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How are ferns and horsetails classified by the Pteridophyte Phylogeny Group?

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Lower vascular plant Aleutian maidenhair fern (*Adiantum aleuticum*) of the Pacific Northwest.

The Pteridophyte Phylogeny Group (PPG I) classifies ferns and horsetails together within the class Polypodiopsida.1

While these plants were historically grouped as "fern allies" and considered part of the now-obsolete Pteridophyta, modern molecular studies have clarified their relationships. Under the PPG I system, the class Polypodiopsida encompasses four subclasses:12
* Polypodiidae: The true ferns.2
* Equisetidae: The horsetails.2
* Marattiidae: A group of ferns.2
* Ophioglossidae: A group of ferns.2

This classification reflects a phylogenetically based consensus, distinguishing these groups from the Lycopodiopsida (lycophytes), which are considered only distantly related to ferns and horsetails.12