Itopsidema, Donwelliacaulis, and Tiania; (2) the thamnopteroid genera Zalesskya, Iegosigopteris, and Petcheropteris are all considered synonymous with Thamnopteris; (3) 12 species of Millerocaulis and Ashicaulis are assigned to modern genera (tribe Osmundeae); (4) the hitherto enigmatic Aurealcaulis is identified as an extinct subgenus of Plenasium; and (5) the poorly known Osmundites tuhajkulensis is assigned to Millerocaulis. In addition, we consider Millerocaulis stipabonettiorum a possible member of Palaeosmunda and Millerocaulis estipularis as probably constituting the earliest representative of the (Todea-)Leptopteris lineage (subtribe Todeinae) of modern Osmundoideae.The Osmundales  originated in the late Paleozoic and is the most ancient surviving lineage of leptosporangiate ferns. In contrast to its low diversity today (less than 20 species in six genera), it has the richest fossil record of any extant group of ferns. The structurally preserved trunks and rhizomes alone are referable to more than 100 fossil species that are classified in up to 20 genera, four subfamilies, and two families. This diverse fossil record constitutes an exceptional source of information on the evolutionary history of the group from the Permian to the present. However, inconsistent terminology, varying formats of description, and the general lack of a uniform taxonomic concept renders this wealth of information poorly accessible. To this end, we provide a comprehensive review of the diversity of structural features of osmundalean axes under a standardized, descriptive terminology. A novel morphological character matrix with 45 anatomical characters scored for 15 extant species and for 114 fossil operational units (species or specimens) is analysed using networks in order to establish systematic relationships among fossil and extant Osmundales rooted in axis anatomy. The results lead us to propose an evolutionary classification for fossil Osmundales and a revised, standardized taxonomy for all taxa down to the rank of (sub)genus. We introduce several nomenclatural novelties: (1) a new subfamily Itopsidemoideae (Guaireaceae) is established to contain  Osmunda as distinct genera , together with morphologically similar sterile fronds  and dispersed spores , are probably the most common organs of Osmundales in the fossil record but, in many cases, these remains are difficult to discriminate with confidence from other fern orders. Moreover, foliage adpressions and spores typically lack the character resolution necessary for confident attribution to families and genera within Osmundales  form the most ancient surviving lineage of leptosporangiate ferns. The order comprises about 20 extant species originally placed in three or four genera with four (three) subgenera. The recently published, first comprehensive classification of the pteridophyte phylogeny group (PPG) recognizes all former subgenera of t genera . Osmundat genera . Detachemundales . Anatomimundales ; Fig. S1Here, we present and discuss the results of a novel systematic–phylogenetic analysis of 129 osmundalean axes, including all currently known fossil records  plus 14 extant representatives. We briefly review the diversity of anatomical features of osmundalean axes using a revised, uniform, descriptive terminology, and introduce a new morphological character matrix consisting of 45 architectural characters of the plant axis. The matrix serves two main purposes: the standardized description and identification of specimens and the characterization of natural groups. The latter incorporates results of customized phylogenetic network analyses of this matrix. Based on these results, we propose an anatomy-based classification for fossil Osmundales that is as natural as possible, with a revised and standardized taxonomy listing diagnostic axis characters for all taxa down to the rank of (sub)genus. For practicality, we also accept several explicitly paraphyletic taxa as valid taxonomic units. Appendices provide the formal taxonomic treatments of nomenclatural novelties (Appendix A), a glossary of terms and abbreviations (Appendix B), a polytomous identification key (Appendix C), and a tutorial and example analysis  to aid tOsmundacaulis) and are surrounded by a prominent mantle of roots and persistent stipe bases (except some Guaireaceae).Osmundalean axes range from small, creeping or shortly erect rhizomes to huge, arborescent trunks, and they display a remarkable diversity in anatomical structure. Features that are common to all osmundalean axes—extant and extinct—include (1) radial symmetry; (2) helical phyllotaxis; (3) a central cylindrical stele; (4) a single peripheral metaxylem siphon surrounding the stem core; and (5) a single, primarily C-shaped , entire vascular bundle in the base of the stipe . Most osIn Guaireaceae and in most Osmundaceae, the stem core consists primarily of a parenchymatous pith, in some cases with varying amounts of additional sclerenchyma and, more rarely, tracheids or “medullary traces”; in thamnopteroid Osmundaceae, the stem core is composed of tracheids that may be short, elongated, or enlarged and “parenchyma-like” . Thus, tGrammatopteris and Rastropteris, considered by some authors to be ancient relatives of Osmundales) nor a true siphonostele, but rather an intermediate between the two  to short parenchyma-like tracheids (most Thamnopteroideae) to short tracheids with interspersed parenchyma (Thamnopteris kidstonii) to parenchyma with interspersed tracheids  to parenchyma  to fully sclerified stem cores   . Thus, t, Todea) , and we Thamnopteris kidstonii), the departing leaf traces usually leave more or less prominent notches or gaps in the stele periphery  may show one or more inverse U-shaped (parts of) xylem segments with the openings directed toward the pith . In C. principalis, this incision may even perforate the xylem siphon completely; thus, except for its stem core consisting presumably of enlarged tracheids rather than parenchyma, the stele of Chasmatopteris equates to the “dictyoxylic siphonostele” found in most other Osmundoideae.Traditionally, the stems of Thamnopteroideae have been described as having a gapless and imperforate stele consistent with their classification as being protostelic  to a highly complex, reticulate network of stelar xylem segments  and Guaireaceae (Guairea in Guaireoideae). This applies also to the opposite extreme: (nearly) imperforate steles are characteristic of Thamnopteroideae in Osmundaceae and of Itopsidemoideae in Guaireaceae. Finally, we argue that an unusually high degree of perforation can also be characteristic of a particular species or even genus within a group with otherwise imperforate or only moderately perforated steles . Altogether, the significance of the degree of stele perforation depends on the context of the individual fossil and its putative relatives.We agree with Guairea, Donwelliacaulis, and Osmundacaulis, in which the siphon is particularly thick and ranges from about 30 to, in extreme cases , more than 70 cells in radial thickness  are unique in that the peripheral xylem siphon consists of a spongy admixture of xylem tracheids with diffusely interspersed “nests” of parenchyma that are not immediately related to protoxylem clusters of early-developing leaf traces . Such particularly complex stele types have been referred to as “dissected-siphonostelic”  and “dictyostelic” (in cases where phloem layers connect) see . This diectioned . In mostectioned .Leptopteris and Todea  the number of protoxylem poles from which a leaf trace is formed; (2) the position in which the protoxylem first appears in the stele; (3) the point at which the protoxylem becomes exposed at the adaxial surface of the trace, i.e., at which the trace becomes endarch; (4) the point at which the protoxylem undergoes first (and subsequent) divisions; and (5) changes in the overall shape of the leaf trace in its course through the stem. Consequently, we score leaf trace development in our matrix in a series of eight characters that describe the number and position(s) of protoxylem poles in the stele, upon departure of the trace, in the central part of the cortex, and upon departure from the stem .Stipe bases of all Osmundales contain a single vascular bundle that is strongly curved adaxially, i.e., with the opening directed toward the stem. In Osmundaceae, the adaxial margins of the bundle can be straight or incurved, giving the bundle a characteristic horseshoe-shape in transverse section; in Guaireaceae, the margins of the bundle are typically recurved or constricted, giving the bundle an inverse omega-shape or a flask-shape in transverse section .Lunea jonesii, it has been suggested that guaireaceaean stipes lack stipular wings in general , and the stipular wings . In the Todea, Leptopteris, Claytosmunda, Osmundastrum, Osmunda, and Plenasium), the composition of this sclerenchyma cylinder is heterogeneous and shows distinctive configurations of patches of thick-walled sclerenchyma cells. Differentiation of the cylinder typically begins with the development of an arch or crescent of thicker-walled cells lining the abaxial periphery of the ring in cross-section. This abaxial arch then begins to differentiate further into characteristic patterns that are diagnostic of particular genera, subgenera, and species , the root vascular bundles may also (rarely) be triarch  and included in In order to resolve systematic relationships among extant and fossil Osmundales based on axis anatomy and to aid description and taxonomic assessment of forthcoming material, we developed a novel morphological character matrix  and S2. Character 1: Stem core: tracheids: (0) absent; (1) accessory; (2) forming main tissue.Character 2: Stem core tracheids (separated and modified from character I.A of Character 3: Stem core: parenchyma: (0) absent; (1) accessory; (2) forming main tissue.Character 4: Stem core: sclerenchyma: (0) absent; (1) present, scattered; (2) variable, up to entirely sclerified (character II.B of Character 5: Stele: (0) solid or diffusely heterogeneous; (1) heterogeneous with a distinct peripheral xylem siphon (Modified from character I.A of Character 6: Thickness of stelar xylem siphon: (0) <30 tracheids thick; (1) >30 tracheids thick : (0) absent; (1) shallow ; (2) prominent (deeper than leaf trace thickness) (Replaces characters 4 and 5 of Character 8: Degree of stele perforation: (0) (nearly) imperforate: ≤3 complete perforations; (1) moderately to densely perforated: 4–15 complete perforations, number of complete perforations per millimeter stele perimeter (PmmS) 0.11–2.45; (2) highly perforated: ≥16 complete perforations and PmmS ≥ 0.30  expressed in the formula,maxN(CP) is the maximum number of complete perforations in a given transverse section of the stelar xylem cylinder; d(Stele) the stele diameter  but very large steles. Consequently, they appear much less perforated than, e.g., Millerocaulis herbstii and Osmunda shimokawaensis with the same maximum number of complete perforations (18), which are much more densely spaced (PmmS = 1.42 compared to 0.11–0.13 in Guairea). Final values range between entirely imperforate to highly perforate with 75 complete perforations (Plenasium dakotense) and 2.45 perforations per mm stele perimeter (Millerocaulis amarjolensis). Inferred cut-off values of the clusters were ≤4, 5–15, and ≥16 for complete perforations and ≤0.56 and ≥0.6 (k = 2) or ≤0.32, 0.36–0.95, and ≥0.99 (k = 3), respectively.The PmmS values (≤0.19); those were scored as “(nearly) imperforate” , comparable to values found frequently in operational units with five or more perforations, hence, these were scored as “moderately perforate.” One of the 35 operational units with ≥16 complete perforations (scored as “highly perforate”) has