The Natural History Museum in Vienna has an exhibit on the Gabonionta, also known as Francevillian biota. They were multicellular organisms that appeared 2.1 billion years ago but then died out due to decreasing atmospheric oxygen levels. The current form of multicellular life didn’t take off until the Cambrian explosion 539 million years ago.
Why have I never heard of this? Because it’s probably not true: There is a big controversy if the Gabonionta are even real fossils of organic origin. No word of that controversy in the museum exhibit though.
Nehaveigur 