 Immediately above the topmost layer of the marine rock sequence forming Murchison's Silurian System was a thin layer of dark sand containing numerous remains of early fish, especially their scales, along with plant debris, spores and microscopic mites.In contrast to the underlying sediments of the Ludlow Series which were deposited in a shallow warm sea some 400 million years ago, the Ludlow Bone Bed represents terrestrial (land) conditions and thus a fundamental change in the landscape.At the time, this was believed to be the earliest occurrence of life on land.