The Osmundaceae (royal fern family) is a family of four to six extant genera and 18–25 known species. It is the only fern family of the order Osmundales an order in the class Polypodiopsida (Filicopsida, Pteridopsida, or Leptosporangiate ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient (known from the Upper Permian) and fairly isolated group that is often known as the "flowering ferns" because of the striking aspect of the ripe sporangia in Osmunda and Osmundastrum. In these genera the sporangia are borne naked on non-laminar pinnules, while Todea and Leptopteris bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns.
Ferns of this family form heavy rootstocks with thick mats of wiry roots. Many species form short trunks; in the case of the genus Todea, they are sometimes considered as tree ferns because of the trunk, although it is relatively short.
The leaf tissue ranges from very coarse, almost leathery in the case of the Cinnamon fern (Osmundastrum cinnamomeum), to delicate and translucent, as in the case of the genus Leptopteris.
In the Pteridophyte Phylogeny Group classification (2016) Osmundales consists of the single family Osmundaceae, six genera, and an estimated 18 species (Christenhusz and Byng give 25 species).
The three genera Osmunda, Leptopteris, and Todea were recognized as members of Osmundaceae by Smith et al. (2006) Of these, the largest genus, Osmunda, had traditionally been treated as three subgenera, Osmunda (3 species), Osmundastrum (2 species), and Plenasium (3–4 species). However there was suspicion that the genus was not monophyletic.
The publication of a detailed phylogeny of the family by Metzgar et al. in 2008 showed that Osmunda as circumscribed was paraphyletic and that Osmunda cinnamomea, despite its morphological similarity to Osmunda claytoniana, was sister to the rest of the family, and resurrected the segregate genus Osmundastrum, by elevating it from subgenus, to contain it and render Osmundamonophyletic, a concept that had been long suggested. In their phylogeny, Todea and Leptopteris were also shown to be sister groups, and Osmunda to contain three separate subclades corresponding to subgenera, Osmunda, Plenasium, and the recently described Claytosmunda with the single species, Osmunda claytoniana. This addition of Osmundastrum as a fourth genus was reflected in the revised classification of Smith et al. in 2008, namely Leptopteris, Osmundastrum, Osmunda and Todea.
The following phylogram shows a likely relationship between the Osmundaceae genera and subtaxa, according to Metzgar et al.:
The circumscription of the order and its families was not changed, and its placement remained the same in subsequent classifications including Chase and Reveal (2009), Christenhusz et al. (2011), and Christenhusz and Chase (2014). The PPG I classification of 2016 continues to place Osmundales in Polypodiidae, and splits Osmunda further by elevating its subgenera to genera as Claytosmunda and Plenasium. This produces the following cladogram;
Jud, Nathan, Gar W. Rothwell, and Ruth A. Stockey (2008). "Todea from the Lower Cretaceous of western North America: implications for the phylogeny, systematics, and evolution of modern Osmundaceae." American Journal of Botany, 95:330-339.
Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants . Second Edition, Academic Press 2009, ISBN 978-0-12-373972-8, p. 437-443