an extremely low PmmS value , hence this species was scored as “moderately perforate” by analogy with its congeners with similar PmmS but fewer complete perforations (12–14). Next lowest PmmS values in the group of operational units scored as “highly perforate” are found in three Osmundacaulis species . The 5–15 complete perforations in the remainder  can be widely or densely spaced. We categorized these as “moderately to densely perforate.”Most operational units in the first cluster have no or only few complete perforations and accordingly low rforate” . The twoCharacter 9: Internal embayments in xylem siphon (indicative of an incipient complete leaf-gap formation): (0) none; (1) fewer than peripheral incisions; (2) as many as or more than peripheral incisions  xylem segments).Character 10: Parenchyma interspersed in stelar xylem, creating “spongy” structure: (0) absent; (1) present .Character 11: Dissected condition, i.e., with external and internal endodermal layers connecting through leaf gaps: (0) absent or very rare; (1) usually present  absent or very rare; (1) usually present. (Modified from character II.D of Character 13: Relative thickness of cortex (RTC): (0) <1.2; (1) ; (2) >2.8. The relative thickness of the cortex was quantified using k-median clustering with three classes (k = 3) was used to define thresholds for each character state.maxd(Stem) is the maximum stem diameter; maxd(Stele) the maximum stele diameter.RTC ranged between 0.14 in Osmundacaulis zululandensis and 13.62 in B. rhomboidea  absent; (1) present (New character accounting for the distinct third cortex layer in outgroup taxa).Character 15: Composition of the inner cortex (the inner one of two main layers): (0) parenchyma only; (1) parenchyma with scattered sclerenchyma cells; (2) parenchyma with a patch of fibres adaxial to each departing leaf trace  absent; (1) thinner than parenchymatous layer; (2) thicker than parenchymatous layer  homogeneous; (1) heterogeneous with a distinct cylinder of thicker-walled sclereids surrounding each leaf trace  one; (1) two; (2) more than two (Character 16 of Character 19: Leaf trace protoxylem initiation in stele: (0) subexarch in peripheral bulge; (1) mesarch  one; (1) two; (2) more than two  lateral; (1) mesarch; (2) endarch (Modified from character I.D of Character 22: Shape of leaf trace immediately after departure from stele: (0) oblong or slightly curved adaxially; (1) strongly curved adaxially; (2) two individual segments  one; (1) two; (2) more than two  mesarch; (1) endarch (Expanded from modified character I.D of Character 25: Number of leaf trace protoxylem strands upon departure from stem: (0) one; (1) two; (2) more than two  oblong or only slightly curved adaxially; (1) strongly curved adaxially.Character 27: Lateral tips of stipe bundle immediately after departure from stem: (0) straight or incurved; (1) recurved.Character 28: Sclerenchyma lining stipe bundle abaxially: (0) absent; (1) interrupted; (2) continuous.Character 29: Sclerenchyma in stipe bundle concavity: (0) absent; (1) free mass; (2) lining band.Character 30: Sclerenchyma in stipe bundle concavity, special states: (0) solid; (1) interrupted/scattered; (2) bifurcating.Character 31: Appearance of sclerenchyma in stipe bundle concavity: (0) in stipe only; (1) extending proximally into stem cortex; (2) extending proximally into stele.Character 32: Scattered sclerenchyma in inner cortex of stipe: (0) absent; (1) present.Character 33: Stipe cortex with distinct outer sclerenchyma cylinder: (0) absent; (1) present.Character 34: Sclerenchyma cylinder of stipe base: (0) homogeneous or diffusely heterogeneous; (1) heterogeneous, having a distinct abaxial arch of thick-walled fibres in basal cross-sections.Character 35: Abaxial arch developing distally into a complete ring: (0) absent; (1) present.Character 36: Abaxial arch differentiating distally into two lateral masses: (0) absent; (1) present.Character 37: Abaxial arch  differentiating distally into two lateral masses and one abaxial mass: (0) absent; (1) present.Character 38: Two lateral masses developing distally into an adaxial arch: (0) absent; (1) present.Character 39: Stipular extensions: (0) none; (1) wings; (2) spines.Character 40: Scattered sclerenchyma fibres in stipular wings: (0) absent; (1) present.Character 41: Distinct sclerenchyma masses in stipular wings: (0) absent; (1) one mass in each wing; (2) two or more masses in each wing.Character 42: Shape or arrangement of distinct sclerenchyma masses in cross-sections of stipular wings: (0) irregular; (1) elongate along lateral extent of stipular wing.Character 43: Point of root emergence: (0) always from stele; (1) from leaf trace upon emission from stele; (2) from leaf trace further out in cortex.Character 44: Number of roots per leaf trace: (0) one, sporadically two; (1) two, sporadically one.Character 45: Predominant orientation of roots in mantle cross-section as an indicator of arborescence: (0) primarily vertical; (1) primarily radial.The more differentiated “operational units”  are coded into a character matrix, the better-substantiated the inferences will be. In this respect, the increasing use of the genus as the basic category in evolutionary studies is problematic because genera are often arbitrarily circumscribed, non-equivalent concepts that do not exist in reality . ArbitraPlenasium dowkeri (Carruth.) Bomfleur, G.W.Grimm & McLoughlin sensu Miller is separated into two operational units: Plenasium dowkeri (Carruth.) Bomfleur, G.W.Grimm & McLoughlin sensu Chandler (see Plenasium chandleri (Arnold) Bomfleur, G.W.Grimm & McLoughlin (see Guairea carnieri (J.Schust.) R.Herbst is separated into two operational units Guairea carnieri (J.Schust.) R.Herbst sensu Miller (see Osmundacaulis braziliensis (H.N.Andrews) C.N.Mill. (see Millerocaulis dunlopii (Kidst. & Gwynne-Vaughan) Tidwell is separated into Millerocaulis dunlopii (Kidst. & Gwynne-Vaughan) Tidwell sensuOsmundites aucklandicus P.Marshall  E.I.Vera is separated into two operational units, one being based on the original type specimen (Osmundastrum precinnamomeum (C.N.Mill.) Bomfleur, G.W.Grimm & McLoughlin, which some authors consider to be a junior synonym of Osmundastrum cinnamomeum (L.) C.Presl  Gualler see  and its ill. see ; (3) Milhall see ; (4) Osmould see ; (5) Milspecimen  and the specimen ; (6) Osmresl see , is coderial see  and fromrial see  and fromrial see .The selection of extant species is adopted from Grammatopteris and Rastropteris belong to a group of filicalean ferns closely allied to Osmundales  from the analyses. This applies to  papuana . The stasing see . Claytosions see  that, hoe matrix .The character coding for the individual operational units is based on data from published literature. Wherever possible, the coding was compiled on the basis of the original protolog of the particular taxon. Data adopted from diagnoses and descriptions were critically checked with information provided by accompanying illustrations. In case information was lacking from the original text, coding of the operational unit was completed as far as possible using accompanying illustrations or additional information from later references; in cases of conflict between figured material and text descriptions, data were corrected according to the illustrations. Common problems that we encountered concern, for instance, erroneous dimensions of stem features resulting from the inconsistent use of the terms “stele,” “stem” , and “trunk” or “rhizome” . Another source of error is the inconsistency in counting stelar xylem segments, depending on whether they are counted as separate already when they are deeply divided or only when they are entirely isolated from another (“Hewitson’s method”). In any case, corrected data are indicated as such in the form of annotations either in the spreadsheet compilation of morphological features  or in thClassification of fossils should be based on morphology rather than theoretical concepts. The study of evolutionary processes, however, requires phylogenetically meaningful (as opposed to merely phenetic) “natural” taxa, and the definition of such “natural” groups requires an implicit phylogenetic framework via the recognition of evolutionary lineages of common ancestry ; Fig. 8ASignals from morphological matrices are generally complex, especially if fossil (presumably primitive) and modern (presumably derived) taxa from different time periods are pooled . With moAltogether, we tried to keep the phylogenetic analyses as simple and straightforward as possible: we used a matrix of unsorted and unweighted characters to infer neighbour-net splits graphs , 2004 baAnomorrhoea fischeri, Claytosmunda nathorstii, Osmunda kidstonii, Osmundacaulis janae, Todea papuana) were excluded from the analyses. For each set analysed, we further excluded all invariant (“constant”) characters before calculating the pairwise distances so that the resulting distance matrices and neighbour-net splits graphs are based only on those characters that are variable (and thus effective) within the focal group; hence, a 0.1 distance between two operational units indicates that they differ in 10% of those characters that are variable within the selected set of operational units. This procedure allowed us to focus on particular aspects of morphological diversity in osmundalean lineages. Furthermore, it helped interpret bootstrap support values as approximations  of the proportion of characters supporting a given phylogenetic split  (Neighbour-nets were inferred from pairwise Hamming (mean) distance matrices, and were visualized using SplitsTree v. 4.13 . Distancic tree) .The distance quality (“tree-likeness”) of the signals from the entire matrix and from taxon subsets was estimated using the matrix Delta Value (mDV) and the range of individual Delta Values (iDV) within ; File S3Mk substitution model for categorical characters, allowing for between-site variation modeled as a Gamma distribution. For LS–BS, we used the BioNJ modification (To estimate robustness of support for inferred (or hypothesized) relationships, we used non-parametric bootstrapping BS;  under thfication  of the nfication  implemenfication , a specification  in whichhttp://dx.doi.org/10.5061/dryad.270gs).All (raw) files for analyses  are provided in the SDA  trait shared by all its members  in a rooted phylogenetic tree  paraphyletic taxa as valid taxonomic units following the concept of “evolutionary classification”  will represent a published work according to the International Code of Nomenclature (ICN) for algae, fungi, and plants, and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.Osmundacaulis skidegatensis) and 0.43 (Millerocaulis tuhajkulensis), and the iDVs are largely decoupled from the proportion of missing data , but increases to BSML/NJ/MP = 56/65/18 for the least-inclusive matrix (Osmundinae) including members of this lineage  provides partly incompatible signals accompanied by substantial intertaxonomic diversity. The maximum pairwise distance found in the Osmundales matrix is 0.81; the matrix Delta Value (mDV) is relatively high with 0.38 , but is ing data . The resing data . Our mating data ; for exa lineage .The high amount of incompatible signals limits the utility of our matrix for inferring explicit trees as phylogenetic scenarios, but it can to some degree be accommodated using neighbour-nets instead. Taxa that belong to the same (putative) phylogenetic lineage (detailed below), defined by notably similar character suites and sharing traits that are not or rarely found outside the (putative) lineage, are typically grouped in the neighbour-nets. The lineage-corresponding edges are usually equivalent to the best-supported alternative(s) found in the BS pseudoreplicates. Furthermore, by adding