I like the "running for higher ground" argument used to explain the differing levels creatures are found at. I've yet to see it explained how it applies to the similar stratification of plantlife.
It all makes sense now. The running animals must have been "petrified" with fear.
If you point that out then they just add more ad-hoc factors. Soon stratification of fossils becomes a consequence of several factors from ability to run up hill, weight of the organism, density, surface area, etc etc. It works great as an obfuscation tactic.