Spores of the first known terrestrial plants have been found in what strata

The answer to this question is the Silurian strata. These are spores of the Psilophytes which appeared at the end of the Silurian period. The spores have been found in early Silurian rocks in Libya. Psilophytes did not have roots or leaves, but has branches carrying spore sacs that grew from an underground vine.