a new taxon to the matrix (or one of the taxon subsets), its systematic affinities can be readily established  are assigned to subfamily Guaireoideae, whereas those that possess gapless steles that are composed of a spongy mix of xylem and parenchyma are assigned to a new subfamily, Itopsidemoideae subfam. nov. Osmundacaulis is included in Osmundaceae  and Millerocaulis, which is characterized by a plesiomorphic character suite including a homogeneous petiolar sclerenchyma ring without distinct fibre patches. The differentiation of this petiolar sclerenchyma ring into distinct arches and patches of thick-walled fibres is here considered diagnostic for the tribe Osmundeae, which comprises all six Osmundaceae genera with extant representatives , most of which belonging to Claytosmunda and some to Osmundastrum. Osmundacaulis estipularis from the Cretaceous of India  Shi-Jun Wang, J.Hilton, Galtier et al.; (2) institution of two new subtribes within tribus Osmundeae, i.e., Todeinae subtribus nov. and Osmundinae subtribus nov.; (3) synonymy of Zalesskya Kidst. & Gwynne-Vaughan, Petcheropteris Zalessky, and Iegosigopteris Zalessky with Thamnopteris Brongn. with the resulting new combinations: Thamnopteris diploxylon (Kidst. & Gwynne-Vaughan) comb. nov., Thamnopteris gracilis (Eichw.) comb. nov., Thamnopteris javorskii  comb. nov., Thamnopteris splendida  comb. nov., Thamnopteris uralica  comb. nov.; (4) recognition of a new species of Millerocaulis that was previously assigned to Osmundites, i.e., Millerocaulis tuhajkulensis (Gorskii ex Pryn.) comb. nov.; (5) several new combinations in Osmundeae (“modern” Osmundoideae) of species that were previously assigned to Ashicaulis and Millerocaulis, including Claytosmunda beardmorensis (J.M.Schopf) comb. nov., Claytosmunda chengii nom. nov. (replacement name for a new combination based on Ashicaulis claytoniites Y.M.Cheng), Claytosmunda johnstonii  comb. nov., Claytosmunda liaoningensis (Wu Zhang & Shao-Lin Zheng) comb. nov., Claytosmunda plumites (N.Tian & Y.D.Wang) comb. nov., Claytosmunda preosmunda  comb. nov., Claytosmunda sinica (Y.M.Cheng & C.S.Li) comb. nov., Claytosmunda tekelili (E.I.Vera) comb. nov., Claytosmunda wangii (N.Tian & Y.D.Wang) comb. nov., Claytosmunda embreei (Stockey & S.Y.Sm.) comb. nov., Osmundastrum indentatum  comb. nov., and Osmunda kidstonii (Stopes) comb. nov.; (6) new combinations in Osmundinae resulting merely from the recent elevation of former subgenera of Osmunda to separate genera  comb. nov., Plenasium arnoldii (C.N.Mill.) comb. nov., Plenasium chandleri (Arnold) comb. nov., and Plenasium dowkeri (Carruth.) comb. nov.; and (7) recognition of Aurealcaulis as a subgenus of Plenasium with the resulting new species combinations, Plenasium bransonii (Tidwell & Medlyn) comb. nov., Plenasium burgii (Tidwell & J.E.Skog) comb. nov., Plenasium crossii (Tidwell & L.R.Parker) comb. nov., Plenasium dakotense (Tidwell & J.E.Skog) comb. nov., Plenasium moorei (Tidwell & Medlyn) comb. nov., and Plenasium nebraskense (Tidwell & J.E.Skog) comb. nov. Formal taxonomic treatment of these nomenclatural novelties is provided in Appendix A.Our analysis warrants the following taxonomic changes for members of fossil Osmundales as presently understood: (1) institution of the new subfamily Itopsidemoideae (Guaireaceae) subfam. nov. to contain e genera , i.e., OMillerocaulis indicus replacing Millerocaulis indica; see Articles 23.5 and 32.2 of the Melbourne Code, 2012); terminations of honorific substantival epithets not in accordance with the sex or number of the person(s) honored, e.g., lutziae replacing lutzii  or stipabonettiorum replacing stipabonettii ; or formation of regular compounds not in accordance with classical usage, e.g., bromeliifolium replacing bromeliaefolium .Several taxon names contained orthographical or typographical errors that were corrected according to Articles 60 and 62 of the ICN for Algae, Fungi, and Plants . Corrected errors include, e.g., terminations of adjectival epithets not in accordance with the gender of the genus , with endarch protoxylem.Status: Holophyletic, extant  with fossil representatives.Known geochronologic range: Late Permian to present.Comments: The holophyly of all Osmundales, extant and extinct, is universally accepted (†) Family Guaireaceae R.Herbst, 1981Diagnostic axis characters: Stem and stipe cortex primarily parenchymatous and not differentiated into distinct layers; where known, stipes lacking stipular wings and stipe bundle with recurved tips ; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Putatively holophyletic, extinct.Known geochronologic range: Late Permian to Early Jurassic.Comments: Our subdivision of Osmundales into two main families—Osmundaceae and Guaireaceae—follows most previous studies since Guairea and Guaireaceae were established  Guaireaceae, but also highlights the relative scarcity of data for members of the family. In the all-inclusive data set, neither family received measurable support (BSML/NJ/MP < 10), nor the alternative proposed by Osmundacaulis. Nonetheless, the character suites found in the earliest Guiareaceae and Osmundaceae indicate that they represent potential sister lineages within the Osmundales and that neither one evolved from the other. The position of the Guaireaceae root is undetermined. By analogy with Osmundaceae, Itopsidemoideae may be basal within Guaireaceae considering the imperforate steles of all its members.ablished . The maiablished  is that 1.1 (†)Subfamily Guaireoideae Z.M.Li, 1993Diagnostic axis characters:Stele perforated; stem and stipe cortex primarily parenchymatous and not differentiated into distinct layers; where known, stipes lacking stipular wings and stipe bundle with strongly recurved tips; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Possibly holophyletic, extinct.Known geochronologic range: Late Permian to Early Jurassic.Comments: We recognize those Guaireaceae taxa that possess perforated (dissected) siphonosteles or dictyosteles  as being sufficiently similar to each other and sufficiently distinct from other Guaireaceae , they are typically restricted to distinct sublineages with a probable inclusive common origin. The split between Guaireoideae and Itopsidemoideae is well supported (BSML/NJ/MP ≥ 70) if the taxon set is restricted to only Guaireacae (ireaceae  that we aireacae .1.1.1 (†)Guairea R.Herbst, 1981(†) Genus Diagnostic axis characters: Stems forming large arborescent trunks (reaching up to about 10 cm in diameter); stelar xylem siphon very thick , moderately perforated; endodermis external and internal and connecting through leaf gaps; pith and stem cortex parenchymatous only and not differentiated into distinct layers; ca 20–35 leaf traces in a given stem transverse section; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Holophyletic or paraphyletic, extinct.Known geochronologic range: Late Permian to Middle Triassic.Comments:Guairea species are very similar to each other and sufficiently distinct from other Permian Osmundales to be recognized as a genus. The close similarity of the individually scored Osmundites braziliensis, which is the reason for the high BS of the according split under ML and LS/NJ (BSML/NJ ≥ 75), supports its synonymy with Guairea carnieri (see Guairea-clade would receive (moderately) high support (BSML/NJ/MP ≥ 67). However, the lack of additional Triassic Guaireaceae fossils makes it difficult to assess the relationship of Guairea and Zhongmingella to the Jurassic Lunea .ieri see . A GuairIncluded species:G. carnierii (J.Schust) R.Herbst, 1981 including Osmundites braziliensis of H.N.Andrews, 1950 .(†) G. milleri R.Herbst, 1981 (Late Permian: Paraguay).(†) References:1.1.2 (†)Lunea Tidwell, 1991Genus Diagnostic characters: Stems rhizomatous to erect; stelar xylem siphon thin , moderately perforated; endodermis external only; pith and cortices of stem and stipes parenchymatous with abundant scattered masses of sclerenchyma fibres; ca 25–35 leaf traces in a given stem transverse section; stipes lacking stipular wings and stipe bundle with strongly recurved tips; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Monotypic, extinct.Known geochronologic range: Early Jurassic.Comments: The Guaireaceae-matrix signal regarding the placement of Lunea is ambiguous  Guairea spp., Zhongmingella even more so, and all are substantially distinct from the members of Itopsidemoideae. There are no traits shared by Lunea and Zhongmingella to the exclusion of Guairea that would support a sister relationship of the former two.mbiguous . The higIncluded species:L. jonesii Tidwell, 1991 .(†) References:1.1.3 (†)Zhongmingella S.Jun Wang, J.Hilton, Xiao Y.He et al., 2014Genus Diagnostic characters: Stems rhizomatous; stelar xylem siphon moderately thick , moderately perforated, dictyostelic with internal and external phloem connecting through leaf gaps; pith and stem cortex parenchymatous with abundant scattered fibre masses; stem cortex not differentiated into distinct layers; more than 40 leaf traces in a given stem transverse section; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Monotypic, extinct.Known geochronologic range: Late Permian (Changhsingian).Comments: The pith and cortex of Zhongmingella contain structural elements that have been described variously as groups or clusters of secretory cells or of sclerenchyma  Shi-Jun Wang, J.Hilton, Xiao-Yuan He et al. 2014 .(†) References:1.2 (†)Subfamily Itopsidemoideae Bomfleur, G.W.Grimm & McLoughlin, subfam. nov.Diagnostic characters: Stelar xylem siphon lacking discrete leaf gaps, composed of a spongy admixture of metaxylem and more or less diffusely interspersed patches of parenchyma; stem and stipe cortex primarily parenchymatous and not differentiated into distinct layers; stipes lacking stipular wings and stipe bundle with recurved tips ; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Holophyletic or paraphyletic, extinct.Known geochronologic range: Late Permian to Middle Triassic.Comments: We consider the unique stele composition and structure of three Guaireaceae taxa—Itopsidema, Donwelliacaulis, and Tiania—to be so substantially different from those of the Guaireoideae to warrant the erection of this new subfamily. Itopsidemoideae might be a holophyletic sister lineage of the Guaireoideae—both lineages having appeared about the same time, and the unusual spongy metaxylem tissue representing a unique trait within the Osmundales—or include members from which the Guaireoideae with their much more complex steles have evolved, analogous to the presumed evolution of Osmundoideae from Thamnopteroideae in Osmundaceae. The less complex steles in combination with traits not found in Osmundaceae, account for the relatively high support for Itopsidemoideae vs. all other splits for the all-inclusive (BSML/NJ/MP = 42/47/32) and Guaireaceae (ireaceae  matrices1.2.1 (†)Itopsidema Daugherty, 1960Genus Diagnostic characters: Pith parenchymatous with interspersed tracheids; stelar xylem siphon thin , lacking discrete leaf gaps, composed of a spongy admixture of metaxylem and more or less diffusely interspersed patches of parenchyma; stem and stipe cortex primarily parenchymatous and not differentiated into distinct layers; numerous (reaching >100) leaf traces visible in a given stem transverse section; stipe bundle with recurved tips ; stipe cortex with masses of secretory cells; surface of stem and stipes covered in multicellular spines with interspersed trichomes; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Monotypic, extinct.Known geochronologic range: Middle Triassic.Comments: Unfortunately, the diagnosis, description, and documentation of Itopsidema and its type species I. vancleavei (T. yunnanense (see below) differs structurally from I. vancleavei; the separation of these taxa in the neighbour-net (Itopsidema + Tiania (BSML/NJ/MP = 38/36/15) as alternative to Itopsidema + Donwelliacaulis .ncleavei  followedbour-net  is partlIncluded species:I. vancleavei Daugherty, 1960 .