This turns up in the Ediacaran fauna before the Cambrian explosion. It is just about what and just about when scientists predicted by looking at specimens of later chordates from the Cambrian. Eighteen specimens are known.
It is part of a growing body of evidence that body plans diversified during the Pre-Cambrian, but organisms were soft and were rarely preserved as fossils. The Cambrian explosion is more a result of developing hard body parts than any actual increase in the rate of evolution.