Nephrozoa

Nephrozoa is a major clade of bilaterians including deuterostomes and protostomes. It is the extant sister clade of Xenacoelomorpha.

The majority of bilaterian animals are split into two groups, the protostomes and deuterostomes. Chordates (which include all the vertebrates) are deuterostomes. It seems very likely that the 555 million year old Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.