(†) References:1.2.2 (†)Donwelliacaulis Ash, 1994Genus Diagnostic characters: Stems exceeding 25 cm in diameter and forming very large arborescent trunks reaching more than 40 cm in diameter; pith parenchymatous with interspersed tracheid bundles; stelar xylem siphon very thick , lacking discrete leaf gaps, composed of a spongy admixture of metaxylem and more or less diffusely interspersed patches of parenchyma; stem cortex primarily parenchymatous with interspersed sclerenchyma masses, not differentiated into distinct layers, containing rather few (ca 20) widely separated leaf traces in a given stem transverse section; stipe bundle inverse omega-shaped with recurved tips.Status: Monotypic, extinct.Known geochronologic range: Middle Triassic.Comments: Hundreds of Donwelliacaulis trunk fragments have been collected from the Holbrook Member of the Moenkopi Formation of east-central Arizona . Their close relationship is reflected by the BS values for the according split .Cells that were originally interpreted as protoxylem are “[…] m thick” : 6. ThesIncluded species:D. chlouberii Ash, 1994 .(†) References:1.2.2 (†)Tiania Shi-Jun Wang, J.Hilton, Galtier et al., 2014Genus Diagnostic characters: Pith primarily parenchymatous; stelar xylem siphon thin , lacking discrete leaf gaps, composed of a spongy admixture of metaxylem and more or less diffusely interspersed patches of parenchyma; stem cortex primarily parenchymatous and not differentiated into distinct layers, containing scattered masses of secretory cells and sclerenchyma; numerous (reaching >100) leaf traces visible in a given stem transverse section; leaf trace protoxylem single and mesarch in stele, dividing into two and becoming endarch before departure from stele; leaf trace with more than four endarch protoxylem strands upon departure from stem; roots commonly arising from abaxial side of leaf trace within the stem cortex.Status: Problematic (possibly synonymous with Itopsidema).Known geochronologic range: Late Permian.Comments:Tiania appears to differ from Itopsidema mainly in preservational aspects and in the accompanying protolog: the former has more fully documented features of the leaf traces but lacks preserved stipes  Shi-Jun Wang, J.Hilton, Galtier et al., 2014a .(†) References:2Family Osmundaceae Martinov 1820Diagnostic axis characters:Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Stipes with a pair of stipular wings; stipe bundle with incurved tips . Peripheral xylem siphon typically with leaf gaps (except some Thamnopteroideae).Status: Natural, possibly paraphyletic with respect to Guaireaceae, extant with fossil representatives.Known geochronologic range: Late Permian to present.Comments: Unlike Osmundacaulis and Shuichengella in Osmundaceae instead of Guaireaceae. We argue that Osmundacaulis may appear affiliated with Guaireaceae, and with Guairea in particular  the two-layered cortex of stems and stipes with an inner, primarily parenchymatous cylinder and an outer sclerenchymatous cylinder ; (2) a C-shaped or horseshoe-shaped vascular bundle with more or less incurved tips in the stipe base (as opposed to omega-shaped or mushroom-shaped bundles with recurved tips); (3) the presence of stipular wings; and (4) the formation of a prominent mantle of many persistent leaf bases and roots (as opposed to a mantle of mainly roots with no or few persistent stipe bases). These features are invariant characteristics distinguishing Osmundaceae (including Osmundacaulis) from Guaireaceae, and represent conserved traits in all extant Osmundaceae. We consider these latter features to have much greater phylogenetic and systematic significance than those used previously, and re-assign Osmundacaulis to Osmundacaeae. Shuichengella is also assigned to Osmundaceae based on the observation that it has a similarly two-layered cortex. The fact that it is not resolved as a close relative in the Osmundales neighbour-net . Hence, if Shuichengella is accurately interpreted as belonging to Osmundaceae, it probably is part of the same lineage that gave rise to Osmundacaulis. If future studies should produce further support for this hypothesis, it may become appropriate to re-institute the subfamily Shuichengelloideae Z.M.Li, 1993 for these two genera.By contrast, bour-net  but nest2.1(†) Subfamily Thamnopteroideae C.N.Mill., 1971Diagnostic characters: Stems typically forming large, arborescent trunks. Stem core consisting primarily of tracheids. Peripheral metaxylem siphon typically entire and imperforate [most taxa] or with few leaf gaps  or sparse perforations [Chasmatopteris]. Leaf trace protoxylem initiating subexarch in peripheral bulge, becoming endarch typically in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Stipes with a pair of stipular wings; stipe bundle with incurved tips .Status: Holophyletic or paraphyletic, extinct.Known geochronologic range: Late Permian.Comments: Species included in this subfamily are rather similar to each other; the group-limited matrix  comprises only 20 partly defined and variable characters (sites). Species included in Thamnopteris (nine variable sites) are generally similar in their preserved traits, hence 10 out of 55 pairwise distances are zero, and little phylogenetic structuring is evident  Genus Synonyms here assigned:Iegosigopteris Zalessky, 1935.Petcheropteris Zalessky, 1931.Zalesskya Kidst. & Gwynne-Vaughan, 1909.Diagnostic characters: Stems typically forming large, arborescent trunks. Stem core consisting primarily of tracheids. Peripheral metaxylem siphon typically entire and imperforate. Leaf trace protoxylem initiating subexarch in peripheral bulge, becoming endarch typically in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Stipes with a pair of stipular wings; stipe bundle with incurved tips .Status: Putatively monophyletic, extinct.Known geochronologic range: Late Permian.Comments: Many species and several genera of large, “protostelic” osmundaceous trunks have been described from Upper Permian strata of the Ural Mountains  and of Zalesskya   . FurtherThamnopteris javorskii (13 out of 20 characters unambiguously defined) and Thamnopteris splendida (15/20). Interspecies relationships are largely unclear; a limiting factor is the lack of overlap in the unambiguously defined characters. For instance, the 0.42 distance between Thamnopteris javorskii and Thamnopteris kidstonii (16/20 characters unambiguously defined) translates into six different out of 14 unambiguously defined characters for both species.Overall, the most distinct species of the genus (pairwise distances of 0.21–0.42 based on the Thamnopteroideae-matrix) are Included species:T. diploxylon  comb. nov. (Late Permian: Russia).(†) T. gracilis  comb. nov. (Late Permian: Russia).(†) T. gwynnevaughanii Zalessky, 1924 (Late Permian: Russia).(†) T. javorskii  comb. nov. (Late Permian: Russia).(†) T. kazanensis Zalessky, 1927 (Late Permian: Russia).(†) T. kidstonii Zalessky, 1924 (Late Permian: Russia).(†) T. schlechtendalii (Eichw.) Brongn., 1849 (Late Permian: Russia).(†) T. splendida  comb. nov. (Late Permian: Russia).(†) T. uralica  comb. nov. (Late Permian: Russia).(†) References:2.1.2Chasmatopteris Zalessky, 1931(†) Genus Diagnostic characters: Stems typically forming large, arborescent trunks. Stem core consisting primarily of tracheids. Peripheral metaxylem siphon with leaf gaps and sparse perforations. Leaf trace protoxylem initiating subexarch in peripheral bulge, becoming endarch typically in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Stipes with a pair of stipular wings; stipe bundle with incurved tips .Status: Monotypic, extinct.Known geochronologic range: Late Permian.Comments: In light of our critical reappraisal of the significance of the degree of stele perforation for systematic classification , into which C. principalis would otherwise be merged. However, given that the entire subfamily Thamnopteroideae is otherwise composed of species with consistently imperforate xylem siphons, we consider the combination of having a stem core of more or less entirely tracheids (“protostelic”) but a truly perforated (“dictyoxylic”) stelar xylem siphon to be so unique as to warrant the separate generic status of C. principalis within Thamnopteroideae. Otherwise, the special significance of C. principalis as the single taxon having a stele construction somewhat intermediate between that of the remaining Thamnopteroideae and those of Osmundoideae might not be adequately emphasized in its systematic classification.see also , one canChasmatopteris is the most distinct (derived) known Thamnopteroideae . Should Chasmatopteris represent an extinct genus in its own right, it probably evolved from a Thamnopteris-like ancestor, rendering the latter genus paraphyletic.Included species:C. principalis Zalessky, 1931[a] (Late Permian: Russia).(†) References:2.2Subfamily Osmundoideae R.Br. ex Sweet, 1826Diagnostic characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon usually thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Inner cortex usually thinner than outer cortex, rarely about equally thick. Stipes with a pair of stipular wings; stipe bundle with incurved tips .Status: Holophyletic, extant with fossil representatives.Known geochronologic range: Permian to present.Comments: The signal of characters instrumental for the recognition of subfamily Osmundoideae is completely outcompeted by the signal from homoplastic characters and differentiation within its sublineages in the case of the all-inclusive Osmundales matrix and the Osmundaceae matrix (BSML/NJ/P < 10). Further taxon-reduction , however, has the effect that the most distinct genera within the Osmundoideae  not only receive higher support from BS , but also has a lesser proportion of missing and ambiguous data cells (16% vs. 20%). from BS  but also2.2.1Palaeosmunda R.E.Gould, 1970(†) Genus Diagnostic characters: Stems erect, forming arborescent trunks. Stem core a primarily parenchymatous pith. Stelar xylem siphon thin with variably developed leaf gaps. Leaf trace protoxylem poles initially single and in mesarch position in stele, becoming endarch and, in some cases, first bifurcating before departure from stele; leaf trace protoxylem further dividing repeatedly within stem cortex; leaf trace departing from stem with at least four (commonly more than 10) protoxylem strands. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder, containing few to many (reaching ca 45) leaf traces in a given stem transverse section; inner cortex about as thick as outer cortex. Stipes with a pair of stipular wings; stipe bundle with incurved tips ; stipe sclerenchyma cylinder somewhat rhombic to fusiform in cross-section, with lateral margins usually thinner than abaxial and adaxial portions, distally becoming extended laterally into flanges that partially or completely replace the stipules.Status: Probably holophyletic, extinct.Known geochronologic range: Late Permian, possibly extending to the Late Triassic.Comments: The position of Palaeosmunda with respect to the other Osmundoideae genera and in particular the (putatively) paraphyletic Millerocaulis is not entirely clear . Nevertheless, the genus seems to represent an early-diverged, distinct sister lineage when analyzed in a larger taxonomic context  we refrain from using Millerocaulis as a “waste-basket taxon” for left-over species that cannot be positively assigned to any of the other taxa of Osmundoideae; (2) all our subset analyses consistently place Millerocaulis stipabonettiorum closer to Palaeosmunda than to any other representative of Millerocaulis ; and (3) generic assignment of this species should thus be considered provisional anyway, we tentatively group Millerocaulis stipabonettiorum with Palaeosmunda and exclude it from our analyses of Millerocaulis .