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

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.

Peer Review #2 of "Fossil fern rhizomes as a model system for exploring epiphyte community structure across geologic time: evidence from Patagonia (v0.1)"

“…In contrast to these rare occurrences, fossil osmundaceous ferns and the communities they may harbor are widespread geographically and stratigraphically (Stewart & Rothwell, 1993;Taylor, Taylor & Krings, 2009). Osmundaceous ferns can be traced back into the Paleozoic (Miller, 1967(Miller, , 1971Stewart & Rothwell, 1993;Taylor, Taylor & Krings, 2009;Bomfleur et al, 2017), a time when terrestrial ecological networks were becoming canalized (DiMichele et al, 1992). Furthermore, osmundaceous morphology has been in stasis since the Paleozoic (Eames, 1936;Stewart & Rothwell, 1993;Bomfleur, McLoughlin & Vajda, 2014).…”

“…A community of organisms similar to that of the Todea rhizome has been described associated with an exceptionally well preserved Osmundastrum pulchellum Bomfleur, G. Grimm et McLoughlin rhizome from the Jurassic of Sweden (Bomfleur, McLoughlin & Vajda, 2014;McLoughlin & Bomfleur, 2016;Bomfleur et al, 2017). Like the Patagonian community, this Jurassic community included fungi, oribatid mites (coprolites) and epiphytic plants.…”

“…Indeed, living osmundaceous fern rhizomes are often covered in epiphytes ( Fig 3C). (2) Osmundaceous ferns have been in morphological stasis since the Permian (> 250 million years ;Miller, 1971;Bomfleur et al, 2017). (3) Osmundaceae were widespread geographically in the geologic past (Miller, 1971;Bomfleur et al, 2017).…”

“…(2) Osmundaceous ferns have been in morphological stasis since the Permian (> 250 million years ;Miller, 1971;Bomfleur et al, 2017). (3) Osmundaceae were widespread geographically in the geologic past (Miller, 1971;Bomfleur et al, 2017). 4Osmundaceous ferns arguably have the richest fossil record of any living fern lineage (Arnold, 1964;Miller, 1971;Tidwell and Ash, 1994;Bomfleur et al, 2017).…”

“…(3) Osmundaceae were widespread geographically in the geologic past (Miller, 1971;Bomfleur et al, 2017). 4Osmundaceous ferns arguably have the richest fossil record of any living fern lineage (Arnold, 1964;Miller, 1971;Tidwell and Ash, 1994;Bomfleur et al, 2017). Additionally, living osmundaceous ferns allow direct comparison of the fossil epiphyte communities to their extant counterparts.…”