 As an example, the boundary between the Silurian and Devonian periods is marked by the first appearance of the graptolite "Mongraptus uniformus uniformus" in a section in Klonk, Czech Republic.In terrestrial deposits, fossils of land mammals and other vertebrates are used as stratigraphic tools, but they have some disadvantages relative to marine fossils.They are seldom evenly distributed through a section, and they tend to occur in isolated pockets with few overlaps between biozones.