(†) P. williamsii R.E.Gould, 1970 .(†) Tentatively included:Millerocaulis”  stipabonettiorum (Late Triassic: Argentina).(†) “References:2.2.2Millerocaulis Erasmus ex Tidwell emend. E.I.Vera, 2008(†) Genus Diagnostic characters: Stems usually rhizomatous to small (semi-)erect. Stem core a primarily parenchymatous pith. Stelar xylem siphon usually thin  and with prominent leaf gaps. Protoxylem poles in stele initially single and mesarch, becoming endarch in stele or (rarely) in cortex. Cortex of stems and stipes two-layered, differentiated into thin inner, primarily parenchymatous cylinder and thick outer sclerenchymatous cylinder. Stipes with a pair of stipular wings; stipe bundle with incurved tips . Sclerenchyma ring in stipe base circular to elliptic in cross-section, homogenous to gradually or diffusely heterogeneous.Status: Probably paraphyletic with respect to Osmundeae.Known geochronologic range: Triassic to mid-Cretaceous.Comments:Millerocaulis Erasmus ex Tidwell emend. AshicaulisMillerocaulis Tidwell emend. E.I.Vera, which we follow here.Millerocaulis, explicitly circumscribed here as a paraphyletic taxon, can be either precursors or belong to sister lineages of the “modern” Osmundaceae (classified as Osmundeae). In the light of the notable size and disparity of the genus, it is tempting to subdivide it further. With the available information, however, a consistent further subdivision is difficult to achieve. The neighbour-net focussing on Millerocaulis is essentially a “spider-web” (http://dx.doi.org/10.5061/dryad.270gs), illustrating the lack of consistent sorting signals in the underlying matrix. The traditional subdivision based on the degree of stele perforation  is, therefore, not supported by any further evidence. Furthermore, the data compiled here show that imperforate and highly perforate steles are end members of a complete transformation series in Osmundoideae and, thus, provide a poor basis for classification above species level. A much deeper understanding of character conservation and evolution in the Osmundoideae would be necessary to propose a finer subdivision. Moreover, it is conspicuous that the network ; we suspect that more complete knowledge of the natural variability of anatomical features of the constituent species might prove some of those to represent variants of the same natural species, as has been suggested for Millerocaulis dunlopii and Millerocaulis aucklandicus . The name Millerocaulis should be reserved for Osmundoideae rhizomes with a distinctly plesiomorphic character suite compared with the other genera of the subfamily and, in particular, Osmundeae.Although Included species:Millerocaulis s.str.” group“ :M. chubutensis (R.Herbst) Tidwell, 1994 (Late Jurassic: Argentina).(†) M. donponii Tidwell & Clifford, 1995 .(†) M. dunlopii (Kidst. & Gwynne-Vaughan) Tidwell, 1994, including Osmundites aucklandicus P.Marshall, 1926 .(†) M. indicus (B.D.Sharma) Tidwell, 1994 (Early Cretaceous: India).(†) M. juandahensis Tidwell & Clifford, 1995 .(†) M. limewoodensis Tidwell & Clifford, 1995 .(†) Ashicaulis” group“ :M. amarjolensis (B.D.Sharma) Tidwell, 1986 (Early Cretaceous: India).(†) M. australis (E.I.Vera) E.I.Vera, 2008 (Early Cretaceous: West Antarctica).(†) M. beipiaoensis  comb. nov. .(†) M. broganii Tidwell, Munzing & M.R.Banks, 1991 .(†) M. gibbianus (Kidst. & Gwynne-Vaughan) Tidwell, 1986 .(†) M. guptai (B.D.Sharma) Tidwell, 1986 (Early Cretaceous: India).(†) M. hebeiensis (Ziquiang Wang) Tidwell, 1986 .(†) M. herbstii (S.Archang. & de la Sota) Tidwell, 1986 (Late Triassic: Argentina).(†) M. livingstonensis (Cantrill) E.I.Vera, 2008 (Late Cretaceous: West Antarctica).(†) M. lutziae (R.Herbst) Herbst, 2006 (Late Triassic: Argentina).(†) M. macromedullosus  E.I.Vera, 2008 .(†) M. patagonicus (S.Archang. & de la Sota) Tidwell, 1986 (Middle–Late Jurassic: Argentina).(†) M. rajmahalensis (K.M.Gupta) Tidwell, 1986 (Early Cretaceous: India).(†) M. richmondii Tidwell, 1992 .(†) M. sahnii (Vishnu-Mittre) Tidwell, 1986 (Early Cretaceous: India).(†) M. santaecrucis (R.Herbst) R.Herbst, 1995 (Middle–Late Jurassic: Argentina).(†) M. spinksii Tidwell, Munzing & M.R.Banks, 1991 .(†) M. swanensis Tidwell, Munzing & M.R.Banks, 1991 .(†) M. wadei (Tidwell & S.R.Rushforth) Tidwell, 1986 .(†) M. websteri Tidwell, Munzing & M.R.Banks, 1991 .(†) M. woolfei  E.I.Vera, 2008 (Middle Triassic: East Antarctica).(†) M. wrightii Tidwell, Munzing & M.R.Banks, 1991 .(†) Millerocaulis kolbei” type“ M. kolbei (Seward) Tidwell, 1986 (Cretaceous: South Africa).(†) References:2.2.3Tribus Osmundeae Hook. ex Duby, 1828 (“modern” Osmundaceae)Diagnostic axis characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon typically thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder. Inner cortex usually thinner than outer cortex. Stipes with a pair of stipular wings; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous: differentiation beginning in the stipe base usually with the formation of an abaxial arch of particularly thick-walled fibers that, distally, may differentiate further into massive sclerotic rings, patches, or arches in characteristic configurations.Status: Natural, holophyletic.Known geochronological range: Triassic to present.Comments: Traditionally, the occurrence of a heterogenous sclerenchymatic ring in the stipe bases has been used to differentiate modern Osmundaceae  from extinct, ancient Osmundoideae . Under the widely shared assumption that all extant Osmundaceae are holophyletic , the lineage including Claytosmunda, Osmunda, Osmundastrum, and Plenasium with two subgenera .phyletic , the hetibes see : Todeina2.2.3.1Subtribus Todeinae Bomfleur, G.W.Grimm & McLoughlin subtrib. nov.Diagnostic stem characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon typically thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex, first bifurcating as leaf trace departs from stem. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex usually thinner than outer stem cortex; outer stem cortex heterogeneous, with a distinct ring of fibers surrounding each leaf trace. Stipes with a pair of stipular wings; stipe bundle with incurved tips ; stipe sclerenchyma ring heterogeneous, differentiating upwards into a thin band of particularly thick-walled fibers forming the outer margin of the sclerenchyma ring.Status: Holophyletic; extant with fossil representatives.Known geochronologic range: Early Cretaceous to present.Comments: Our concept of a Todeinae subtribe inferred solely on the basis of stem anatomy is equivalent to the informal “leptopteroid clade” of i) can be derived from the basic type represented in Osmundinae (“paraphyletic Osmunda” scenario discussed by ii) represent their actual sister lineage (“monophyletic Osmunda” scenario of Todea-type vs. Claytosmunda/Osmundastrum-type) found from the Triassic onwards  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex, first bifurcating as leaf trace departs from stem. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex usually thinner than outer stem cortex, containing a patch of thick-walled fibers adaxial to each leaf trace; outer stem cortex heterogeneous, with a distinct ring of fibers surrounding each leaf trace. Stipes with a pair of stipular wings; stipe bundle with incurved tips ; stipe inner cortex with numerous small sclerenchyma strands scattered throughout; stipe sclerenchyma ring heterogeneous, differentiating upwards into a thin band of particularly thick-walled fibers forming the outer margin of the sclerenchyma ring.Status: Holophyletic, extant with one putative fossil representative.Known geochronologic range: Early Cretaceous to present.Comments: Recent phylogenetic analyses support the inclusion of the fossil Todea tidwellii in the Todea–Leptopteris lineage, but are ambiguous regarding the question whether it represents a potential precursor or relative of modern Todea (making Todea as defined here holophyletic) or related to the common ancestor of Todea and Leptopteris  , which provides ample support for the conclusion of Todea tidwellii is an early respresentative of the modern genus Todea.hyletic) . The Earcenarios . In the see also , and a cIncluded species:T. barbara (L.) Moore, 1857 .T. papuana Hennipman, 1968 (Extant: Papua New Guinea).T. tidwellii Jud, G.W.Rothwell & Stockey, 2008 .(†) References:2.2.3.1.2Leptopteris C.Presl, 1845Genus Diagnostic axis characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon typically thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex, first bifurcating as leaf trace departs from stem. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex usually thinner than outer cortex, lacking sclerenchyma patches; outer stem cortex heterogeneous, with a distinct ring of fibers surrounding each leaf trace. Stipes with a pair of stipular wings; stipe bundle with incurved tips ; stipe inner cortex lacking sclerenchyma strands; stipe sclerenchyma ring heterogeneous, differentiating distally into a thin band of particularly thick-walled fibers forming the outer margin of the sclerenchyma ring.Status: Holophyletic, extant with one equivocal fossil record.Known geochronologic range: Possibly Early Cretaceous to present.Comments: A single, rather poorly preserved specimen of a fossil osmundaceous stem from Lower Cretaceous strata of the Rajmahal Hills, India, was described under the name Osmundacaulis estipularis B.D.Sharma, D.Bohra & R.Singh, 1979. The species was supposed to be distinct from most other Osmundaceae in lacking stipular wings (hence the epithet estipularis = lat. “without stipules”). We disagree with this assumption, and point out several features of the specimen based on which we propose an alternative interpretation: (1) preservation of the holotype is imperfect, and parenchyma is mostly lacking throughout the specimen; (2) there are abundant transverse sections of unusually thick roots visible between the sclerenchyma cylinders of stipes throughout the mantle of the trunk; and (3) the composition of the outer stem cortex appears heterogeneous with clearly distinct rings surrounding each leaf trace already in the outer cortex. Based on these observations we argue that the type and only specimen of Osmundacaulis estipularis represents a poorly preserved fossil of an arborescent osmundoid trunk in which the strong vertical roots penetrating downwards through the mantle, together with the imperfect preservation of parenchyma, hamper the identification of stipular wings. Furthermore, the outer stem cortex is heterogeneous in a manner typical of Todea and Leptopteris, and what is visible of the inner cortex shows no evidence of sclerenchyma patches that would be diagnostic of Todea; in fact, the entire aspect of the specimen is very similar to the basal stem section of the extant arborescent Leptopteris wilkesiana reproduced by Leptopteris yet discovered, but definite assignment should await a re-investigation of the type material.Todea, character suites of Leptopteris rhizomes provide a highly ambiguous phylogenetic signal as reflected in the much lower supports for a potential Leptopteris clade . Under ML, a competing, presumably inaccurate split . Mutual holophyly of Leptopteris and Todea, however, is well-supported by distance data  C.Presl, 1845 .L. hymenophylloides (A.Rich.) C.Presl, 1845 .L. superba (Colenso) C. Presl, 1848 .L. wilkesiana (Brack.) Christ, 1897 .Tentatively included:Leptopteris estipularis  comb. nov. (Early Cretaceous: India).