English: Dennstaedtia americana Knowlton, 1910 - fossil fern from the Tertiary of Montana, USA.

Plants are multicellular, photosynthesizing eucaryotes. Most species occupy terrestrial environments, but they also occur in freshwater and saltwater aquatic environments. The oldest known land plants in the fossil record are Ordovician to Silurian. Land plant body fossils are known in Silurian sedimentary rocks - they are small and simple plants (e.g., Cooksonia). Fossil root traces in paleosol horizons are known in the Ordovician. During the Devonian, the first trees and forests appeared. Earth's initial forestation event occurred during the Middle to Late Paleozoic. Earth's continents have been partly to mostly covered with forests ever since the Late Devonian. Occasional mass extinction events temporarily removed much of Earth's plant ecosystems - this occurred at the Permian-Triassic boundary (251 million years ago) and the Cretaceous-Tertiary boundary (65 million years ago).

The most conspicuous group of living plants is the angiosperms, the flowering plants. They first unambiguously appeared in the fossil record during the Cretaceous. They quickly dominated Earth's terrestrial ecosystems, and have dominated ever since. This domination was due to the evolutionary success of flowers, which are structures that greatly aid angiosperm reproduction.

Shown here is a fossil fern. Ferns are not angiosperms - they are a large group of plants with about 12,000 fossil and modern species. They first appear in the Devonian fossil record. Ferns have true leaves, stems, and roots. The leaf is called a megaphyll. The specimen shown above is a fossil species of "true fern" (Filicales). True ferns first appear in the Carboniferous. Numerous taxonomic classification schemes have been proposed.

Classification: Plantae, Pteridophyta, Pteridopsida, Filicales (Polypodiales), Dennstaedtiaceae

Stratigraphy: Fort Union Formation, Paleocene, lower Tertiary

Locality: unrecorded/undisclosed site in Yellowstone County, southern Montana, USA