Life forms changed dramatically during this period:
Over a half-billion years ago, life on Earth was composed of simple ocean organisms unlike anything living in today’s oceans. Then, beginning about 540 million years ago, animal structures changed dramatically.
During this time, ancestors of many animal groups we know today appeared, such as primitive crustaceans and worms, yet for years scientists did not know how these two seemingly unrelated communities of animals were connected, until now. An analysis of tubular fossils by scientists led by Jim Schiffbauer at the University of Missouri provides evidence of a 550 million-year-old digestive tract — one of the oldest known examples of fossilized internal anatomical structures — and reveals what scientists believe is a possible answer to the question of how these animals are connected…
“Not only are these structures the oldest guts yet discovered, but they also help to resolve the long-debated evolutionary positioning of this important fossil group,” said Schiffbauer, an associate professor of geological sciences in the MU College of Arts and Science and director of the X-ray Microanalysis Core facility. “These fossils fit within a very recognizable group of organisms — the cloudinids — that scientists use to identify the last 10 to 15 million years of the Ediacaran Period, or the period of time just before the Cambrian Explosion. We can now say that their anatomical structure appears much more worm-like than coral-like.”
University of Missouri-Columbia, “ Scientists find oldest-known fossilized digestive tract — 550 million years” at ScienceDaily
Wouldn’t this appear to push the origin of worms back a bit?
https://commons.wikimedia.org/wiki/File:Cloudina.svg
The paper is open access.
See also: Stasis: When life goes on but evolution does not happen