(†)?References:2.2.3.2Subtribus Osmundinae Bomfleur, G.W.Grimm & McLoughlin subtrib. nov.Diagnostic stem characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon typically thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings commonly containing strands of thick-walled fibers of various shapes and sizes; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous: differentiation typically initiating in the stipe base with the formation of an abaxial arch of particularly thick-walled fibers.Status: Holophyletic  lack the derived features typical of its modern relatives, and have many presumably primitive traits shared with species of Millerocaulis. Consequently, pairwise distances between (early) members of the Osmundinae and of Millerocaulis can be smaller than those within each lineage, which may explain the poor support for an Osmundinae clade and the Osmundastrum- and Osmunda-lineages , whereas Claytosmunda, which probably represents the most primitive suite of axis anatomical characters  Metzgar & Rouhan, 2016Genus Diagnostic axis characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon typically thin  and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex, first bifurcating in outermost cortex or outside the stem. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings commonly containing strands of thick-walled fibers of various shapes and sizes; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous with an abaxial arch of particularly thick-walled fibers that may further develop into two opposite, separate or thinly connected lateral masses.Status: Partly ancestral (paraphyletic per definition) to Osmunda, Plenasium, and possibly Osmundastrum; extant with fossil representatives.Known geochronologic range: Middle Triassic to present.Comments: In a previous phylogenetic analysis with fewer characters and fewer operational units, we concluded that “[…] et al. and Claytosmunda generates two taxa without discriminating anatomical and morphological featuresa subdivision into two putatively monophyletic subgenera Osmunda sensu Yatabe ”  anatomical characters as outlined above. The plesiomorphic features of Claytosmunda include the protoxylem division occurring only outside the stem (shared with Osmundastrum) and the sporadic occurrence of mesarch leaf traces in the inner or rarely outer cortex . The main diagnostic rhizome trait is the composition of the heterogenous sclerenchymatic ring in the leaf trace.eatures” : 15f. BaMillerocaulis are here formally transferred to Claytosmunda. In most cases, the similarity and inferred relationship of each of these species to Osmundinae in general and to genus Claytosmunda in particular  genera of the Osmundinae .Several fossils currently included in raphyly” : 217. Alhe genus . Precursmundinae  and 15. Claytosmunda nathorstii from the Paleogene of Svalbard is known only from isolated stipes and was thus excluded from the phylogenetic analyses owing to it lacking too many characters, the species is clearly allied with Claytosmunda based on its characteristic stipe structure with two lateral masses of thick-walled fibers in the sclerenchyma cylinder and with an elongate mass of fibers in each stipular wing  Metzgar & Rouhan, 2016 (Extant: East Asia and eastern North America).C. beardmorensis  comb. nov. (Middle Triassic: East Antarctica).(†) C. chengii nom. nov. .(†) C. johnstonii  comb. nov. .(†) C. embreii  comb. nov. .(†) C. liaoningensis  comb. nov. .(†) C. nathorstii  comb. nov. .(†) C. plumites (N.Tian & Y.D.Wang 2014[a]) comb. nov. .(†) C. preosmunda  comb. nov. .(†) C. sinica  comb. nov. .(†) C. tekelili  comb. nov. (Early Cretaceous: West Antarctica).(†) C. wangii  comb. nov. .(†) C. wehrii  comb. nov. .(†) References:2.2.3.2.2Osmundastrum C.Presl, 1847Genus Diagnostic stem characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon thin and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele, first bifurcating in outermost cortex or outside the stem. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings commonly containing strands of thick-walled fibers of various shapes and sizes; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous with an abaxial arch or mass and two opposite, separate or thinly connected lateral masses of particularly thick-walled fibers.Status: Holophyletic; extant with fossil representatives.Known geochronologic range: Triassic to present.Comments: The recently described Osmunda pulchella Bomfleur, G.W.Grimm & McLoughlin from the Lower Jurassic of Sweden is sufficiently similar to extant and other fossil Osmundastrum species to warrant assignment to this genus , and accordingly placed in the neighbour-nets.is genus ; Fig. 16Osmundastrum precinnamomeum from the Paleocene of North America  fossil Osmundastrum cinnamomeum are better treated as distinct species requires a better understanding of the extant diversity in this species. We note that the diversity evident in the fossils is as high or higher than between species of other (extinct and extant) genera  nevertheless form a coherent group distinct from other Osmundinae or Osmundoideae; a corresponding clade would receive moderate support under ML and LS  comb. nov. from the Lower Jurassic of Sweden is a probable precursor of modern Osmundastrum, which ranges from the Late Cretaceous onwards (Osmundastrum and early representatives with putatively primitive axis anatomy as seen in early Claytosmunda. This placement underscores its great similarity to its more derived congeners (MD ≥ 0.08) and to early members of the Claytosmunda–Osmunda(–Plenasium) lineage (MD ≥ 0.10). If forced into a tree, Osmundastrum pulchellum would be placed close to modern Osmundastrum (http://dx.doi.org/10.5061/dryad.270gs), as expected for an early member of this lineage. America  is more see also ; a Osmun onwards . Hence, Osmundastrum and the remainder of the Osmundinae , this species is most similar to the Jurassic Claytosmunda embreii, and to a lesser degree, Claytosmunda sinica and Claytosmunda preosmunda; corresponding clades would receive low but consistent support based on our matrices  or generally homoplastic features encoded in our matrix among the earliest members of both lineages within Osmundinae . This example cautions that classification of fossil taxa should always consider the actual traits shared or not by two taxa resolved as potential sisters in trees or grouped in networks. A further hint to the existence of two already differentiated, modern lineages in the Triassic is the higher diversity between the Triassic Osmundinae species  in comparison to the situation from the Jurassic onwards .Molecular dating using the frond fossil record confirms a Triassic split between mundinae . Triassindentata , which hmatrices . Given tIncluded species:O. cinnamomeum (L.) C.Presl, 1847 .O. precinnamomeum , comb. nov. .(†) O. pulchellum  comb. nov. (Early Jurassic: Sweden).(†) O. indentatum  comb. nov. .(†) References:2.2.3.2.3Osmunda L., 1753Diagnostic axis characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon thin and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or rarely in cortex; leaf trace either arising with single protoxylem strand that bifurcates in inner or outer cortex or rarely arising from stele already with two protoxylem strands. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings commonly containing strands of thick-walled fibers of various shapes and sizes; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous developing two opposite lateral masses that may extend and conjoin into an adaxial arch of particularly thick-walled fibers.Status: Holophyletic; extant with fossil representatives.Known geochronologic range: Paleocene to present.Comments: Based only on axis anatomy, it is difficult to distinguish clearly between Osmunda and the paraphyletic Claytosmunda, which comprises stem group members of the Osmundinae and members of the actual lineage leading to the extant Claytosmunda claytoniana (note the placement of the extant Osmunda lancea in neighbour-nets). According to a recent molecular dating that makes use of the entire rhizome and frond fossil record of modern Osmundaceae, the Claytosmunda claytoniana-lineage diverged from the Osmunda–Plenasium lineage sometime in the Jurassic  still appear intermediate between extant members of Osmunda and Claytosmunda in their development of leaf trace protoxylem and development of fibre masses in the sclerenchyma ring. This is also well reflected by their intermediate position in the neighbour-nets. Some fossil axes show the modern Osmunda feature of the leaf trace protoxylem dividing already in the inner cortex, but also the retention of just two separate lateral fibre masses in the sclerenchyma ring instead of an adaxial arch, as is typical of Claytosmunda. Hence, we restrict Osmunda to only those species that have the adaxial arch in the sclerenchyma ring, and retain other early potential members of the Osmunda-lineage in Claytosmunda until more explicit phylogenetic hypotheses can be presented.Jurassic , with OsIncluded species:O. japonica Thunb., 1780 (Extant: East Asia).O. lancea Thunb., 1784 (Extant: East Asia).O. regalis L., 1753 .O. ilianensis C.N.Mill., 1967 .(†) O. oregonensis (C.A.Arnold) C.N.Mill., 1967 .(†) O. pluma C.N.Mill., 1967 .(†) O. shimokawaensis M.Matsumoto & H.Nishida, 2003 .(†) References:2.2.3.2.4Plenasium C.Presl, 1836Diagnostic stem characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele; leaf trace adaxially curved upon departure from stele, usually arising with two protoxylem strands from two adjacent stelar xylem segments. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings typically containing numerous, small scattered strands of thick-walled fibres; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous, differentiating distally into a thin band of particularly thick-walled fibres forming the outer margin of the sclerenchyma ring.Status: Holophyletic; extant with fossil representatives.Known geochronologic range: Early Cretaceous to present.Comments:Plenasium species differ from all remaining Osmundoideae in that their leaf traces originate typically from two independent protoxylem poles in two adjacent xylem segments.2.2.3.2.4.1Plenasium subgenus Aurealcaulis (Tidwell & L.R.Parker), 1987, comb. et stat. nov.(†) Diagnostic characters: Stems reaching great size; steles highly perforate and dissected, comparatively thick ; leaf traces usually originating from two independent protoxylem poles from two adjacent stelar xylem segments (similar to Plenasium), departing from stele usually in the form of two separate bundles that fuse into a single C-shaped strand in their course through the cortex. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings typically containing a single or numerous distinct masses, commonly in elongate shape or arrangement; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous, differentiating distally into a thin band of particularly thick-walled fibres forming the outer margin of the sclerenchyma ring.Status: Possibly ancestral (paraphyletic per definition) to subgenus Plenasium; extinct.Known geochronologic range: Early Cretaceous to ?mid-Eocene.Comments: Since Aurealcaulis was established  and wall thickness (10–25 μm). However, a layer of fibres near the edge of the sclerotic ring is much thicker-walled and appears to be more resistant to decay than the remainder of the ring”  a typical member of Osmundaceae with mesarch protoxylem in the stem and endarch protoxylem in the leaf traces; (2) a member of Osmundeae (modern Osmundoideae) with a heterogeneous stipe sclerenchyma ring similar to those of Todea, Leptopteris, and Plenasium; (3) an extinct member of Osmundinae owing to its homogeneous outer cortex  comb. nov. .(†) P. burgii  comb. nov. .(†) P. crossii  comb. nov. .(†) P. dakotense  comb. nov. .(†) P. moorei  comb. nov. .(†) P. nebraskense  comb. nov. .