This is strange because the subsoil composition of today vs. 65+ million years ago, let alone 250 million years ago, is substantially different. Mostly different insects, different bacteria, and though Earthworms did exist for much of that period, other types of worms did not. Lignin was already in trees and mushrooms were already in the ground, but soil as a living ecosystem was far less developed and alive than it is today - it was probably closer to regolith at least in the 250-100 million year ago period.