Fossil layers give clues to the environment of the time and area where they were laid down.
There are fossil layers identified as Mezonic where the coal layers are found. These sedimentary layers give evidence of dense tropical fern forests that have been changed to coal. The temperature of these areas must have been high, with a high level of humidity and carbon dioxide levels.
The average temperature as revealed in the coal layer fossils is much higher than the temperatures on earth today. Also the Carbon Dioxide levels were much higher than today providing for the dense plant growth revealed in the fossil layers.
Fossils give clues about the environment where the fossils lived before the fossils were buried and became part of a sedimentary layer. These clues provide information about how the earth was different at the time the fossils were formed.