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The Osmundales (Royal Fern order) 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  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.

peerj

The fossil Osmundales (Royal Ferns)—a phylogenetic network analysis, revised taxonomy, and evolutionary classification of anatomically preserved trunks and rhizomes