Placement of Devonian Period
The Phanerozoic Eon spans all of the time of "visible life" from the Cambrian Period (540 Ma) to the Recent. As far as we can tell, life before the Cambrian was mostly forms of cyanobacteria and soft bodied organisms whose hard parts only preserve in rare instances. Rocks from the Cambrian Period are the earliest rocks with abundant shelly fossils. These shelly organisms are much more prone to fossilize than soft bodied organisms, and thus are more "visible" in the rock record.
Devonian Period rocks were deposited from approximately 416 Ma until 359 Ma. In a complete succession they are sandwiched between Silurian Period rocks (444 – 416 Ma) below and Mississippian Epoch rocks (359 – 318 Ma) above. It is further subdivided into smaller time slices based on first and last occurrences of particular types of fossils.