Want to see some weird fossils? Check out ediacaran biota
This is the period before the Cambrian, just after the "Snowball earth." Finally the earth had warm enough temperatures and enough oxygen to really start playing with complex multi-cellular life.
See:
https://www.scientificamerican.com/article/what-sparked-the-cambrian-explosion1/
Also:
https://www.scientificamerican.com/article/reverse-engineering-mysterious-500-million-year-old-fossils-that-confound-our-tree-of-life/