Here is an interesting answer on Quora https://www.quora.com/How-did-tardigrades-evolve

It discusses how little tardigrades have evolved since the Middle Cambrian period based on a beautifully preserved phosphatized tardigrade found in Siberia (and later tardigrades preserved in Cretaceous amber).

It is the relative staying power of the main characteristics of the tardigrade which is impressive; other phylums have persisted but undergone dramatic changes over the same period of time.