a lot of microscopic Archaea, but nothing like today's tube worms and crabs
https://en.wikipedia.org/wiki/Hydrothermal_vent#Animal-bacterial_symbiosis