1. Estimating by the appearance of the fossils they are all discovered in a rock layer are from the Devonian era. The appearance of some of the fossils re even estimated to be even before then the Devonian era, however from the Devonian era we can find the clear instances of all types of fossil fuel.
2. The fossil which was received from the Devonian Era could be referred to as Index fossils because they come from the class of animals that was most extensive in that geological era. In the Devonian, the aggressive group of mammals was the corals, brachiopods, and trilobites, so this accumulation of fossils would be known as unique index fossils.