Smith et al. (2006) carried out the first higher-level
pteridophyte classification published in the
molecular phylogenetic era, creating four
classes of
ferns (Polypodiopsida). At that time they used the term Polypodiopsida
sensu stricto to apply to the largest of these. Later the term Polypodiopsida
sensu lato was used to refer to all four subclasses, and the large subclass renamed
Polypodiidae. This is also referred to informally as the
leptosporangiate ferns. The Polypodiidae contain seven
orders whose
phylogenic relationship is shown in the following
cladogram, where Osmundales is seen as a
sister to all other members of the subclass. }}
Subdivision 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 first molecular phylogeny showed that
Osmunda as traditionally
circumscribed was
paraphyletic and that
Osmunda cinnamomea, despite its morphological similarity to
Osmunda claytoniana, was
sister to the rest of the family. 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.
Todea and
Leptopteris are consistently resolved as sister groups, and
Osmunda was found to contain three separate subclades corresponding to subgenera (now genera)
Osmunda,
Plenasium, and the recently described
Claytosmunda with the single species,
Osmunda claytoniana. The following phylogram shows the 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 find of an exceptionally well preserved Jurassic fossil intermediate between genus
Osmunda (as shown above) and
Osmundastrum lead to a re-analysis of Metzgar et al.'s data, which revealed that the Osmundaceae root used above may be wrong and a
tree-branching artefact (all other ferns are genetically very distant from the Osmundaceae), and allowing the following classification: }} A molecular dating study using Metzgar et al.'s data and a comprehensive set of rhizome and leaf fossils estimated that (sub)generic differentiation within Osmundaceae started by the
Triassic and was finished by the
Early Cretaceous with the formation of
Osmunda and
Plenasium. Accordingly, the PPG I classification of 2016 continues to place Osmundales in Polypodiidae, but splits
Osmunda further by elevating its subgenera to genera (
Claytosmunda,
Plenasium). The following cladogram reproduces the PPG I concept for the extant members of the family: }} The new system was used in a comprehensive taxonomic evaluation of Osmundales rhizome fossils, 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) }} Notes: a
Millerocaulis is a likely paraphyletic genus that includes forms ancestral to the modern Osmundaceae (classified as Osmundeae) as well as their potential sister lineages. 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 genus comprises 12 fossil rhizome species in addition to the sole surviving species. The monophyly of Osmundaceae has faced recent challenges, with a study highlighting unresolved affinities among several taxa. Fossil-based phylogenetic inference presents two key difficulties: (1) fossil placements are often highly unstable (yielding ambiguous relationships), and (2) morphological characters exhibiting logical dependencies—which violates the assumption of data independence essential to phylogenetic and statistical methods. Morphological phylogenetic analyses accounting for such dependencies recovered multiple fossils, traditionally considered outside Osmundaceae (e.g.,
Bathypteris), as nested within the family. Furthermore, several fossils assigned to Osmundoideae showed poorly resolved relationships, both with the Guaireaceae and other Osmundaceae members. Consequently, the monophyly of Osmundaceae itself—alongside the placement of
incertae sedis fossils—remains a persistent and complex problem. In the Triassic foliage becomes frequent which is similar to identical to the fronds of modern-day
Todea,
Osmundastrum and
Claytosmunda. 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. and literature cited therein). The first fossil representatives of the modern Osmundaceae (= tribus Osmundeae) include rhizome fossils with
Claytosmunda-anatomy or with structural features characteristic for
Osmundastrum cinnomomeum and its precursors. The same holds for the equally old leaf fossil record of the group. The main diagnostic feature of Osmundeae (modern Osmundaceae) is a heterogenous
sclerenchymatic ring in the
stipe basis. This potential
synapomorphy is the only character differentiating between Osmundeae and the paraphyletic genus collecting their potential early ancestors and sister lineages,
Millerocaulis. The basic
Bauplan shared with 'Millerocaulis' was generally kept within the Osmundeae-lineage and only slightly to moderately modified in the last 200 million years. Triassic-Jurassic rhizome and frond morphologies remained essentially unaltered in the lineage leading to
Claytosmunda claytoniana. This makes it impossible to discern direct ancestors of
C. claytoniana from the ancestors of its sister lineages
Osmunda and
Plenasium), or their shared ancestors. Molecular dating placed the split between
Osmunda and
Plenasium, and their divergence from
Claytosmunda in the
Early Cretaceous, co-eval with the divergence between the sister genera
Todea and
Leptopteris. The
Osmundastrum-lineage diverged much earlier (probably Middle Triassic), which fits with the new classification of re-evaluated rhizome fossils originally included in
Millercaulis. A comprehensive list of rhizome and leaf fossils associated with modern Osmundaceae (Osmundeae in the classification of Bomfleur et al.) can be found at datadryad.org. The only explicit reconstruction regarding the evolution of morphological traits in Osmundaceae can be found in Miller's groundbreaking work. 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, 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. Another general trend is that the Permian Osmundales were much larger than their modern counterparts. Today, the widespread species of Osmundaceae are
rhizomatous with small, low-dissected steles. ==References==