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In the Northern Hemisphere, Plagiothecium cavifolium is currently one of the most widely distributed species. This taxon has been described as extremely variable for decades, but the reasons for this variability have not been investigated in detail. The analysis of original materials and diagnoses, as well as a detailed analysis of the history of names considered as synonyms of P. cavifolium sensu lato, showed that in terms of qualitative and quantitative characteristics, a number of the names of this complex differ significantly from the diagnosis of Hypnum cavifolium (basionym of P. cavifolium). The most important features distinguishing individual taxa include: julaceous stems; imbricate leaves, their symmetry, concavity; serration of leaf apices; the length of the cells from the middle part of the leaf; and the orientation of the capsules. Thus, the research conducted within P. cavifolium sensu lato made it possible to distinguish seven separate taxa: P. cavifolium (= P. cavifolium sensu stricto), P. flaccidum, P. tenue (being a new combination), P. ikegamii, P. subjulaceum, P. sakuraii and P. otii (four resurrected species). In addition, the analysis of original materials and the diagnosis of several taxa allowed them to be excluded from the described complex, and here we propose their synonymization with other taxa, such as P. longisetum and Hygrohypnum luridum. Photographic documentation and a key to distinguishing species within the described complex are attached. For two names (P. sakuraii and P. succulentum var. longifolium) lectotypes are proposed.

A re-examination of the original collection of Plagiothecium novae-seelandiae described by Brotherus in 1916 indicated that this material is not homogeneous. Re-examination of the diagnosis of this species and morphological analysis supports that two separate taxa should be distinguished – Plagiothecium novae-seelandiae var. novae-seelandiae and P. novae-seelandiae var. brotheri var. nov. Also, comparisons with the original collection of Hypnum lamprostachys (= P. lamprostachys ) showed differences, which supported their treatment as separate taxa. Revision of the genus Plagiothecium from Australasia (CANB, CHR, HO, MEL, WELT) and types of other species described from this part of the world ( P. funale and P. lucidum ) supported by the study of their diagnoses, qualitative and quantitative characteristics as well as mathematical analyses (PCA, HCA) allowed the division of the examined material into six separate groups – six separate taxa. Thereby, three distinct taxa are proposed – P. cordatum sp. nov. , P. semimortuum sp. nov. , and P. semimortuum var. macquariense var. nov. All taxa mentioned above are described in detail, their current known distribution and ecological preferences are also included. In addition, images illustrating their most important taxonomic features, as well as an original key to distinguish individual taxa are presented.

Hypnum denticulatum var. obtusifolium was described by Turner in the early nineteenth century. This taxon, now known as Plagiothecium denticulatum var. obtusifolium, has not been studied in detail or received a detailed description. During the revision in the Natural History Museum (herbarium BM), a specimen described there as a type (BM000890810) was found, but a careful analysis showed that it is currently not a type specimen. On the other hand, the conducted research showed that the holotype of this taxon is the figure attached by Turner to the newly described taxon. However, because the holotype does not contain all taxonomically significant features, the specimen found was used to designate the epitype of this name (BM000890810). Therefore, this paper provides the first complete description of the qualitative and quantitative characteristics of this taxon, which is today known as P. denticulatum var. obtusifolium. Forty qualitative and quantitative characteristics analyzed made it possible to make redescription of the examined taxon. Moreover, analysis of two morphologically similar, currently distinct taxa, P. sandbergii and P. denticulatum var. auritum, revealed no differences between them. Therefore, these taxa were proposed as new synonyms of P. denticulatum var. obtusifolium. Additionally, for both taxa mentioned above, lectotypes were proposed for P. denticulatum var. auritum, specimen PC0132639, and for P. sandbergii, specimen PC0132604, both from the Muséum National d’Histoire Naturelle, Paris, France (Herbarium PC).

