In the Devonian, Laurussia was known as the "old red sandstone" continent. It represented a large stratigraphic section of sediment that was not marred by an abundance of unconformities or differening sequences.
To start off with, sandstone is not the best for preserving fossils. It is generally from a higher energy environment with high oxygen levels. This does not bode well for preservation, as the preferred conditions are a low energy, anoxic environment.
Another problem arises when considering the large straitgraphic section that these fossils belong to. If fossils are found, it is extremly hard to correlate them with finds from other localities because of the similarity of the host rock over such an extended period of time.
This also makes problem for stratigraphers trying to define
the boarder between the Devonian and the Mississippian.