David: Recently discovered, and very complex:

https://www.livescience.com/nightmare-creature-multitool-head.html?utm_source=ls-newsle...

"... have found dozens of fossils of this species in recent years that include preserved soft tissue of the mouthparts, along with the animals' multiple legs and bulbous eyes.

Comment: very early in the Cambrian and way more complex than anything in the Ediacaran, despite all efforts to imply the gap is not that large..

Tony: How is there 'preserved soft tissue' after 500 million years?

The usual explanation is that a cast was formed around the soft tissues. Trilobite brains were described that way recently.