About 10 yers ago, the paleo guys at Virginia Tech published a proposal that the "Cambrian Explosion" be presented as an event occuring (and defining) the base of the Cambrian, wherein organisms began sporting "hard parts".
At the ame time they proposed that similar "complex animals" were first seen to occur in an earlier event (650 + MYCE) roughly the age that included the end of the Cryogenian and included ALL of the Ediacaran.
This, having been accepted (pretty much) in the International Strat Conf, staets in plain Wnglish that
"Besides the Cambrian explosion which defined complex organisms with their framework (skeletons and shells), we now introduce the "AVALON EXPLOSION" which occured about 75 -100 million yeqrs erlir nd defined the appearance of complex organisms except without frameworks composed of hard parts"
(Of course Ive sorta paraphrased to keep out the jargon)
The "Cambrian Explosion" has, in the last 10 years , lost an important bit of status to the Fundamental minds out there. Complex organisms appear in the forms of annelids and other animals sans shells and bones.AND, as the atmosphere attained saturation in N and O2 , these same animals began having hard covers and various types of skeletal structures.
Most of this change in historical classification is due to the ability to see complex structures within such fossils as Dickinsonia
and several others that appear to be direct linneages of arthropods.