I first heard about this huge eruption of complex lifeforms in the book “A Short History of Nearly Everything” by the celebrated author Bill Bryson. It seems that almost overnight (or about 12 million years in geological time), complex lifeforms just sprang into existance. Some of them are your ancestors, but some of them were a bit weird. For instance Hallucigenia, a creature so weird it looks alien. Maybe it was.
The head end contained no mouth or sensory organs, where you’d expect them. It was suggested that the tentacles had tubes in them which directly fed the entity with whatever food it ate. It was also suggested that Hallucigenia was part of another animal.