Supported by the examination of specimens from the entire range and by the analysis of type specimens and the diagnosis of individual names, morphological and genetic studies of the Plagiothecium curvifolium complex resulted in the conclusion that this taxon should be recognized as four separate taxa. In addition to P . curvifolium s.str., there is a variety that is proposed as a new combination– P . curvifolium var. recurvum ; resurrection of the forgotten P . decursivifolium ; and the description of a new species– P . imbricatum . The features that distinguish individual taxa focus primarily on: plant size; arrangement of leaves on the stem; the symmetry, dimensions, shape, concavity and folding of leaves; cell length; serration of the leaf apex; the shape of the decurrencies; the length of the sporophyte and the shape of the operculum. For all described taxa, the distribution, ecological preferences, key to their identification and detailed photographic documentation have been provided.

Plagiothecium longisetum was described by Lindberg in 1872, based on Maximowicz materials from Japan. In the 1970s, this species was synonymized with P. nemorale. However, a polyphasic approach applied to the investigation of the P. nemorale sensu lato showed a clear separation between the specimens of former P. longisetum and the type of P. nemorale. Morphological features and molecular analyses provide evidence that those two groups are distinct, as well as allowed to describe the new species. The results are strongly supported by the statistical analyses of morphometric features and phylogenetic analyses based on concatenated nuclear and chloroplast DNA markers. The maximum likelihood (ML) and Bayesian inference (BI) analyses of ITS, rps4 and rpl16 regions place both species outside the P. nemorale group. The distinctions between individual species, reflected by the morphological features-easy to observe-and the molecular data, provide a scientific foundation for the resurrection of P. longisetum Lindb. and establishment of a new species-P. angusticellum sp. nov.

Re-assessment of the lectotype of Hypnum sylvaticum Brid. (≡ Plagiothecium sylvaticum (Brid.) Schimp.) (B 31 0915 01) showed that this specimen is characterised by dense, 6–10 cm long stems, pale green, yellowish-green to dark green and dull foliage; with complanate, ovate, not imbricate and not julaceous, 2.0–3.0 × 1.0–1.6 mm leaves; acute and denticulate, often eroded apices; 75.0–160.0 × 12.5–20.0 μm laminal cells at mid-leaf, which form diagonal rows, and decurrencies of 3–4 rows of rectangular to square, inflated cells, forming distinct auricules. Thus, this specimen represents the characteristics of the taxon currently referred to as Plagiothecium platyphyllum Mönk. Taking into account the above and the fact that the name H. sylvaticum was published first, the correct name for the species is Plagiothecium sylvaticum . Whereas the later one ( P. platyphyllum ) is a synonym. Additionally, in this article for the name P. platyphyllum , a lectotype is designated and a new synonym (Plagiothecium ruthei f. submersum) is proposed for the resurrected P. sylvaticum .

This manuscript presents current knowledge of the taxonomy, variability and distribution of taxa of the genus Plagiothecium in Europe. Currently the genus comprises 31 taxa: 17 species and 14 varieties. In this study I designated 10 lectotypes and proposes six new synonyms for the described taxa, in addition five new nomenclatural combinations: P. denticulatum var. pseudosylvaticum, P. denticulatum var. pungens, P. laetum var. hercinicum, P. succulentum var. cryptarum, and P. sylvaticum var. immersum. Detailed descriptions and distribution data for each taxon, illustrations highlighting key taxonomic features and a diagnostic key are also provided to facilitate the identification of individual taxa.

