About 150 plant species are known from the Florissant fossil beds. These species are represented by leaves, flowers, seeds, pollen, cones, and stumps [click here to learn more about stumps]. Plants are often used to reconstruct past climates and forest ecology. To understand this principal think about how different the plants in Alaska are from the ones in Florida. This variation is because plants, unlike animals, cannot move and so evolve through adaptations that allow them to live in a certain stationary environment.
Scientists are able to look at those adaptations and determine paleoclimate [to learn more about calculating past climates from leaf fossils click here]. Fossil plants help us reconstruct what kind of forest lived at Florissant during the late Eocene (34 million years ago), which is very different than the Florissant forest today. Modern Florissant consists mainly of pine and aspen trees, with much less species variety than there was 34 million years ago.
What was the ancient forest like?
Eocene Florissant was a warm temperate forest with a rich variety of plant and animal species. The ancient Florissant forest had canopy, understory, and ground layers. Near the lake, tall conifers like redwood and false-cypress formed the canopy. Under the canopy in the understory grew hardwoods like hickory, soapberry, maple, willow, and extinct genera from the elm and beech families. The ground-layer in the near-lake microecosystem had grasses, shrubs, and roses. On the shores of the lake were rare ferns and horsetails, and lily pads floated on the surface of the water. Distant from the lake on the drier hillsides grew oak, pine, and mountain mahogany. Can you see these microecosystems in the picture below?
Finding plant fossils
This is an artist's informed interpretation of the ancient Eocene landscape of Florissant. The topography looks remarkably similar to that of modern Florissant.
Volcanic debris flows (lahars) and ashfall 34 million years ago are the reason that so many fossils are pristinely preserved and available for you to view. Massive redwood trees were encased in lahars, which provided mineral material for permineralization (fossilization process
). Excavation of petrified stumps is no longer practiced, in fact, some stumps have been reburied for their protection.
Identifying Fossil Plants
Macrofossils (visible to the naked eye): Periodic volcanic eruptions and diatom (a kind of alga) mats formed thinly layered rock referred to as paper shale in the ancient Florissant lake. This shale is split along its layers to look for macrofossils.
Microfossils (invisible to the naked eye): Microscopic pollen, the most common microfossil at Florissant, is found by dissolving rock material using a series of acids which leave behind pollen grain fossils. These fossils can then be mounted on a slide and viewed using a microscope.
Modern botanists use entire plants to describe a species, which includes leaves as well as any seed, nuts, flowers, fruits, stems, etc. that belong to the plant. Paleobotanists (scientists that study fossil plants) don't always have all of these organs present in the fossils that they work with. So how do they identify fossil plants?
This method of classifying fossil plants is based on leaf architecture: the physical traits of leaves such as size, shape, margin type (smooth or toothed), and venation. You can clearly see some of these features on these specimens.
As you browse some of the plants found at Florissant, notice how similar they look to plants that exist today. You can use the common name function to look for species that you might already know, like oak, beech, or pine.
Fossil plants are similar to modern plants-- this is how paleobotanists are able to observe characteristics of modern leaves in order to develop informative methods for describing and identifying plant fossils. Modern leaves are chemically "cleared" and stained to expose their physical traits and venation. Here is a comparison of a cleared modern leaf and a fossilized leaf of similar species.
Image a. is a picture of a fossil leaf, while image b. is a picture of a cleared modern leaf. Can you see the similarities?
The different parts of plants are referred to as organs. Examples of plant organs would be leaves, flowers, fruits, and seeds. Though very rare, fossil plants with multiple organs attached have been found in the fossil record. When this is available paleobotanists use everything they can see to classify the fossil plant.
These are some leaves with attached male and female reproductive organs.
.Male Fagopsis longifolia
.Female Fagopsis longifolia
Microscopic Plant Material
Microscopic plant fossils, pollen and spores, are often preserved in locations without any macrofossils because they are so small and have a layer (sporopollenin) that is very resistant to decay. The shapes of microscopic pollen grains are distinct to species groups.
Microscopic pollen is usually stained for visibility and is often a beautiful photographic subject.
Salixipollenites sp. B
Abiespollenites (?) sp.
Many of the species preserved as pollen and spores are not present in the macrofossils. The shapes of microscopic pollen grains are distinct to genera or species groups. Examining both microfossils and macrofossils together portrays a more complete representation of the ancient forest.
Water lillies from Florissant are known only from pollen.
A confirmed 20 Florissant plant families are known only from pollen.
is known only from macrofossils at Florissant.
A confirmed 12 Florissant plant families are known only from leaf and fruit fossils.