Which are fossil plants?
The large black slab shows late-Middle to Late Pennsylvanian (~308 to 300 million years old) plants from the famous St. Clair fossil beds of Pennsylvania. The leaf fronds are from the genus Alethopteris, and although they look like ferns, they are actually seed ferns. Seed ferns are not closely related to ferns and went extinct long ago.The small rectangular slab is from the famous limestone fossil quarries of Solnhofen, Germany where Archaeopteryx comes from. But these marks are not fossils. They are what are called manganese-oxide dendrites. These are naturally-forming features that occur when minerals in solution percolate into tiny fissures in rocks and are deposited in a tree-like form. This beautiful plant-mimic grows along fractures in the bedding planes of Solnhofen rocks, and since this is also where the fossils are found, dendrites are often found radiating out from a genuine fossil making it look like it’s covered in moss.