(†) References:2.2.3.2.4.2Plenasium subgenus PlenasiumDiagnostic stem characters: Stem core a primarily parenchymatous pith. Stelar xylem siphon thin  and with prominent leaf gaps; leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele; leaf trace strongly curved adaxially upon departure from stele, usually arising with two protoxylem strands from two adjacent stelar xylem segments. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings typically containing numerous, small scattered strands of thick-walled fibres; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous, differentiating distally into a thin band of particularly thick-walled fibres forming the outer margin of the sclerenchyma ring.Status: Holophyletic; extant with fossil representatives.Known geochronologic range: Paleocene to present.Included species:P. banksiifolium (C.Presl) C.Presl, 1836 (Extant: East and Southeast Asia).P. bromeliifolium (C.Presl) C.Presl, 1836 (Extant: East and Southeast Asia).P. javanicum (Blume) C.Presl, 1848 (Extant: Southeast Asia).P. vachelii C.Presl, 1848 (Extant: Southeast Asia).P. arnoldii , comb. nov. .(†) References:2.2.3.2.4.?Plenasium subgenus indet.Comments:Plenasium dowkeri and Plenasium chandleri from the Palaeogene of North America and Europe occupy intermediate positions between species of subgenus Plenasium and species of subgenus Aureacaulis in the neighbour-nets  comb. nov. .(†) P. dowkeri  comb. nov. .(†) 2.?Family Osmundaceae, subfamily unknown2.?.1Osmundacaulis C.N.Mill., 1967 emend. Tidwell, 1986(†) Genus Diagnostic characters: Axes usually arborescent or erect, rarely rhizomatous. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex thicker than outer stem cortex. Peripheral xylem siphon with prominent leaf gaps, very thick , commonly highly perforated ; phloem external and internal and sporadically connecting through leaf gaps . Leaf trace protoxylem initially single and in mesarch position in stele, in most cases dividing repeatedly in stele and inner cortex (except Osmundacaulis bamfordae); leaf trace strongly curved already upon departure from stele. Stipes with a pair of stipular wings; stipe bundle with incurved tips .Status: Putatively holophyletic, extinct.Known geochronologic range: Jurassic to Cretaceous.Comments:Osmundacaulis is clearly distinct from other Osmundaceae in having an inner cortex that is thicker than the outer cortex, in having very thick and typically highly perforated stelar metaxylem cylinders, and in having leaf traces that are already strongly curved at the point of departure from the stele. These features served to distinguish first the species Osmundites skidegatensis Penh.  to only a few , or no perforations at all (Osmundacaulis lemonii).enh. see  and lates group”  that subIncluded species:O. andrewii Tidwell & Pigg, 1993 .(†) O. atherstonei (Schelpe) C.N.Mill., 1971 (Early Cretaceous: South Africa).(†) O. bamfordae R.Herbst, 2015 (Early Cretaceous: South Africa).(†) O. griggsii Tidwell & Pigg, 1993 .(†) O. hoskingii R.E.Gould, 1973 .(†) O. janae Tidwell & Pigg, 1993 .(†) O. jonesii Tidwell, 1987 .(†) O. lemonii Tidwell, 1990 .(†) O. natalensis (Schelpe) C.N.Mill., 1971 (Early Cretaceous: South Africa).(†) O. nerii Tidwell & Ross Jones, 1987 .(†) O. pruchnickii Tidwell & Pigg, 1993 .(†) O. richmondii Tidwell & Pigg, 1993 .(†) O. skidegatensis (Penh.) C.N.Mill., 1967 .(†) O. tasmanensis Tidwell & Pigg, 1993 .(†) O. tehuelchensis R.Herbst, 2003 (Middle Jurassic: Argentina).(†) O. tidwellii R.Herbst, 2015 (Early Cretaceous: South Africa).(†) O. whittlesii McKenzie, A.Smith, G.W.Rothwell et al., 2015 .(†) O. zululandensis R.Herbst, 2015 (Early Cretaceous: South Africa).(†) References:2.?.2Shuichengella Z.M.Li, 1993(†) Genus Diagnostic characters: Axes large, erect to arborescent. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex much thicker than outer stem cortex, containing numerous (up to 60) leaf traces in a given transverse section. Peripheral xylem siphon with sporadic narrow, complete leaf gaps, thin . Leaf trace strongly curved upon departure from stele, containing three protoxylem clusters in inner cortex.Status: Monotypic, extinct.Known geochronologic range: Late Permian.Comments: The precise systematic affinities of Shuichengella primitiva within Osmundaceae are unknown. However, it is notable that the genus shares some rather unusual characters with Osmundacaulis, e.g., an inner cortex that is much thicker than the outer cortex, and the strong curvature and multiple protoxylem clusters of leaf traces already differentiated upon departure from the stele. Shuichengella to be too poorly known to warrant erection of a separate subfamily.Included species:S. primitiva (Z.M.Li) Z.M.Li, 1993 .(†) References:2.?.3Anomorrhoea Eichw., 1860(†) Genus Status:Nomen dubium, extinct.Known geochronologic range: Late Permian.Comments: The holotype is a fragment of the mantle of roots and stipe bases with a small portion of outer cortex attached; stele, inner cortex, and most of the outer cortex is missing. It is, thus, too fragmentary to be identified with certainty .(†) References.2.?.4Bathypteris Eichw., 1860(†) Genus Diagnostic characters: Stems forming large, arborescent trunks. Stem core consisting of tracheids with scalariform thickenings. Peripheral metaxylem siphon entire and imperforate. Cortex of stems and stipes two-layered, differentiated into inner parenchymatous cylinder and outer sclerenchymatous cylinder. Leaf trace protoxylem single. Stipes lacking stipular wings but bearing multicellular spines; stipe bundle with incurved tips , surrounded by sclerotic cells that form a narrow lining band. Sclerenchyma ring in stipe base circular to elliptic in cross-section, homogenous.Status: Monotypic, extinct.Known geochronologic range: Late Permian.Comments: The affinities of Bathypteris cannot be determined precisely; it has a two-layered cortex and horseshoe-shaped stipe bundle typical of Osmundaceae, but its petiole surfaces bear multicellular spines instead of the characteristic stipular wings—a feature otherwise known only from Itopsidema among Osmundales  Eichw., 1860 (Late Permian: Russia).(†) References:This study provides a comprehensive comparative dataset and the methodological tool-kit to describe, identify and classify fossil osmundalean axes. A phylogenetic framework assigns explicit biological meaning to the names of fossil and extant taxa. The design of our character matrix is optimized for these purposes. Information on the anatomy of forthcoming finds of fossil osmundalean axes can easily be incorporated into the matrix in order to facilitate systematic placement, comparisons, and character analysis. We propose natural groups that can be identified based on diagnostic character suites, and we aim to reconcile the extensive fossil record of the group in the form of anatomically preserved axes with modern phylogenies and systematic treatments of extant taxa based on molecular data. In light of the wealth of information about the axis anatomy in extant and fossil Osmundales, the use of artificial form-genera of osmundalean axes is, in our opinion, no longer needed; overall, axis anatomy has proven so informative for the systematic interpretation of fossil Osmundales that in case an anatomically preserved axis cannot be identified with certainty, it should be described in open nomenclature instead of being formally described. With the artifice of accepting also paraphyletic taxa as valid taxonomic units, we expect our systematic framework to be particularly stable and widely applicable, yet adaptable in case new data warrant erection of new groups. Furthermore, we hope to have set a new basis for in-depth analyses of evolutionary trends in Osmundales. The many homoplasious characters in our matrix make it difficult to infer explicit scenarios for the evolution of the order; a first necessary step will, therefore, be to assess which characters or character suites are compatible with the molecular tree. Ideally, this will make it possible to estimate probabilities for character changes, which can then be included in subsequent tree inferences . Total evidence (TE) analysis would benefit from using approaches that treat fossils not as terminal taxa, but place them according to their age , but seeGuaireaceae subfam. Itopsidemoideae subfam. nov.Name-bringing genus:Itopsidema Daugherty .Diagnosis: Stems radially symmetrical with spiral phyllotaxis. Stem core a parenchymatic pith with variable amounts of interspersed sclereids and tracheids; stele with a distinct peripheral metaxylem siphon lacking discrete leaf gaps and consisting of a spongy admixture of metaxylem and more or less diffusely interspersed patches of parenchyma. Stem cortex primarily parenchymatous and not differentiated into distinct layers; stipe without stipular wings and sclerenchyma ring; stipe vascular bundle with recurved tips . Roots commonly arising from abaxial side of leaf trace within the stem cortex.Tribus Osmundeae subtribus Osmundinae subtrib. nov.Name-bringing genus:Osmunda L. (in Species Plantarum 2: 1063. 1753)Diagnosis: Stem core a primarily parenchymatous pith. Stelar xylem siphon comparatively thin and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; outer stem cortex homogeneous, thicker than inner stem cortex. Stipes with a pair of stipular wings commonly containing strands of thick-walled fibres of various shapes and sizes; stipe bundle with incurved tips . Stipe sclerenchyma ring heterogeneous: differentiation typically initiating in the stipe base with the formation of an abaxial arch of particularly thick-walled fibres.Tribus Osmundeae subtribus Todeinae subtrib. nov.Name-bringing genus:Todea Willd. ex Bernh. (in Schraders Jahrbuch der Botanik 1800(2): 126. 1801)Diagnosis: Stem core a primarily parenchymatous pith. Stelar xylem siphon comparatively thin and with prominent leaf gaps. Leaf protoxylem poles initiating in mesarch position in stele, becoming endarch in stele or (rarely) in cortex, first bifurcating as leaf trace departs from stem. Cortex of stems and stipes two-layered, differentiated into inner, primarily parenchymatous cylinder and outer sclerenchymatous cylinder; inner stem cortex usually thinner than outer stem cortex; outer stem cortex heterogeneous, with a distinct ring of fibres surrounding each leaf trace. Stipes with a pair of stipular wings; stipe bundle with incurved tips ; stipe sclerenchyma ring heterogeneous, differentiating upwards into a thin band of particularly thick-walled fibres forming the outer margin of the sclerenchyma ring.Millerocaulis tuhajkulensis  comb. nov.Basionym:Osmundites tuhajkulensis Pryn. in Gorskii  ex Pryn. .Synonym:Osmundites jelkinensis Pryn. in Gorskii .Comments: Presumably due to its publication in Russian, the species has been largely overlooked in the international literature  comb. nov.Basionym:Osmundacaulis beardmorensis J.M.Schopf .Synonyms:Millerocaulis beardmorensis (J.M.Schopf) Tidwell (in SIDA 11: 402. 1986)Ashicaulis beardmorensis (J.M.Schopf) Tidwell (in SIDA 16: 256. 1994).Comments: This species is known from large, silicified root mounds from the Middle Triassic Fremouw Formation of the Transantarctic Mountains .Synonyms:Claytosmunda claytoniites  comb. nov. .Ashicaulis advencensis Y.-M. Cheng .Claytosmunda embreei  comb. nov.Basionym:Millerocaulis embreei Stockey & S.Y.Sm. .Claytosmunda johnstonii  comb. nov.Basionym:Millerocaulis johnstonii Tidwell, Munzing & M.R.Banks .Synonyms:Ashicaulis johnstonii  Tidwell (in SIDA 16: 256. 1994).Claytosmunda liaoningensis  comb. nov.Basionym:Millerocaulis liaoningensis Wu Zhang & Shao-Lin Zheng .Synonym:Ashicaulis liaoningensis (Wu Zhang & Shao-Lin Zheng) Tidwell (in SIDA 16: 256. 