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 Claytosmunda, Osmunda, Osmundastrum, and Plensium (subtribe Osmundinae[1]).
[3] The mantle of sclerenchymatous leaf bases and intermixed roots[4] can form a woody trunk when the stem emerges above ground, up to 1 meter (3.3 ft) in Todea barbara.
[2] Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, creating four classes of ferns (Polypodiopsida).
This was later confirmed by a detailed species-level phylogeny of the family by Metzgar et al. (2008) leading to the resurrection of the segregate genus Osmundastrum, by elevating it from subgenus, to contain it and render Osmunda monophyletic.
The following cladogram reproduces the PPG I concept for the extant members of the family:[9] Osmundastrum (Alternative A: outgroup-inferred root in multigene trees) Todea Leptopteris Osmundastrum (Alternative B[20][21]) Claytosmunda (=Osmunda claytoniana) Plenasium 4 species Osmunda 4 species The new system was used in a comprehensive taxonomic evaluation of Osmundales rhizome fossils,[1] who provide a polytomous key using anatomical features of Osmundaceae rhizomes and an updated ‘evolutionary’ (non-cladistic) classification of fossil and extant Osmundales (see classification concepts for groups including extinct members), which can be tentatively transferred into the following cladogram (monophyla in bold, polytomies reflect unresolved relationships) Bathypteris † monotypic, Late Permian Guirea † mono- or paraphyletic with 2 species, Late Permian to Middle Triassic Zhongmingella † monotypic, late Permian Changhsingian Lunea † monotypic, Early Jurassic Donwelliacaulis † monotypic, Middle Triassic Itopsidema (incl.
bThe morphology of Claytosmunda is primitive within the Osmundinae, and total evidence indicates that Osmunda and Plenasium likely evolved from a Claytosmunda-type ancestor, rendering the latter genus paraphyletic when Osmundaceae fossil should be considered.
The better understood rhizome fossil record indicates that the group was most diverse in the Permian, and already much reduced in the Triassic regarding the number of substantially different forms (see Bomfleur et al.[1] and literature cited therein).
[21] The Osmundastrum-lineage diverged much earlier (probably Middle Triassic), which fits with the new classification of re-evaluated rhizome fossils originally included in Millercaulis.
[24] Notable is that Permian rhizomes of both families in the Osmundales, the extinct Guaireaceae and the Osmundaceae including the extant species, show already relatively complex stele anatomies in comparison to the surviving members of the group.
The rhizome fossil record also indicates several independent radiations of likely arborescent lineages, the Guaireaceae and Thamnopterioideae in the Permian, Osmundacaulis in the Triassic, and Plenasium (subgenus Aurealcaulis) in the late Cretaceous to Paleogene.
As already noted by Miller,[24] highly derived forms not directly related to the extant species and genera, can be found in the Jurassic and Cretaceous, such as Millerocaulis (Osmundacaulis) kolbii.