Several nomenclatural changes in the Hypnales are proposed in order to purify the circumscription of Pseudotaxiphyllum and Taxiphyllum. Three species of Pseudotaxiphyllum (P. falcifolium, P. homomallifolium and P. richardsii) do not fit well in any current genus, or belong to distinctive subgeneric lineages, which necessitates the description of novel taxa (Margrethypnum gen. nov., Redfearnia gen. nov., and Taxiphyllum subgen. Parataxiphyllum subgen. nov.). A fourth species, Pseudotaxiphyllum distichaceum [≡ Stereodon distichaceus], is transferred to Longiella, leaving only species that produce branch-like gemmae in Pseudotaxiphyllum. Five species of Taxiphyllum are transferred to other genera: T. fluitans and T. gabonense to Ectropothecium, T. laevifolium [≡ E. laevifolium] to Phyllodon, T. ligulaefolium [≡ Glossadelphus ligulaefolius] to Mittenothamnium, and T. torrentium [≡ Amblystegium torrentium] to Bryocrumia. A 50-taxon ITS-based phylogeny is included for illustrative purposes. Plagiothecium sect. Stansvikia is established to accommodate a group of medium-sized to large, glossy, autoicous species with an austral oceanic distribution. Included taxa are P. subantarcticum sp. nov. [= P. Donianum sensu Mitten 1869] from Tierra del Fuego, P. falklandicum, P. georgicoantarcticum, P. georgicoantarcticum var. antarcticum comb. nov. [≡ Hypnum antarcticum] from the Kerguelen Islands, and P. novae-seelandiae. Phylogenetically, sect. Stansvikia belongs to the core of the genus and is related to both sect. Plagiothecium and sect. Leptophyllum (P. lucidum s.l.).

An annotated checklist of the pleurocarpous moss genus Plagiothecium in Eurasia is presented for the first time based on a thorough review of the literature. Data have been compiled from previous relevant works conducted on the genus over more than 70 years and published up to the end of June 2020 for 107 Eurasian countries (and islands). Sectional classification is based on molecular phylogeny of the genus published recently. A total of 41 taxa are reported, including 29 species and 12 infraspecific taxa (nine varieties and three forms) belonging to eight sections. The highest numbers of taxa were found in China (20 taxa), the Russian Federation (20 taxa) and Japan (18 taxa), while the smallest numbers of taxa were recorded in the Middle East, Central Asia and the islands area. Not a single species of Plagiothecium was recorded in 26 regions, whereas P. denticulatum, P. nemorale and P. cavifolium turned out to be the most widespread species in the entire study area. They were recorded in most of the surveyed countries and islands. For each accepted taxon, information on relevant literature, synonyms, distribution within Eurasia and globally are provided. Comments on each taxon, ecological preferences, and notes on doubtful records are also included. Additionally, distribution maps for each recognised taxon are supplied. This checklist can enlighten and foster a better understanding of the distribution, diversity, and ecology of Plagiothecium in Eurasia and provides an incentive for future research on the genus.

• Premise of the study: Within the Hypnales—the most derived and species‐rich order of pleurocarpous mosses — phylogenies at or below the family level often show poor resolution. In preparation for a phylogeny of the genus Plagiothecium, we wished to identify the DNA markers best suited for evolutionary reconstruction in this group of hypnalean pleurocarps. • Methods: For each of 25 collections of Plagiothecium and associated taxa, 16 DNA regions were sequenced: nuclear ITS and 26S, and plastid rps4, rps4‐trnL, trnL‐F, trnK (matK)‐psbA, psbA‐trnH, trnM‐V, trnD‐T, rbcL, atpB‐rbcL, psbT‐H, rpoC1 exon 2 (partial), the trnG intron, the rpl16 intron and the plastid ribosomal spacer DNA (cpITS). Each region was evaluated on the basis of its ability to resolve clades, the amount of homoplasy present in the data set, and the relative ease of obtaining the data. Descriptive statistics for each region are given. • Key results: Under‐utilized plastid markers for bryophytes such as trnK‐psbA, rps4‐trnL, and trnD‐T outperformed more traditional markers such as trnL‐F and rps4. Individual plastid topologies were similar, suggesting that only a limited amount of plastid data are needed to recover a backbone phylogeny. Adding a small amount of nuclear ribosomal data to a large plastid matrix restructured the recovered topology, emphasizing the importance of sampling multiple genomes and the need for new low‐copy nuclear markers in bryophyte systematics. • Conclusions: Future genus‐level phylogenies of pleurocarpous mosses should target under‐utilized plastid markers such as trnK‐psbA and rps4‐trnL in conjunction with low‐copy nuclear markers.