1994).Claytosmunda plumites (N.Tian & Y.D.Wang 2014) comb. nov.Basionym:Ashicaulis plumites N.Tian & Y.D.Wang .Claytosmunda preosmunda  comb. nov.Basionym:Millerocaulis preosmunda Y.M.Cheng, Yu F. Wang & C.S.Li .Claytosmunda sinica  comb. nov.Basionym:Millerocaulis sinica Y.M.Cheng & C.S.Li .Claytosmunda tekelili  comb. nov.Basionym:Millerocaulis tekelili E.I.Vera (in Alcheringa 36: 37. 2012).Claytosmunda wangii  comb. nov.Basionym:Ashicaulis wangii N.Tian & Y.D.Wang (in Science China Earth Sciences 57: 673. 2014b).Claytosmunda wehrii  comb. nov.Basionym:Osmunda wehrii C.N.Mill. .Osmunda kidstonii  comb. nov.Basionym:Osmundites kidstonii Stopes .Synonyms:Osmundacaulis kidstoni (Stopes) C.N.Mill. ex C.N.Mill. .Millerocaulis kidstonii (Stopes) Tidwell (in SIDA 11: 403. 1986).Ashicaulis kidstonii (Stopes) Tidwell (in SIDA 16: 256. 1994).Comments: The species is incompletely known; information on the structure of the pith, stele, and cortex is lacking. However, the anatomy of the stipe bases is so clear and so similar to modern Osmunda that we have no hesitation in assigning the fossil to this genus.Osmundastrum indentatum  comb. nov.Basionym:Australosmunda indentata R.S.Hill, S.M.Forsyth & F.Green .Synonyms:Millerocaulis indentata  Tidwell (in SIDA 16: 255. 1994).Osmundastrum precinnamomeum  comb. nov.Basionym:Osmunda precinnamomea C.N.Mill. .Osmundastrum pulchellum  comb. nov.Basionym:Osmunda pulchella Bomfleur, G.W.Grimm & McLoughlin (in BMC Evolutionary Biology 15: 126. 2015).Plenasium subgenus Aurealcaulis stat. et comb. nov.Original name and status: Genus Aurealcaulis Tidwell & L.R.Parker .Plenasium  bransonii  comb. nov.Basionym:Aurealcaulis bransonii Tidwell & Medlyn .Plenasium  burgii  comb. nov.Basionym:Aurealcaulis burgii Tidwell & J.E.Skog .Plenasium  crossii  comb. nov.Synonym:Plenasium  crossii (Tidwell & L.R.Parker) comb. nov.Basionym:Aurealcaulis crossii Tidwell & L.R.Parker .Plenasium  dakotense  comb. nov.Basionym:Aurealcaulis dakotensis Tidwell & J.E.Skog .Plenasium  moorei  comb. nov.Basionym:Aurealcaulis moorei Tidwell & Medlyn .Plenasium  nebraskense  comb. nov.Basionym:Aurealcaulis nebraskensis Tidwell & J.E.Skog .Plenasium (Plenasium) arnoldii  comb. nov.Basionym:Osmunda arnoldii C.N.Mill. .Plenasium chandleri  comb. nov.Basionym:Osmundites chandleri Arnold .Comments: In her detailed treatment of Osmunda dowkeri, it seems reasonably certain that Osmundites chandleri should be transferred to the living Osmunda and to the sub-genus Plenasium within it.” We concur with this transferal.Plenasium dowkeri  comb. nov.Basionym:Osmundites dowkeri Carruth. .Synonym:Osmunda dowkeri (Carruth.) Chandler , Geology 10: 142. 1965).Thamnopteris diploxylon  comb. nov.Basionym:Zalesskya diploxylon Kidst. & Gwynne-Vaughan .Thamnopteris gracilis  comb. nov.Basionym:Chelepteris gracilis Eichw. (in Lethaea Rossica I: 98. 1860).Synonym:Zalesskya gracilis (Eichw.) Kidst. & Gwynne-Vaughan .Thamnopteris javorskii  comb. nov.Basionym:Iegosigopteris javorskii Zalessky (in Bulletin de l’Académie des Sciences de l’URSS: 747. 1935).Thamnopteris splendida  comb. nov.Basionym:Petcheropteris splendida Zalessky (in Bulletin de l’Académie des Sciences de l’URSS: 705. 1931b).Thamnopteris uralica  comb. nov.Basionym:Zalesskya uralica Zalessky .Axis: The most general term applied to the axial organs of Osmundales; includes the stem and its surrounding mantle of roots and persistent stipe bases; may be, for instance, arborescent (trunk) or non-arborescent (rhizome).Complete gap: A leaf gap that completely perforates the stelar xylem siphon; note that gap formation may be complete even though the complete aspect of the perforation may not be visible in any single TS, e.g., in cases where gaps are small and transect the siphon obliquely at an acute angle.Delayed gap: Mode of formation of a complete leaf gap in which the gap breaks through to the pith only after the trace has departed from the stele.Dictyostele: Type of stele with external and internal phloem layers connecting through (at least some) leaf gaps, thus completely enveloping stelar xylem segments (or groups thereof) in TS of axis. Example: Osmundacaulis skidegatensis.Dictyostelic: Having a dictyostele.Dictyoxylic: Having a type of reticulate stele in which leaf gaps completely perforate the stelar metaxylem siphon such that in TS the siphon will appear like an interrupted ring of two or more separate stelar xylem segments.Dissected stele: Type of stele with external and internal endodermis layers that connect through (at least some) leaf gaps, thus completely enveloping stelar xylem segments (or groups thereof) in some TS of axis. Example: Guairea milleri.Heterogeneous sclerenchyma ring: A sclerenchyma ring in the stipe that, in its distal course away from the stem, develops distinct patches of particularly thick-walled fibres with characteristic arrangements beginning usually with an abaxial arch; proposed here as diagnostic of modern Osmundoideae.Immediate gap: Mode of formation of a complete leaf gap in which the gap breaks through to the pith immediately upon the departure of the trace from the stele; usually indicated already further below in the stele in the form of a notch or incision along the internal surface of the siphon.Incomplete gap: A leaf gap that does not perforate the stelar xylem siphon; formation of only incomplete gaps may be assumed in cases where the internal surface of the siphon is perfectly smooth and shows no notches, incisions, or embayments in a given TS.“Hewitson method”: Method introduced by xylem segments in a given TS, in which only those xylem masses are recognized as individual segments that are completely separated from adjacent masses ; alternative approaches to determine the number of xylem segments might include, for instance, counting instead the number of protoxylem clusters in a given TS of the stelar xylem siphon.Homogeneous sclerenchyma cylinder/ring: Stipe sclerenchyma cylinder that does not show a clear differentiation into regions composed of particularly thick-walled or completely occluded sclerenchyma cells .Leaf trace: Within stem portion of a vascular bundle supplying a leaf from the point where, in TS of axis, it appears completely separated from the stele up to the point where, in TS of axis, it departs from the stem to enter the stipe base and become the stipe vascular bundle.Leaf gap: Notch, incision, or perforation left in the external surface of the stelar xylem cylinder above a departed leaf trace; may be, for example, complete versus incomplete or immediate versus delayed.Mantle: Outer part of the axis enveloping the actual stem, composed usually of roots and persistent stipe bases; trunks of arborescent forms usually characterized by thick mantle with more or less vertical roots ; rhizomes of non-arborescent Osmundales usually characterized by mantle with roots radiating out- and downwards in sinuous course , in some cases forming conspicuous mounds.Petiole:Stipe.Pith: Type of stem core consisting primarily of parenchyma, in some cases with varying amounts of sclerenchyma or interspersed tracheids.Plenasoid: Term initially introduced to illustrate one representative example for a mode of leaf trace formation in which each trace is formed from two protoxylem poles originating from two adjacent stelar xylem segments; then adopted as one of a few “types” of leaf trace formation in Osmundales; here considered obsolete as a nominal category.Rhizome: The creeping to (semi-)erect axis characterizing a non-arborescent osmundalean plant.Root trace: Within stem portion of a vascular bundle supplying a root, from the point where, in TS of axis, it appears completely separated from the stele or leaf trace up to the point where, in TS of axis, it has departed from the stem and become the root vascular bundle.RS: Radial section.Sclerenchyma cylinder: a cylinder of sclerenchyma in the outer stipe cortex that derives from the outer stem cortex; may be homogeneous or heterogeneous; absent in Guaireaceae.Sclerenchyma ring: aspect of a sclerenchyma cylinder in a given TS.Semi-plenasoid (here considered obsolete): A mode of leaf trace formation in which each trace originates from a single protoxylem pole that begins to divide already in the stele, i.e., before the leaf trace separates from the stele.Stele: Central part of the stem, excluding stem cortex; including all tissues contained inside the  stelar endodermis, such as stem core and stelar metaxylem siphon.Stem: Central part of the axis consisting of stele and cortex, excluding the mantle of roots and stipe bases.Stem core: The central part of the stele that is enveloped by the stelar metaxylem siphon; may consist primarily of tracheids or of a primarily parenchymatic pith with varying amounts of tracheids and/or sclerenchyma.Stem cortex: The outer part of the stem surrounding the stele, extending from external endodermis out to stem epidermis; traversed by root traces and leaf traces.Stipe (petiole): Leafless portion of a frond connecting the frond blade (lamina) to the stem; may bear stipular wings or spines.Trunk: The tall, upright form of axis characterizing an arborescent plant.TS: Transverse section.Xylem segment, stelar x.s.: Anastomosing portion of the reticulate peripheral xylem siphon of a dictyoxylic stele that, in a given TS, appears as an isolated element separated from adjacent elements at each side by a complete (“Hewitson method”) leaf gap.Xylem siphon, stelar x.s.: Distinct tube of xylem at the stele periphery; encloses the stem core; may show variable development of leaf gaps; shared by all Osmundales.A polytomous key for identification and classification of fossil osmundalean axes is provided in 10.7717/peerj.3433/supp-1Supplemental Information 1Click here for additional data file.10.7717/peerj.3433/supp-2Supplemental Information 2Petiolar parenchyma in yellow-green; outer and inner surface of sclerenchymatic outer cortex in light grey; xylem in brown; parenchyma of pith and inner cortices left transparent and most roots omitted for clarity reasons (unlabelled version of Click here for additional data file.10.7717/peerj.3433/supp-3Supplemental Information 3Fully labelled version of Click here for additional data file.10.7717/peerj.3433/supp-4Supplemental Information 4Fully labelled raw version of Click here for additional data file.10.7717/peerj.3433/supp-5Supplemental Information 5Fully labelled raw version of Click here for additional data file.10.7717/peerj.3433/supp-6Supplemental Information 6Oum, Osmundastrum. Primarily parenchymatic tissues in pale yellow; stelar metaxylem in dark brown; sclerenchyma in light brown; note that individual fibres or fibre patches in pith or cortical tissues are omitted.Each circular diagram represents a simplified stem cross-section of the relevant species that gives basic information on the structure and proportional sizes of the stem core, xylem siphon, cortex or cortical layers and the entire stem excluding the mantle, all to the same scale. Taxon names are given in the form of six-letter labels that are contractions formed from the three first letters of the genus name in bold followed by the first three letters of the specific epithet; Click here for additional data file.10.7717/peerj.3433/supp-7Supplemental Information 7Click here for additional data file.10.7717/peerj.3433/supp-8Supplemental Information 8Click here for additional data file.10.7717/peerj.3433/supp-9Supplemental Information 9Click here for additional data file.10.7717/peerj.3433/supp-10Supplemental Information 10Click here for additional data file.10.7717/peerj.3433/supp-11Supplemental Information 11Click here for additional data file.10.7717/peerj.3433/supp-12Supplemental Information 12Click here for additional data file.10.7717/peerj.3433/supp-13Supplemental Information 13Click here for additional data file.10.7717/peerj.3433/supp-14Supplemental Information 14Click here for additional data file.