Answer: Plant fossils
Fossils are formed when dead organisms, sink below the heap of sediments. The decomposers in the soil feed upon the soft body parts and hard remains are left. These hard remains includes wood, pollen, bones, shells and others. These remains gets preserved in the rocks as fossils.
Pelaeobotany is the branch of botany that deals with the recovery and identification of the plant remains. It is useful for the reconstruction of the geological past events. It also defines how new plant species evolved from the primitive plant species, on the basis of the structures and features observed in the excavated plant fossil evidences.
In the given situation, the pine trees are evident to produce needles and cones which both are non-decomposing evidences. The fossils of these evidences proves that gymnosperms where the primitive plants, from which angiosperms or flowering plants have evolved.
Therefore, plant fossils would provide the most reliable information about the periods when the two types of plants appeared on Earth.
