Abstract
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.
Highlights
The Royal Ferns (Osmundales) form the most ancient surviving lineage of leptosporangiate ferns
The maximum pairwise distance found in the Osmundales matrix is 0.81; the matrix Delta Value is relatively high with 0.38 (File S3), but is within the range expected for morphological matrices including mainly fossil taxa
A tree derived from a matrix with a matrix Delta Value (mDV) > 0.2 (Table S2) can be assumed to represent the signal in the underlying dataset only inadequately, occasionally including clades that are incompatible with molecular data (Denk & Grimm, 2005; Denk, Grimm & Hemleben, 2005; Manos et al, 2007; Friis et al, 2009, see Sareela et al 2007)
Summary
The Royal Ferns (Osmundales) form the most ancient surviving lineage of leptosporangiate ferns. Osmundales has arguably the richest and most informative fossil record of any extant group of ferns (Arnold, 1964; Miller, 1971; Tidwell & Ash, 1994). Preserved trunks and rhizomes of Osmundales occur in the fossil record since the Permian (matching the stratigraphic range of fossil foliage and spores), and show an extraordinary taxic and structural diversity (Arnold, 1964; Miller, 1971; Tidwell & Ash, 1994; Tian, Wang & Jiang, 2008; Wang et al, 2014b; Fig. S1). Miller compiled, analysed, and evaluated an unparalleled amount of data on the axis structure of most extant and all fossil Osmundaceae known at that time, and provided systematic descriptions and taxonomic treatments for all included taxa in a standardized format; he carefully designed and analysed character matrices in order to reconstruct the phylogenetic history of the group from the Permian to the present. Not all subsequent authors adopted Miller’s standardized terminology and format of description; studies of the anatomy of fossil Osmundales are riddled with inconsistent terminology and taxonomy, which renders any attempt at an exhaustive analysis problematic
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